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In physics and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids—liquids and gases. It has several subdisciplines, including aerodynamics (the study of air and other gases in motion) and hydrodynamics (the study of liquids in motion). Fluid dynamics has a wide range of applications, including calculating forces and moments on aircraft, determining the mass flow rate of petroleum through pipelines, predicting weather patterns, understanding nebulae in interstellar space and modelling fission weapon detonation.
{{short description|Branch of dynamics concerned with studying the motion of air}}
{{redirect|Aerodynamic}}
[[Image:Airplane vortex edit.jpg|300px|thumb|upright=1.6|A NASA [[wake turbulence]] study at [[Wallops Flight Facility|Wallops Island]] in 1990. A [[vortex]] is created by passage of an aircraft wing, revealed by smoke. Vortices are one of the many phenomena associated with the study of aerodynamics.]]


Fluid dynamics offers a systematic structure—which underlies these practical disciplines—that embraces empirical and semi-empirical laws derived from flow measurement and used to solve practical problems. The solution to a fluid dynamics problem typically involves the calculation of various properties of the fluid, such as flow velocity, pressure, density, and temperature, as functions of space and time.
'''Aerodynamics''', from [[Greek language|Greek]] ἀήρ ''aero'' (air) + δυναμική (dynamics), is the study of motion of [[air]], particularly when affected by a solid object, such as an [[airplane]] wing. It is a sub-field of [[fluid dynamics]] and [[gas dynamics]], and many aspects of aerodynamics theory are common to these fields. The term ''aerodynamics'' is often used synonymously with gas dynamics, the difference being that "gas dynamics" applies to the study of the motion of all gases, and is not limited to air.
The formal study of aerodynamics began in the modern sense in the eighteenth century, although observations of fundamental concepts such as [[aerodynamic drag]] were recorded much earlier. Most of the early efforts in aerodynamics were directed toward achieving [[Aircraft#Heavier than air – aerodynes|heavier-than-air flight]], which was first demonstrated by [[Otto Lilienthal]] in 1891.<ref>{{cite web |title=How the Stork Inspired Human Flight |url=http://www.flyingmag.com/how-stork-inspired-human-flight.html |publisher=flyingmag.com }}{{Dead link|date=June 2020 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> Since then, the use of aerodynamics through [[mathematical]] analysis, empirical approximations, [[wind tunnel]] experimentation, and [[computer simulation]]s has formed a rational basis for the development of heavier-than-air flight and a number of other technologies. Recent work in aerodynamics has focused on issues related to [[compressible flow]], [[turbulence]], and [[boundary layer]]s and has become increasingly [[Computational fluid dynamics|computational]] in nature.


Before the twentieth century, hydrodynamics was synonymous with fluid dynamics. This is still reflected in names of some fluid dynamics topics, like magnetohydrodynamics and hydrodynamic stability, both of which can also be applied to gases.[1]
==History==
{{main article|History of aerodynamics}}
Modern aerodynamics only dates back to the seventeenth century, but aerodynamic forces have been harnessed by humans for thousands of years in sailboats and windmills,<ref>{{cite web |title=Wind Power's Beginnings (1000 BC – 1300 AD) Illustrated History of Wind Power Development |url=http://telosnet.com/wind/early.html |publisher=Telosnet.com |access-date=2011-08-24 |archive-date=2010-12-02 |archive-url=https://web.archive.org/web/20101202073417/http://telosnet.com/wind/early.html |url-status=dead }}</ref> and images and stories of flight appear throughout recorded history,<ref>{{cite book |first=Don |last=Berliner |year=1997 |url=https://books.google.com/books?id=Efr2Ll1OdqMC&pg=PA128 |title=Aviation: Reaching for the Sky |publisher= The Oliver Press, Inc. |page=128 |isbn= 1-881508-33-1}}</ref> such as the [[Ancient Greek]] legend of [[Icarus]] and [[Daedalus]].<ref>{{cite book |author1=Ovid |author2=Gregory, H. | title=The Metamorphoses | publisher=Signet Classics | year=2001 | isbn=0-451-52793-3 | oclc=45393471}}</ref> Fundamental concepts of [[Continuum mechanics|continuum]], [[Aerodynamic drag|drag]], and [[pressure gradient]]s appear in the work of [[Aristotle]] and [[Archimedes]].<ref name = "andersonhist"/>


Contents
In [[1726]], [[Isaac Newton|Sir Isaac Newton]] became the first person to develop a theory of air resistance,<ref>{{cite book | author=Newton, I. | title=Philosophiae Naturalis Principia Mathematica, Book II | year=1726}}</ref> making him one of the first aerodynamicists. [[Netherlands|Dutch]]-[[Switzerland|Swiss]] [[mathematician]] [[Daniel Bernoulli]] followed in 1738 with ''Hydrodynamica'' in which he described a fundamental relationship between pressure, density, and flow velocity for incompressible flow known today as [[Bernoulli's principle]], which provides one method for calculating aerodynamic lift.<ref>{{cite web | URL =http://www.britannica.com/EBchecked/topic/658890/Hydrodynamica#tab=active~checked%2Citems~checked&title=Hydrodynamica%20--%20Britannica%20Online%20Encyclopedia | title= Hydrodynamica | access-date=2008-10-30 |publisher= Britannica Online Encyclopedia }}</ref> In 1757, [[Leonhard Euler]] published the more general [[Euler equations (fluid dynamics)|Euler equations]] which could be applied to both compressible and incompressible flows. The Euler equations were extended to incorporate the effects of viscosity in the first half of the 1800s, resulting in the [[Navier–Stokes equations]].<ref>{{cite journal | author=Navier, C. L. M. H. | title=Memoire Sur les Lois du Mouvement des fluides | journal=Mémoires de l'Académie des Sciences |volume=6|pages=389–440 | year=1827}}</ref><ref>{{cite journal | author=Stokes, G. | title=On the Theories of the Internal Friction of Fluids in Motion|url=https://archive.org/details/cbarchive_39179_onthetheoriesoftheinternalfric1849 | journal=Transactions of the Cambridge Philosophical Society |volume=8|pages=287–305 | year=1845}}</ref> The Navier-Stokes equations are the most general governing equations of fluid flow and but are difficult to solve for the flow around all but the simplest of shapes.
1 Equations
1.1 Conservation laws
1.2 Compressible versus incompressible flow
1.3 Newtonian versus non-Newtonian fluids
1.4 Inviscid versus viscous versus Stokes flow
1.5 Steady versus unsteady flow
1.6 Laminar versus turbulent flow
1.7 Subsonic versus transonic, supersonic and hypersonic flows
1.8 Reactive versus non-reactive flows
1.9 Magnetohydrodynamics
1.10 Relativistic fluid dynamics
1.11 Other approximations
2 Terminology
2.1 Terminology in incompressible fluid dynamics
2.2 Terminology in compressible fluid dynamics
3 See also
3.1 Fields of study
3.2 Mathematical equations and concepts
3.3 Types of fluid flow
3.4 Fluid properties
3.5 Fluid phenomena
3.6 Applications
3.7 Fluid dynamics journals
3.8 Miscellaneous
3.9 See also
4 References
5 Further reading
6 External links
Equations
The foundational axioms of fluid dynamics are the conservation laws, specifically, conservation of mass, conservation of linear momentum, and conservation of energy (also known as First Law of Thermodynamics). These are based on classical mechanics and are modified in quantum mechanics and general relativity. They are expressed using the Reynolds transport theorem.


In addition to the above, fluids are assumed to obey the continuum assumption. Fluids are composed of molecules that collide with one another and solid objects. However, the continuum assumption assumes that fluids are continuous, rather than discrete. Consequently, it is assumed that properties such as density, pressure, temperature, and flow velocity are well-defined at infinitesimally small points in space and vary continuously from one point to another. The fact that the fluid is made up of discrete molecules is ignored.
[[Image:WB Wind Tunnel.jpg|thumb|A replica of the [[Wright brothers]]' [[wind tunnel]] is on display at the Virginia Air and Space Center. Wind tunnels were key in the development and validation of the laws of aerodynamics.]]


For fluids that are sufficiently dense to be a continuum, do not contain ionized species, and have flow velocities small in relation to the speed of light, the momentum equations for Newtonian fluids are the Navier–Stokes equations—which is a non-linear set of differential equations that describes the flow of a fluid whose stress depends linearly on flow velocity gradients and pressure. The unsimplified equations do not have a general closed-form solution, so they are primarily of use in computational fluid dynamics. The equations can be simplified in a number of ways, all of which make them easier to solve. Some of the simplifications allow some simple fluid dynamics problems to be solved in closed form.[citation needed]
In 1799, [[George Cayley|Sir George Cayley]] became the first person to identify the four aerodynamic forces of flight ([[weight]], [[Lift (force)|lift]], [[Aerodynamic drag|drag]], and [[thrust]]), as well as the relationships between them,<ref>{{cite web|title=U.S Centennial of Flight Commission – Sir George Cayley. |url=http://www.centennialofflight.gov/essay/Prehistory/Cayley/PH2.htm |access-date=2008-09-10 |quote=Sir George Cayley, born in 1773, is sometimes called the Father of Aviation. A pioneer in his field, he was the first to identify the four aerodynamic forces of flight – weight, lift, drag, and thrust and their relationship. He was also the first to build a successful human-carrying glider. Cayley described many of the concepts and elements of the modern airplane and was the first to understand and explain in engineering terms the concepts of lift and thrust. |archive-url=https://web.archive.org/web/20080920052758/http://centennialofflight.gov/essay/Prehistory/Cayley/PH2.htm |archive-date=20 September 2008 |url-status=dead }}</ref><ref name="AerNav123">''Cayley, George''. "On Aerial Navigation" [http://www.aeronautics.nasa.gov/fap/OnAerialNavigationPt1.pdf Part 1] {{webarchive|url=https://web.archive.org/web/20130511071413/http://www.aeronautics.nasa.gov/fap/OnAerialNavigationPt1.pdf |date=2013-05-11 }}, [http://www.aeronautics.nasa.gov/fap/OnAerialNavigationPt2.pdf Part 2] {{webarchive|url=https://web.archive.org/web/20130511041814/http://www.aeronautics.nasa.gov/fap/OnAerialNavigationPt2.pdf |date=2013-05-11 }}, [http://www.aeronautics.nasa.gov/fap/OnAerialNavigationPt3.pdf Part 3] {{webarchive|url=https://web.archive.org/web/20130511052409/http://www.aeronautics.nasa.gov/fap/OnAerialNavigationPt3.pdf |date=2013-05-11 }} ''Nicholson's Journal of Natural Philosophy'', 1809–1810. (Via [[NASA]]). [http://invention.psychology.msstate.edu/i/Cayley/Cayley.html Raw text]. Retrieved: 30 May 2010.</ref> and in doing so outlined the path toward achieving heavier-than-air flight for the next century. In 1871, [[Francis Herbert Wenham]] constructed the first [[wind tunnel]], allowing precise measurements of aerodynamic forces. Drag theories were developed by [[Jean le Rond d'Alembert]],<ref>{{cite book | author=d'Alembert, J. | title=Essai d'une nouvelle theorie de la resistance des fluides | year=1752}}</ref> [[Gustav Kirchhoff]],<ref>{{cite journal | author=Kirchhoff, G. | title=Zur Theorie freier Flussigkeitsstrahlen | journal=Journal für die reine und angewandte Mathematik |volume=1869| issue=70 |pages=289–298 | year=1869| doi=10.1515/crll.1869.70.289 | s2cid=120541431 | url=https://zenodo.org/record/1448898 }}</ref> and [[John Strutt, 3rd Baron Rayleigh|Lord Rayleigh]].<ref>{{cite journal | author=Rayleigh, Lord | title=On the Resistance of Fluids | journal=Philosophical Magazine |volume=2| issue=13 |pages=430–441 |doi=10.1080/14786447608639132| year=1876| url=https://zenodo.org/record/1431123 }}</ref> In 1889, [[Charles Renard]], a French aeronautical engineer, became the first person to reasonably predict the power needed for sustained flight.<ref>{{cite journal | author=Renard, C. | title=Nouvelles experiences sur la resistance de l'air | journal=L'Aéronaute |volume=22|pages= 73–81 | year=1889}}</ref> [[Otto Lilienthal]], the first person to become highly successful with glider flights, was also the first to propose thin, curved airfoils that would produce high lift and low drag. Building on these developments as well as research carried out in their own wind tunnel, the [[Wright brothers]] flew the first powered airplane on December 17, 1903.


In addition to the mass, momentum, and energy conservation equations, a thermodynamic equation of state that gives the pressure as a function of other thermodynamic variables is required to completely describe the problem. An example of this would be the perfect gas equation of state:
During the time of the first flights, [[Frederick W. Lanchester]],<ref>{{cite book | author=Lanchester, F. W. | title=Aerodynamics | url=https://archive.org/details/aerodynamicscons00lanc | year=1907}}</ref> [[Martin Kutta]], and [[Nikolay Yegorovich Zhukovsky|Nikolai Zhukovsky]] independently created theories that connected [[Circulation (fluid dynamics)|circulation]] of a fluid flow to lift. Kutta and Zhukovsky went on to develop a two-dimensional wing theory. Expanding upon the work of Lanchester, [[Ludwig Prandtl]] is credited with developing the mathematics<ref>{{cite book | author=Prandtl, L. | title=Tragflügeltheorie | publisher=Göttinger Nachrichten, mathematischphysikalische Klasse, 451–477 | year=1919}}</ref> behind thin-airfoil and lifting-line theories as well as work with [[boundary layer]]s.


{\displaystyle p={\frac {\rho R_{u}T}{M}}}p={\frac {\rho R_{u}T}{M}}
As aircraft speed increased, designers began to encounter challenges associated with air [[compressibility]] at speeds near the speed of sound. The differences in airflow under such conditions lead to problems in aircraft control, increased drag due to [[shock wave]]s, and the threat of structural failure due to [[Aeroelasticity|aeroelastic flutter]]. The ratio of the flow speed to the speed of sound was named the [[Mach number]] after [[Ernst Mach]] who was one of the first to investigate the properties of the [[supersonic]] flow. [[Macquorn Rankine]] and [[Pierre Henri Hugoniot]] independently developed the theory for flow properties before and after a [[shock wave]], while [[Jakob Ackeret]] led the initial work of calculating the lift and drag of supersonic airfoils.<ref>{{cite journal | author=Ackeret, J. | title=Luftkrafte auf Flugel, die mit der grosser also Schallgeschwindigkeit bewegt werden | journal=Zeitschrift für Flugtechnik und Motorluftschiffahrt |volume=16|pages=72–74 | year=1925}}</ref> [[Theodore von Kármán]] and [[Hugh Latimer Dryden]] introduced the term [[transonic]] to describe flow speeds between the [[critical Mach number]] and Mach 1 where drag increases rapidly. This rapid increase in drag led aerodynamicists and aviators to disagree on whether supersonic flight was achievable until the [[sound barrier]] was broken in 1947 using the [[Bell X-1]] aircraft.
where p is pressure, ρ is density, T the absolute temperature, while Ru is the gas constant and M is molar mass for a particular gas.


Conservation laws
By the time the sound barrier was broken, aerodynamicists' understanding of the subsonic and low supersonic flow had matured. The [[Cold War]] prompted the design of an ever-evolving line of high-performance aircraft. [[Computational fluid dynamics]] began as an effort to solve for flow properties around complex objects and has rapidly grown to the point where entire aircraft can be designed using computer software, with wind-tunnel tests followed by flight tests to confirm the computer predictions. Understanding of [[supersonic]] and [[hypersonic]] aerodynamics has matured since the 1960s, and the goals of aerodynamicists have shifted from the behaviour of fluid flow to the engineering of a vehicle such that it interacts predictably with the fluid flow. Designing aircraft for supersonic and hypersonic conditions, as well as the desire to improve the aerodynamic efficiency of current aircraft and propulsion systems, continues to motivate new research in aerodynamics, while work continues to be done on important problems in basic aerodynamic theory related to flow turbulence and the existence and uniqueness of analytical solutions to the Navier-Stokes equations.
Three conservation laws are used to solve fluid dynamics problems, and may be written in integral or differential form. The conservation laws may be applied to a region of the flow called a control volume. A control volume is a discrete volume in space through which fluid is assumed to flow. The integral formulations of the conservation laws are used to describe the change of mass, momentum, or energy within the control volume. Differential formulations of the conservation laws apply Stokes' theorem to yield an expression which may be interpreted as the integral form of the law applied to an infinitesimally small volume (at a point) within the flow.


Mass continuity (conservation of mass)
==Fundamental concepts==
The rate of change of fluid mass inside a control volume must be equal to the net rate of fluid flow into the volume. Physically, this statement requires that mass is neither created nor destroyed in the control volume,[2] and can be translated into the integral form of the continuity equation:
[[File:aeroforces.svg|thumb|Forces of flight on an [[airfoil]]]]
{\displaystyle {\frac {\partial }{\partial t}}\iiint _{V}\rho \,dV=-\,{}}{\displaystyle {\frac {\partial }{\partial t}}\iiint _{V}\rho \,dV=-\,{}} \oiint{\displaystyle {\scriptstyle S}}{\scriptstyle S} {\displaystyle {}\,\rho \mathbf {u} \cdot d\mathbf {S} }{}\,\rho \mathbf {u} \cdot d\mathbf {S}
Understanding the motion of air around an object (often called a flow field) enables the calculation of forces and [[Moment (physics)|moments]] acting on the object. In many aerodynamics problems, the forces of interest are the fundamental forces of flight: [[Lift (force)|lift]], [[Aerodynamic drag|drag]], [[thrust]], and [[weight]]. Of these, lift and drag are aerodynamic forces, i.e. forces due to air flow over a solid body. Calculation of these quantities is often founded upon the assumption that the flow field behaves as a continuum. Continuum flow fields are characterized by properties such as [[flow velocity]], [[pressure]], [[density]], and [[temperature]], which may be functions of position and time. These properties may be directly or indirectly measured in aerodynamics experiments or calculated starting with the equations for conservation of mass, [[momentum]], and energy in air flows. Density, flow velocity, and an additional property, [[viscosity]], are used to classify flow fields.
Above, ρ is the fluid density, u is the flow velocity vector, and t is time. The left-hand side of the above expression is the rate of increase of mass within the volume and contains a triple integral over the control volume, whereas the right-hand side contains an integration over the surface of the control volume of mass convected into the system. Mass flow into the system is accounted as positive, and since the normal vector to the surface is opposite the sense of flow into the system the term is negated. The differential form of the continuity equation is, by the divergence theorem:
{\displaystyle \ {\frac {\partial \rho }{\partial t}}+\nabla \cdot (\rho \mathbf {u} )=0}{\displaystyle \ {\frac {\partial \rho }{\partial t}}+\nabla \cdot (\rho \mathbf {u} )=0}
Conservation of momentum
Newton's second law of motion applied to a control volume, is a statement that any change in momentum of the fluid within that control volume will be due to the net flow of momentum into the volume and the action of external forces acting on the fluid within the volume.
{\displaystyle {\frac {\partial }{\partial t}}\iiint _{\scriptstyle V}\rho \mathbf {u} \,dV=-\,{}}{\frac {\partial }{\partial t}}\iiint _{\scriptstyle V}\rho \mathbf {u} \,dV=-\,{} \oiint{\displaystyle _{\scriptstyle S}}_{\scriptstyle S} {\displaystyle (\rho \mathbf {u} \cdot d\mathbf {S} )\mathbf {u} -{}}(\rho \mathbf {u} \cdot d\mathbf {S} )\mathbf {u} -{} \oiint{\displaystyle {\scriptstyle S}}{\scriptstyle S} {\displaystyle {}\,p\,d\mathbf {S} }{}\,p\,d\mathbf {S} {\displaystyle \displaystyle {}+\iiint _{\scriptstyle V}\rho \mathbf {f} _{\text{body}}\,dV+\mathbf {F} _{\text{surf}}}\displaystyle {}+\iiint _{\scriptstyle V}\rho \mathbf {f} _{\text{body}}\,dV+\mathbf {F} _{\text{surf}}
In the above integral formulation of this equation, the term on the left is the net change of momentum within the volume. The first term on the right is the net rate at which momentum is convected into the volume. The second term on the right is the force due to pressure on the volume's surfaces. The first two terms on the right are negated since momentum entering the system is accounted as positive, and the normal is opposite the direction of the velocity u and pressure forces. The third term on the right is the net acceleration of the mass within the volume due to any body forces (here represented by fbody). Surface forces, such as viscous forces, are represented by Fsurf, the net force due to shear forces acting on the volume surface. The momentum balance can also be written for a moving control volume.[3] The following is the differential form of the momentum conservation equation. Here, the volume is reduced to an infinitesimally small point, and both surface and body forces are accounted for in one total force, F. For example, F may be expanded into an expression for the frictional and gravitational forces acting at a point in a flow.
{\displaystyle \ {\frac {D\mathbf {u} }{Dt}}=\mathbf {F} -{\frac {\nabla p}{\rho }}}{\displaystyle \ {\frac {D\mathbf {u} }{Dt}}=\mathbf {F} -{\frac {\nabla p}{\rho }}}
In aerodynamics, air is assumed to be a Newtonian fluid, which posits a linear relationship between the shear stress (due to internal friction forces) and the rate of strain of the fluid. The equation above is a vector equation in a three-dimensional flow, but it can be expressed as three scalar equations in three coordinate directions. The conservation of momentum equations for the compressible, viscous flow case are called the Navier–Stokes equations.[2]
Conservation of energy
Although energy can be converted from one form to another, the total energy in a closed system remains constant.
{\displaystyle \ \rho {\frac {Dh}{Dt}}={\frac {Dp}{Dt}}+\nabla \cdot \left(k\nabla T\right)+\Phi }{\displaystyle \ \rho {\frac {Dh}{Dt}}={\frac {Dp}{Dt}}+\nabla \cdot \left(k\nabla T\right)+\Phi }
Above, h is the specific enthalpy, k is the thermal conductivity of the fluid, T is temperature, and Φ is the viscous dissipation function. The viscous dissipation function governs the rate at which mechanical energy of the flow is converted to heat. The second law of thermodynamics requires that the dissipation term is always positive: viscosity cannot create energy within the control volume.[4] The expression on the left side is a material derivative.
Compressible versus incompressible flow
All fluids are compressible to an extent; that is, changes in pressure or temperature cause changes in density. However, in many situations the changes in pressure and temperature are sufficiently small that the changes in density are negligible. In this case the flow can be modelled as an incompressible flow. Otherwise the more general compressible flow equations must be used.


Mathematically, incompressibility is expressed by saying that the density ρ of a fluid parcel does not change as it moves in the flow field, that is,
===Flow classification===
Flow velocity is used to classify flows according to speed regime. Subsonic flows are flow fields in which the air speed field is always below the local speed of sound. Transonic flows include both regions of subsonic flow and regions in which the local flow speed is greater than the local speed of sound. Supersonic flows are defined to be flows in which the flow speed is greater than the speed of sound everywhere. A fourth classification, hypersonic flow, refers to flows where the flow speed is much greater than the speed of sound. Aerodynamicists disagree on the precise definition of hypersonic flow.


{\displaystyle {\frac {\mathrm {D} \rho }{\mathrm {D} t}}=0\,,}{\frac {\mathrm {D} \rho }{\mathrm {D} t}}=0\,,
[[Compressibility|Compressible flow]] accounts for varying density within the flow. Subsonic flows are often idealized as incompressible, i.e. the density is assumed to be constant. Transonic and supersonic flows are compressible, and calculations that neglect the changes of density in these flow fields will yield inaccurate results.
where
D
/
Dt
is the material derivative, which is the sum of local and convective derivatives. This additional constraint simplifies the governing equations, especially in the case when the fluid has a uniform density.


For flow of gases, to determine whether to use compressible or incompressible fluid dynamics, the Mach number of the flow is evaluated. As a rough guide, compressible effects can be ignored at Mach numbers below approximately 0.3. For liquids, whether the incompressible assumption is valid depends on the fluid properties (specifically the critical pressure and temperature of the fluid) and the flow conditions (how close to the critical pressure the actual flow pressure becomes). Acoustic problems always require allowing compressibility, since sound waves are compression waves involving changes in pressure and density of the medium through which they propagate.
Viscosity is associated with the frictional forces in a flow. In some flow fields, viscous effects are very small, and approximate solutions may safely neglect viscous effects. These approximations are called inviscid flows. Flows for which viscosity is not neglected are called viscous flows. Finally, aerodynamic problems may also be classified by the flow environment. External aerodynamics is the study of flow around solid objects of various shapes (e.g. around an airplane wing), while internal aerodynamics is the study of flow through passages inside solid objects (e.g. through a jet engine).


Newtonian versus non-Newtonian fluids
====Continuum assumption====
Unlike liquids and solids, gases are composed of discrete [[molecule]]s which occupy only a small fraction of the volume filled by the gas. On a molecular level, flow fields are made up of the collisions of many individual of gas molecules between themselves and with solid surfaces. However, in most aerodynamics applications, the discrete molecular nature of gases is ignored, and the flow field is assumed to behave as a [[Continuum mechanics|continuum]]. This assumption allows fluid properties such as density and flow velocity to be defined everywhere within the flow.


Flow around an airfoil
The validity of the [[continuum assumption]] is dependent on the density of the gas and the application in question. For the continuum assumption to be valid, the [[mean free path]] length must be much smaller than the length scale of the application in question. For example, many aerodynamics applications deal with aircraft flying in atmospheric conditions, where the mean free path length is on the order of micrometers and where the body is orders of magnitude larger. In these cases, the length scale of the aircraft ranges from a few meters to a few tens of meters, which is much larger than the mean free path length. For such applications, the continuum assumption is reasonable. The continuum assumption is less valid for extremely low-density flows, such as those encountered by vehicles at very high altitudes (e.g. 300,000&nbsp;ft/90&nbsp;km)<ref name = "andersonhist">{{cite book|last = Anderson|first = John David | title = A History of Aerodynamics and its Impact on Flying Machines| publisher = Cambridge University Press |year = 1997| location=New York, NY | isbn=0-521-45435-2}}</ref> or satellites in [[Low Earth orbit]]. In those cases, [[statistical mechanics]] is a more accurate method of solving the problem than is continuum aerodynamics. The [[Knudsen number]] can be used to guide the choice between statistical mechanics and the continuous formulation of aerodynamics.
All fluids are viscous, meaning that they exert some resistance to deformation: neighbouring parcels of fluid moving at different velocities exert viscous forces on each other. The velocity gradient is referred to as a strain rate; it has dimensions T−1. Isaac Newton showed that for many familiar fluids such as water and air, the stress due to these viscous forces is linearly related to the strain rate. Such fluids are called Newtonian fluids. The coefficient of proportionality is called the fluid's viscosity; for Newtonian fluids, it is a fluid property that is independent of the strain rate.


Non-Newtonian fluids have a more complicated, non-linear stress-strain behaviour. The sub-discipline of rheology describes the stress-strain behaviours of such fluids, which include emulsions and slurries, some viscoelastic materials such as blood and some polymers, and sticky liquids such as latex, honey and lubricants.[5]
===Conservation laws===
The assumption of a [[Continuum mechanics|fluid continuum]] allows problems in aerodynamics to be solved using [[Fluid dynamics#Conservation laws|fluid dynamics conservation laws]]. Three conservation principles are used:
; [[Conservation of mass]]: Conservation of mass requires that mass is neither created nor destroyed within a flow; the mathematical formulation of this principle is known as the [[Continuity equation#Fluid dynamics|mass continuity equation]].
; [[Conservation of momentum]]: The mathematical formulation of this principle can be considered an application of [[Newton's Second Law]]. Momentum within a flow is only changed by external forces, which may include both [[surface force]]s, such as viscous ([[friction]]al) forces, and [[body force]]s, such as [[gravity|weight]]. The momentum conservation principle may be expressed as either a [[Vector space|vector]] equation or separated into a set of three [[Scalar (mathematics)|scalar]] equations (x,y,z components).
; [[Conservation of energy]]: The energy conservation equation states that energy is neither created nor destroyed within a flow, and that any addition or subtraction of energy to a volume in the flow is caused by [[heat transfer]], or by [[Work (physics)|work]] into and out of the region of interest.


Inviscid versus viscous versus Stokes flow
Together, these equations are known as the [[Navier-Stokes equations]], although some authors define the term to only include the momentum equation(s). The Navier-Stokes equations have no known analytical solution and are solved in modern aerodynamics using [[computational fluid dynamics|computational techniques]]. Because computational methods using high speed computers were not historically available and the high computational cost of solving these complex equations now that they are available, simplifications of the Navier-Stokes equations have been and continue to be employed. The [[Euler equations (fluid dynamics)|Euler equations]] are a set of similar conservation equations which neglect viscosity and may be used in cases where the effect of viscosity is expected to be small. Further simplifications lead to [[Laplace's equation]] and [[potential flow]] theory. Additionally, [[Bernoulli's principle|Bernoulli's equation]] is a solution in one dimension to both the momentum and energy conservation equations.
The dynamic of fluid parcels is described with the help of Newton's second law. An accelerating parcel of fluid is subject to inertial effects.


The Reynolds number is a dimensionless quantity which characterises the magnitude of inertial effects compared to the magnitude of viscous effects. A low Reynolds number (Re ≪ 1) indicates that viscous forces are very strong compared to inertial forces. In such cases, inertial forces are sometimes neglected; this flow regime is called Stokes or creeping flow.
The [[ideal gas law]] or another such [[equation of state]] is often used in conjunction with these equations to form a determined system that allows the solution for the unknown variables.<ref>"Understanding Aerodynamics: Arguing from the Real Physics" Doug McLean John Wiley & Sons, 2012 Chapter 3.2 "The main relationships comprising the NS equations are the basic conservation laws for mass, momentum, and energy. To have a complete equation set we also need an equation of state relating temperature, pressure, and density..." https://play.google.com/books/reader?id=_DJuEgpmdr8C&printsec=frontcover&source=gbs_vpt_reviews&pg=GBS.PA191.w.0.0.0.151</ref>


In contrast, high Reynolds numbers (Re ≫ 1) indicate that the inertial effects have more effect on the velocity field than the viscous (friction) effects. In high Reynolds number flows, the flow is often modeled as an inviscid flow, an approximation in which viscosity is completely neglected. Eliminating viscosity allows the Navier–Stokes equations to be simplified into the Euler equations. The integration of the Euler equations along a streamline in an inviscid flow yields Bernoulli's equation. When, in addition to being inviscid, the flow is irrotational everywhere, Bernoulli's equation can completely describe the flow everywhere. Such flows are called potential flows, because the velocity field may be expressed as the gradient of a potential energy expression.
==Branches of aerodynamics==
[[File:3840x1080_F16_OpenFOAM.jpg|thumb|computational modelling]]
Aerodynamic problems are classified by the flow environment or properties of the flow, including [[flow speed]], [[compressibility]], and [[viscosity]]. ''External'' aerodynamics is the study of flow around solid objects of various shapes. Evaluating the [[Lift (force)|lift]] and [[Drag (physics)|drag]] on an [[airplane]] or the [[shock wave]]s that form in front of the nose of a [[rocket]] are examples of external aerodynamics. ''Internal'' aerodynamics is the study of flow through passages in solid objects. For instance, internal aerodynamics encompasses the study of the airflow through a [[jet engine]] or through an [[air conditioning]] pipe.


This idea can work fairly well when the Reynolds number is high. However, problems such as those involving solid boundaries may require that the viscosity be included. Viscosity cannot be neglected near solid boundaries because the no-slip condition generates a thin region of large strain rate, the boundary layer, in which viscosity effects dominate and which thus generates vorticity. Therefore, to calculate net forces on bodies (such as wings), viscous flow equations must be used: inviscid flow theory fails to predict drag forces, a limitation known as the d'Alembert's paradox.
Aerodynamic problems can also be classified according to whether the [[flow speed]] is below, near or above the [[speed of sound]]. A problem is called subsonic if all the speeds in the problem are less than the speed of sound, [[transonic]] if speeds both below and above the speed of sound are present (normally when the characteristic speed is approximately the speed of sound), [[supersonic]] when the characteristic flow speed is greater than the speed of sound, and [[hypersonic]] when the flow speed is much greater than the speed of sound. Aerodynamicists disagree over the precise definition of hypersonic flow; a rough definition considers flows with [[Mach number]]s above 5 to be hypersonic.<ref name = "andersonhist"/>


A commonly used[citation needed] model, especially in computational fluid dynamics, is to use two flow models: the Euler equations away from the body, and boundary layer equations in a region close to the body. The two solutions can then be matched with each other, using the method of matched asymptotic expansions.
The influence of [[viscosity]] on the flow dictates a third classification. Some problems may encounter only very small viscous effects, in which case viscosity can be considered to be negligible. The approximations to these problems are called [[inviscid flow]]s. Flows for which viscosity cannot be neglected are called viscous flows.


Steady versus unsteady flow
===Incompressible aerodynamics===
{{further information|incompressible flow}}
An incompressible flow is a flow in which density is constant in both time and space. Although all real fluids are compressible, a flow is often approximated as incompressible if the effect of the density changes cause only small changes to the calculated results. This is more likely to be true when the flow speeds are significantly lower than the speed of sound. Effects of compressibility are more significant at speeds close to or above the speed of sound. The [[Mach number]] is used to evaluate whether the incompressibility can be assumed, otherwise the effects of compressibility must be included.


Hydrodynamics simulation of the Rayleigh–Taylor instability [6]
====Subsonic flow====
A flow that is not a function of time is called steady flow. Steady-state flow refers to the condition where the fluid properties at a point in the system do not change over time. Time dependent flow is known as unsteady (also called transient[7]). Whether a particular flow is steady or unsteady, can depend on the chosen frame of reference. For instance, laminar flow over a sphere is steady in the frame of reference that is stationary with respect to the sphere. In a frame of reference that is stationary with respect to a background flow, the flow is unsteady.
Subsonic (or low-speed) aerodynamics describes fluid motion in flows which are much lower than the speed of sound everywhere in the flow. There are several branches of subsonic flow but one special case arises when the flow is [[inviscid]], [[Compressibility|incompressible]] and [[irrotational]]. This case is called [[potential flow]] and allows the [[differential equations]] that describe the flow to be a simplified version of the equations of [[fluid dynamics]], thus making available to the aerodynamicist a range of quick and easy solutions.<ref name=":0">{{cite book|last=Katz|first=Joseph|title=Low-speed aerodynamics: From wing theory to panel methods|series=McGraw-Hill series in aeronautical and aerospace engineering|year=1991|publisher=McGraw-Hill
|location=New York|isbn=0-07-050446-6|oclc=21593499}}</ref>


Turbulent flows are unsteady by definition. A turbulent flow can, however, be statistically stationary. The random velocity field U(x, t) is statistically stationary if all statistics are invariant under a shift in time.[8]:75 This roughly means that all statistical properties are constant in time. Often, the mean field is the object of interest, and this is constant too in a statistically stationary flow.
In solving a subsonic problem, one decision to be made by the aerodynamicist is whether to incorporate the effects of compressibility. Compressibility is a description of the amount of change of [[density]] in the flow. When the effects of compressibility on the solution are small, the assumption that density is constant may be made. The problem is then an incompressible low-speed aerodynamics problem. When the density is allowed to vary, the flow is called compressible. In air, compressibility effects are usually ignored when the [[Mach number]] in the flow does not exceed 0.3 (about 335 feet (102&nbsp;m) per second or 228 miles (366&nbsp;km) per hour at 60&nbsp;°F (16&nbsp;°C)). Above Mach 0.3, the problem flow should be described using compressible aerodynamics.


Steady flows are often more tractable than otherwise similar unsteady flows. The governing equations of a steady problem have one dimension fewer (time) than the governing equations of the same problem without taking advantage of the steadiness of the flow field.
===Compressible aerodynamics===
{{main article|Compressible flow}}
According to the theory of aerodynamics, a flow is considered to be compressible if the [[density]] changes along a [[Streamlines, streaklines and pathlines|streamline]]. This means that – unlike incompressible flow – changes in density are considered. In general, this is the case where the [[Mach number]] in part or all of the flow exceeds 0.3. The Mach 0.3 value is rather arbitrary, but it is used because gas flows with a Mach number below that value demonstrate changes in density of less than 5%. Furthermore, that maximum 5% density change occurs at the [[stagnation point]] (the point on the object where flow speed is zero), while the density changes around the rest of the object will be significantly lower. Transonic, supersonic, and hypersonic flows are all compressible flows.


Laminar versus turbulent flow
====Transonic flow====
Turbulence is flow characterized by recirculation, eddies, and apparent randomness. Flow in which turbulence is not exhibited is called laminar. The presence of eddies or recirculation alone does not necessarily indicate turbulent flow—these phenomena may be present in laminar flow as well. Mathematically, turbulent flow is often represented via a Reynolds decomposition, in which the flow is broken down into the sum of an average component and a perturbation component.
{{main article|Transonic}}
The term Transonic refers to a range of flow velocities just below and above the local [[speed of sound]] (generally taken as [[Mach Number|Mach]] 0.8–1.2). It is defined as the range of speeds between the [[critical mach|critical Mach number]], when some parts of the airflow over an aircraft become [[supersonic]], and a higher speed, typically near [[Mach number|Mach 1.2]], when all of the airflow is supersonic. Between these speeds, some of the airflow is supersonic, while some of the airflow is not supersonic.


It is believed that turbulent flows can be described well through the use of the Navier–Stokes equations. Direct numerical simulation (DNS), based on the Navier–Stokes equations, makes it possible to simulate turbulent flows at moderate Reynolds numbers. Restrictions depend on the power of the computer used and the efficiency of the solution algorithm. The results of DNS have been found to agree well with experimental data for some flows.[9]
{{anchor|Supersonic aerodynamics}}<!-- anchor for the link that leads directly to the section -->


Most flows of interest have Reynolds numbers much too high for DNS to be a viable option,[8]:344 given the state of computational power for the next few decades. Any flight vehicle large enough to carry a human (L > 3 m), moving faster than 20 m/s (72 km/h; 45 mph) is well beyond the limit of DNS simulation (Re = 4 million). Transport aircraft wings (such as on an Airbus A300 or Boeing 747) have Reynolds numbers of 40 million (based on the wing chord dimension). Solving these real-life flow problems requires turbulence models for the foreseeable future. Reynolds-averaged Navier–Stokes equations (RANS) combined with turbulence modelling provides a model of the effects of the turbulent flow. Such a modelling mainly provides the additional momentum transfer by the Reynolds stresses, although the turbulence also enhances the heat and mass transfer. Another promising methodology is large eddy simulation (LES), especially in the guise of detached eddy simulation (DES)—which is a combination of RANS turbulence modelling and large eddy simulation.
====Supersonic flow====
{{main article|Supersonic}}
Supersonic aerodynamic problems are those involving flow speeds greater than the speed of sound. Calculating the lift on the [[Concorde]] during cruise can be an example of a supersonic aerodynamic problem.


Subsonic versus transonic, supersonic and hypersonic flows
Supersonic flow behaves very differently from subsonic flow. Fluids react to differences in pressure; pressure changes are how a fluid is "told" to respond to its environment. Therefore, since [[sound]] is, in fact, an infinitesimal pressure difference propagating through a fluid, the [[speed of sound]] in that fluid can be considered the fastest speed that "information" can travel in the flow. This difference most obviously manifests itself in the case of a fluid striking an object. In front of that object, the fluid builds up a [[stagnation pressure]] as impact with the object brings the moving fluid to rest. In fluid traveling at subsonic speed, this pressure disturbance can propagate upstream, changing the flow pattern ahead of the object and giving the impression that the fluid "knows" the object is there by seemingly adjusting its movement and is flowing around it. In a supersonic flow, however, the pressure disturbance cannot propagate upstream. Thus, when the fluid finally reaches the object it strikes it and the fluid is forced to change its properties – [[temperature]], [[density]], [[pressure]], and [[Mach number]]—in an extremely violent and [[reversible process (thermodynamics)|irreversible]] fashion called a [[shock wave]]. The presence of shock waves, along with the compressibility effects of high-flow velocity (see [[Reynolds number]]) fluids, is the central difference between the supersonic and subsonic aerodynamics regimes.
While many flows (such as flow of water through a pipe) occur at low Mach numbers, many flows of practical interest in aerodynamics or in turbomachines occur at high fractions of M = 1 (transonic flows) or in excess of it (supersonic or even hypersonic flows). New phenomena occur at these regimes such as instabilities in transonic flow, shock waves for supersonic flow, or non-equilibrium chemical behaviour due to ionization in hypersonic flows. In practice, each of those flow regimes is treated separately.


Reactive versus non-reactive flows
====Hypersonic flow====
Reactive flows are flows that are chemically reactive, which finds its applications in many areas, including combustion (IC engine), propulsion devices (rockets, jet engines, and so on), detonations, fire and safety hazards, and astrophysics. In addition to conservation of mass, momentum and energy, conservation of individual species (for example, mass fraction of methane in methane combustion) need to be derived, where the production/depletion rate of any species are obtained by simultaneously solving the equations of chemical kinetics.
{{main article|Hypersonic}}
In aerodynamics, hypersonic speeds are speeds that are highly supersonic. In the 1970s, the term generally came to refer to speeds of Mach 5 (5 times the speed of sound) and above. The hypersonic regime is a subset of the supersonic regime. Hypersonic flow is characterized by high temperature flow behind a shock wave, viscous interaction, and chemical dissociation of gas.


Magnetohydrodynamics
==Associated terminology==
Main article: Magnetohydrodynamics
[[File:Types of flow analysis in fluid mechanics.svg|thumb|Different types flow analysis around an airfoil:
Magnetohydrodynamics is the multidisciplinary study of the flow of electrically conducting fluids in electromagnetic fields. Examples of such fluids include plasmas, liquid metals, and salt water. The fluid flow equations are solved simultaneously with Maxwell's equations of electromagnetism.
{{legend|#f3f3fd|[[Potential flow]] theory}}
{{legend|#ff9665|[[Boundary layer|Boundary layer flow]] theory}}
{{legend|#3b3bde|[[Turbulence|Turbulent wake]] analysis}}]]


Relativistic fluid dynamics
The incompressible and compressible flow regimes produce many associated phenomena, such as boundary layers and turbulence.
Relativistic fluid dynamics studies the macroscopic and microscopic fluid motion at large velocities comparable to the velocity of light.[10] This branch of fluid dynamics accounts for the relativistic effects both from the special theory of relativity and the general theory of relativity. The governing equations are derived in Riemannian geometry for Minkowski spacetime.


Other approximations
===Boundary layers===
There are a large number of other possible approximations to fluid dynamic problems. Some of the more commonly used are listed below.
{{main article|Boundary layer}}
The concept of a [[boundary layer]] is important in many problems in aerodynamics. The viscosity and fluid friction in the air is approximated as being significant only in this thin layer. This assumption makes the description of such aerodynamics much more tractable mathematically.


The Boussinesq approximation neglects variations in density except to calculate buoyancy forces. It is often used in free convection problems where density changes are small.
===Turbulence===
Lubrication theory and Hele–Shaw flow exploits the large aspect ratio of the domain to show that certain terms in the equations are small and so can be neglected.
{{main article|Turbulence}}
Slender-body theory is a methodology used in Stokes flow problems to estimate the force on, or flow field around, a long slender object in a viscous fluid.
In aerodynamics, turbulence is characterized by chaotic property changes in the flow. These include low momentum diffusion, high momentum convection, and rapid variation of pressure and flow velocity in space and time. Flow that is not turbulent is called [[laminar flow]].
The shallow-water equations can be used to describe a layer of relatively inviscid fluid with a free surface, in which surface gradients are small.
Darcy's law is used for flow in porous media, and works with variables averaged over several pore-widths.
In rotating systems, the quasi-geostrophic equations assume an almost perfect balance between pressure gradients and the Coriolis force. It is useful in the study of atmospheric dynamics.
Terminology
The concept of pressure is central to the study of both fluid statics and fluid dynamics. A pressure can be identified for every point in a body of fluid, regardless of whether the fluid is in motion or not. Pressure can be measured using an aneroid, Bourdon tube, mercury column, or various other methods.


Some of the terminology that is necessary in the study of fluid dynamics is not found in other similar areas of study. In particular, some of the terminology used in fluid dynamics is not used in fluid statics.
==Aerodynamics in other fields==
{{refimprove section|date=March 2018}}
===Engineering design===
{{Further information|Automotive aerodynamics}}
Aerodynamics is a significant element of [[Automotive engineering|vehicle design]], including [[Car|road cars]] and [[Truck|trucks]] where the main goal is to reduce the vehicle [[drag coefficient]], and [[Auto racing|racing cars]], where in addition to reducing drag the goal is also to increase the overall level of [[downforce]].<ref name=":0" /> Aerodynamics is also important in the prediction of forces and moments acting on [[sailing|sailing vessels]]. It is used in the design of mechanical components such as [[hard drive]] heads. [[Structural engineering|Structural engineers]] resort to aerodynamics, and particularly [[aeroelasticity]], when calculating [[wind]] loads in the design of large buildings, [[bridge]]s, and [[Wind turbine design|wind turbines]]


Terminology in incompressible fluid dynamics
The aerodynamics of internal passages is important in [[HVAC|heating/ventilation]], [[Duct (HVAC)|gas piping]], and in [[Internal combustion engine|automotive engines]] where detailed flow patterns strongly affect the performance of the engine.
The concepts of total pressure and dynamic pressure arise from Bernoulli's equation and are significant in the study of all fluid flows. (These two pressures are not pressures in the usual sense—they cannot be measured using an aneroid, Bourdon tube or mercury column.) To avoid potential ambiguity when referring to pressure in fluid dynamics, many authors use the term static pressure to distinguish it from total pressure and dynamic pressure. Static pressure is identical to pressure and can be identified for every point in a fluid flow field.


A point in a fluid flow where the flow has come to rest (that is to say, speed is equal to zero adjacent to some solid body immersed in the fluid flow) is of special significance. It is of such importance that it is given a special name—a stagnation point. The static pressure at the stagnation point is of special significance and is given its own name—stagnation pressure. In incompressible flows, the stagnation pressure at a stagnation point is equal to the total pressure throughout the flow field.
===Environmental design===
Urban aerodynamics are studied by [[Urban planning|town planners]] and designers seeking to improve [[amenity]] in outdoor spaces, or in creating urban microclimates to reduce the effects of urban pollution. The field of environmental aerodynamics describes ways in which [[atmospheric circulation]] and flight mechanics affect ecosystems.


Terminology in compressible fluid dynamics
Aerodynamic equations are used in [[numerical weather prediction]].
In a compressible fluid, it is convenient to define the total conditions (also called stagnation conditions) for all thermodynamic state properties (such as total temperature, total enthalpy, total speed of sound). These total flow conditions are a function of the fluid velocity and have different values in frames of reference with different motion.


To avoid potential ambiguity when referring to the properties of the fluid associated with the state of the fluid rather than its motion, the prefix "static" is commonly used (such as static temperature and static enthalpy). Where there is no prefix, the fluid property is the static condition (so "density" and "static density" mean the same thing). The static conditions are independent of the frame of reference.
===Ball-control in sports===
Sports in which aerodynamics are of crucial importance include [[Association football|soccer]], [[table tennis]], [[cricket]], [[baseball]], and [[golf]], in which expert players can control the trajectory of the ball using the "[[Magnus effect#In sport|Magnus effect]]".


Because the total flow conditions are defined by isentropically bringing the fluid to rest, there is no need to distinguish between total entropy and static entropy as they are always equal by definition. As such, entropy is most commonly referred to as simply "entropy".
==See also==
* [[Aeronautics]]
* [[Aerostatics]]
* [[Aviation]]
* [[Insect flight]] – how bugs fly
* [[List of aerospace engineering topics]]
* [[List of engineering topics]]
* [[Nose cone design]]


See also
==References==
Fields of study
{{reflist|30em}}
Acoustic theory

Aerodynamics
==Further reading==
Aeroelasticity
{{Refbegin|2}}
Aeronautics
'''General aerodynamics'''
Computational fluid dynamics
* {{cite book | author=Anderson, John D.| author-link=John D. Anderson | title=Fundamentals of Aerodynamics | publisher=McGraw-Hill | edition=4th |year=2007 | isbn=978-0-07-125408-3 | oclc=60589123}}
Flow measurement
* {{cite book |author1=Bertin, J. J. |author2=Smith, M. L. | title=Aerodynamics for Engineers | publisher=Prentice Hall | edition=4th | year=2001 | isbn=0-13-064633-4 | oclc=47297603}}
Geophysical fluid dynamics
* {{cite book | author=Smith, Hubert C. | title=Illustrated Guide to Aerodynamics | publisher=McGraw-Hill | edition=2nd | year=1991 | isbn=0-8306-3901-2 | oclc=24319048 | url-access=registration | url=https://archive.org/details/illustratedguide0000smit }}
Hemodynamics
* {{cite book | author=Craig, Gale | title=Introduction to Aerodynamics | publisher=Regenerative Press | year=2003 | isbn=0-9646806-3-7 | oclc=53083897 | url-access=registration | url=https://archive.org/details/introductiontoae0000crai }}
Hydraulics

Hydrology
'''Subsonic aerodynamics'''
Hydrostatics
* {{cite book |author1=Katz, Joseph |author2=Plotkin, Allen | title=Low-Speed Aerodynamics | publisher=Cambridge University Press | edition=2nd | year=2001 | isbn=0-521-66552-3 | oclc=43970751 }}
Electrohydrodynamics
* Obert, Ed (2009). {{Google books|V1DuJfPov48C|Aerodynamic Design of Transport Aircraft}}. Delft; About practical aerodynamics in industry and the effects on design of aircraft. {{ISBN|978-1-58603-970-7}}.
Magnetohydrodynamics

Metafluid dynamics
'''Transonic aerodynamics'''
Quantum hydrodynamics
* {{cite book | author=Moulden, Trevor H. | title=Fundamentals of Transonic Flow | publisher=Krieger Publishing Company | year=1990 | isbn=0-89464-441-6 | oclc=20594163}}
Mathematical equations and concepts
* {{cite book |author1=Cole, Julian D |author2=Cook, L. Pamela|author2-link=Pamela Cook | title=Transonic Aerodynamics | publisher=North-Holland | year=1986 | isbn=0-444-87958-7 | oclc=13094084}}
Airy wave theory

Benjamin–Bona–Mahony equation
'''Supersonic aerodynamics'''
Boussinesq approximation (water waves)
* {{cite book | author=Ferri, Antonio | author-link=Antonio Ferri | title=Elements of Aerodynamics of Supersonic Flows | publisher=Dover Publications | edition=Phoenix | year=2005 | isbn=0-486-44280-2 | oclc=58043501}}
Different types of boundary conditions in fluid dynamics
* {{cite book | last = Shapiro | first = Ascher H. | author-link=Ascher H. Shapiro| title = The Dynamics and Thermodynamics of Compressible Fluid Flow, Volume 1 | year = 1953 | publisher = Ronald Press | isbn = 978-0-471-06691-0 | oclc = 11404735 }}
Helmholtz's theorems
* {{cite book | author=Anderson, John D. | author-link=John D. Anderson | title = Modern Compressible Flow | year = 2004 | publisher = McGraw-Hill | isbn = 0-07-124136-1 | oclc = 71626491 }}
Kirchhoff equations
* {{cite book | last1 = Liepmann | first1 = H. W. | author-link1=H. W. Liepmann | last2 = Roshko | first2 = A. | author-link2=A. Roshko | title = Elements of Gasdynamics | year = 2002 | publisher = Dover Publications | isbn = 0-486-41963-0 | oclc = 47838319 }}
Knudsen equation
* {{cite book | last = von Mises | first = Richard | author-link=Richard von Mises | title = Mathematical Theory of Compressible Fluid Flow | year = 2004 | publisher = Dover Publications | isbn = 0-486-43941-0 | oclc = 56033096 }}
Manning equation
* {{cite book | last = Hodge | first = B. K. |author2= Koenig K. | title = Compressible Fluid Dynamics with Personal Computer Applications | year = 1995 | publisher = Prentice Hall | isbn = 0-13-308552-X | oclc = 31662199 }}
Mild-slope equation

Morison equation
'''Hypersonic aerodynamics'''
Navier–Stokes equations
* {{cite book | author=Anderson, John D. | author-link=John D. Anderson | title=Hypersonic and High Temperature Gas Dynamics | publisher=AIAA | edition=2nd | year=2006 | isbn=1-56347-780-7 | oclc=68262944}}
Oseen flow
* {{cite book | last1 = Hayes | first1 = Wallace D. | author-link1=Wallace D. Hayes | last2 = Probstein | first2 = Ronald F. | author-link2=Ronald F. Probstein | title=Hypersonic Inviscid Flow | publisher=Dover Publications | year=2004 | isbn=0-486-43281-5 | oclc=53021584}}
Poiseuille's law

Pressure head
'''History of aerodynamics'''
Relativistic Euler equations
* {{cite book | author=Chanute, Octave| author-link=Octave Chanute | title=Progress in Flying Machines | publisher=Dover Publications | year=1997 | isbn=0-486-29981-3 | oclc=37782926}}
Stokes stream function
* {{cite book | author=von Karman, Theodore | author-link=Theodore von Karman |title=Aerodynamics: Selected Topics in the Light of Their Historical Development | publisher=Dover Publications | year=2004 | isbn=0-486-43485-0 | oclc=53900531}}
Stream function
* {{cite book | author=Anderson, John D.| author-link=John D. Anderson | title=A History of Aerodynamics: And Its Impact on Flying Machines | publisher=Cambridge University Press | year=1997 | isbn=0-521-45435-2 | oclc=228667184 }}
Streamlines, streaklines and pathlines

Torricelli's Law
'''Aerodynamics related to engineering'''
Types of fluid flow

Aerodynamic force
''Ground vehicles''
Cavitation
* {{cite book | author=Katz, Joseph | title=Race Car Aerodynamics: Designing for Speed | publisher=Bentley Publishers | year=1995 | isbn=0-8376-0142-8 | oclc=181644146 }}
Compressible flow
* {{cite book | author=Barnard, R. H. | title=Road Vehicle Aerodynamic Design | publisher=Mechaero Publishing | edition=2nd | year=2001 | isbn=0-9540734-0-1 | oclc=47868546 | url-access=registration | url=https://archive.org/details/roadvehicleaerod0000barn }}
Couette flow

Effusive limit
''Fixed-wing aircraft''
Free molecular flow
* {{cite book |author1=Ashley, Holt |author2=Landahl, Marten | title=Aerodynamics of Wings and Bodies | publisher=Dover Publications | edition=2nd | year=1985 | isbn=0-486-64899-0 | oclc=12021729}}
Incompressible flow
* {{cite book |author1=Abbott, Ira H. |author2=von Doenhoff, A. E. | title=Theory of Wing Sections: Including a Summary of Airfoil Data | publisher=Dover Publications | year=1959 | isbn=0-486-60586-8 | oclc=171142119}}
Inviscid flow
* {{cite book | author=Clancy, L.J. |author-link=L. J. Clancy| title=Aerodynamics | publisher=Pitman Publishing Limited | year=1975 | isbn=0-273-01120-0 | oclc=16420565}}
Isothermal flow

Open channel flow
''Helicopters''
Pipe flow
* {{cite book | author=Leishman, J. Gordon | title=Principles of Helicopter Aerodynamics | publisher=Cambridge University Press | edition=2nd | year=2006 | isbn=0-521-85860-7 | oclc=224565656 }}
Secondary flow
* {{cite book | author=Prouty, Raymond W. | title=Helicopter Performance, Stability, and Control | publisher=Krieger Publishing Company Press | year=2001 | isbn=1-57524-209-5 | oclc=212379050 }}
Stream thrust averaging
* {{cite book |author1=Seddon, J. |author2=Newman, Simon | title=Basic Helicopter Aerodynamics: An Account of First Principles in the Fluid Mechanics and Flight Dynamics of the Single Rotor Helicopter | publisher=AIAA | year=2001 | isbn=1-56347-510-3 | oclc=47623950 }}
Superfluidity

Transient flow
''Missiles''
Two-phase flow
* {{cite book | author=Nielson, Jack N. | title=Missile Aerodynamics | publisher=AIAA | year=1988 | isbn=0-9620629-0-1 | oclc=17981448}}
Fluid properties

List of hydrodynamic instabilities
''Model aircraft''
Newtonian fluid
* {{cite book | author=Simons, Martin | title=Model Aircraft Aerodynamics | publisher=Trans-Atlantic Publications, Inc. | edition=4th | year=1999 | isbn=1-85486-190-5 | oclc=43634314 }}
Non-Newtonian fluid

Surface tension
'''Related branches of aerodynamics'''
Vapour pressure

Fluid phenomena
''Aerothermodynamics''
Balanced flow
* {{cite book | author=Hirschel, Ernst H. | title=Basics of Aerothermodynamics | publisher=Springer | year=2004 | isbn=3-540-22132-8 | oclc=228383296 }}
Boundary layer
* {{cite book | author=Bertin, John J. | title=Hypersonic Aerothermodynamics | publisher=AIAA | year=1993 | isbn=1-56347-036-5 | oclc=28422796}}
Coanda effect

Convection cell
''Aeroelasticity''
Convergence/Bifurcation
* {{cite book |author1=Bisplinghoff, Raymond L. |author2=Ashley, Holt |author3=Halfman, Robert L. | title=Aeroelasticity | publisher=Dover Publications | year=1996 | isbn=0-486-69189-6 | oclc=34284560}}
Darwin drift
* {{cite book | author=Fung, Y. C. | title=An Introduction to the Theory of Aeroelasticity | publisher=Dover Publications | edition=Phoenix | year=2002 | isbn=0-486-49505-1 | oclc=55087733}}
Drag (force)

Droplet vaporization
''Boundary layers''
Hydrodynamic stability
* {{cite book | author=Young, A. D. | title=Boundary Layers | publisher=AIAA | year=1989 | isbn=0-930403-57-6 | oclc=19981526}}
Kaye effect
* {{cite book | author=Rosenhead, L. | title=Laminar Boundary Layers | publisher=Dover Publications | year=1988 | isbn=0-486-65646-2 | oclc=17619090 }}
Lift (force)

Magnus effect
''Turbulence''
Ocean current
* {{cite book | author1=Tennekes, H. | author-link1=Hendrik Tennekes | author2=Lumley, J. L. | author-link2=John L. Lumley |title=A First Course in Turbulence | publisher=The MIT Press | year=1972 | isbn=0-262-20019-8 | oclc=281992}}
Ocean surface waves
* {{cite book | author=Pope, Stephen B. | title=Turbulent Flows | publisher=Cambridge University Press | year=2000 | isbn=0-521-59886-9 | oclc=174790280 }}
Rossby wave
{{Refend}}
Shock wave

Soliton
==External links==
Stokes drift
{{commons category|Aerodynamics}}
Thread breakup
* [http://www.grc.nasa.gov/WWW/K-12/airplane/bga.html NASA Beginner's Guide to Aerodynamics]
Turbulent jet breakup
* [http://howthingsfly.si.edu Smithsonian National Air and Space Museum's How Things Fly website]
Upstream contamination
* [http://www.aerodynamics4students.com Aerodynamics for Students]
Venturi effect
* [https://web.archive.org/web/20090617225411/http://selair.selkirk.bc.ca/Training/Aerodynamics/index.html Aerodynamics for Pilots]
Vortex
* [https://web.archive.org/web/20090413073637/http://www.240edge.com/performance/tuning-aero.html Aerodynamics and Race Car Tuning]
Water hammer
* [http://www.aerodyndesign.com Aerodynamic Related Projects]
Wave drag
* [http://www.efluids.com/efluids/pages/bicycle.htm eFluids Bicycle Aerodynamics]
Wind
* [https://web.archive.org/web/20100312225152/http://www.forumula1.net/2006/f1/features/car-design-technology/aerodynamics/ Application of Aerodynamics in Formula One (F1)]
Applications
* [http://www.nas.nasa.gov/About/Education/Racecar/ Aerodynamics in Car Racing]
Acoustics
* [http://wings.avkids.com/Book/Animals/intermediate/birds-01.html Aerodynamics of Birds]
Aerodynamics

Cryosphere science

Fluidics
{{Authority control}}
Fluid power

Geodynamics
[[Category:Aerodynamics| ]]
Hydraulic machinery
[[Category:Aerospace engineering|Dynamics]]
Meteorology
[[Category:Energy in transport]]
Naval architecture
Oceanography
Plasma physics
Pneumatics
3D computer graphics
Fluid dynamics journals
Annual Review of Fluid Mechanics
Journal of Fluid Mechanics
Physics of Fluids
Experiments in Fluids
European Journal of Mechanics B: Fluids
Theoretical and Computational Fluid Dynamics
Computers and Fluids
International Journal for Numerical Methods in Fluids
Flow, Turbulence and Combustion
Miscellaneous
Important publications in fluid dynamics
Isosurface
Keulegan–Carpenter number
Rotating tank
Sound barrier
Beta plane
Immersed boundary method
Bridge scour
Finite volume method for unsteady flow
See also
Aileron – Aircraft control surface used to induce roll
Airplane – A powered, flying vehicle with wings
Angle of attack
Banked turn – Inclination of road or surface other than flat
Bernoulli's principle – principle relating to fluid dynamics
Bilgeboard
Boomerang – Thrown tool and weapon
Centerboard
Chord (aircraft)
Circulation control wing – Aircraft high-lift device
Currentology – A science that studies the internal movements of water masses
Diving plane
Downforce
Drag coefficient – Dimensionless parameter to quantify fluid resistance
Fin – Flight control surface
Flipper (anatomy) – Flattened limb adapted for propulsion and maneuvering in water
Flow separation
Foil (fluid mechanics)
Fluid coupling
Gas kinetics
Hydrofoil – A type of fast watercraft and the name of the technology it uses
Keel – Lower centreline structural element of a ship or boat hull (hydrodynamic)
Küssner effect
Kutta condition
Kutta–Joukowski theorem
Lift coefficient
Lift-induced drag
Lift-to-drag ratio
Lifting-line theory – Mathematical model to quantify lift
NACA airfoil
Newton's third law
Propeller – Device that transmits rotational power into linear thrust on a fluid
Pump – Device that imparts energy to the fluids by mechanical action
Rudder – Control surface for fluid-dynamic steering in the yaw axis
Sail – Fabric or other surface supported by a mast to allow wind propulsion (aerodynamics)
Skeg – Extension of a boat's keel at the back, also a surfboard's fin
Spoiler (automotive)
Stall (flight)
Surfboard fin
Surface science – Study of physical and chemical phenomena that occur at the interface of two phases
Torque converter
Trim tab – Small surfaces connected to the trailing edge of a larger control surface on a boat or aircraft, used to control the trim of the controls
Wing – Surface used for flight, for example by insects, birds, bats and airplanes
Wingtip vortices
References
Eckert, Michael (2006). The Dawn of Fluid Dynamics: A Discipline Between Science and Technology. Wiley. p. ix. ISBN 3-527-40513-5.
Anderson, J. D. (2007). Fundamentals of Aerodynamics (4th ed.). London: McGraw–Hill. ISBN 978-0-07-125408-3.
Nangia, Nishant; Johansen, Hans; Patankar, Neelesh A.; Bhalla, Amneet Pal S. (2017). "A moving control volume approach to computing hydrodynamic forces and torques on immersed bodies". Journal of Computational Physics. 347: 437–462. arXiv:1704.00239. Bibcode:2017JCoPh.347..437N. doi:10.1016/j.jcp.2017.06.047. S2CID 37560541.
White, F. M. (1974). Viscous Fluid Flow. New York: McGraw–Hill. ISBN 0-07-069710-8.
Wilson, DI (February 2018). "What is Rheology?". Eye. 32 (2): 179–183. doi:10.1038/eye.2017.267. PMC 5811736. PMID 29271417.
Shengtai Li, Hui Li "Parallel AMR Code for Compressible MHD or HD Equations" (Los Alamos National Laboratory) [1] Archived 2016-03-03 at the Wayback Machine
"Transient state or unsteady state? -- CFD Online Discussion Forums". www.cfd-online.com.
Pope, Stephen B. (2000). Turbulent Flows. Cambridge University Press. ISBN 0-521-59886-9.
See, for example, Schlatter et al, Phys. Fluids 21, 051702 (2009); doi:10.1063/1.3139294
Landau, Lev Davidovich; Lifshitz, Evgenii Mikhailovich (1987). Fluid Mechanics. London: Pergamon. ISBN 0-08-033933-6.
Further reading
Acheson, D. J. (1990). Elementary Fluid Dynamics. Clarendon Press. ISBN 0-19-859679-0.
Batchelor, G. K. (1967). An Introduction to Fluid Dynamics. Cambridge University Press. ISBN 0-521-66396-2.
Chanson, H. (2009). Applied Hydrodynamics: An Introduction to Ideal and Real Fluid Flows. CRC Press, Taylor & Francis Group, Leiden, The Netherlands, 478 pages. ISBN 978-0-415-49271-3.
Clancy, L. J. (1975). Aerodynamics. London: Pitman Publishing Limited. ISBN 0-273-01120-0.
Lamb, Horace (1994). Hydrodynamics (6th ed.). Cambridge University Press. ISBN 0-521-45868-4. Originally published in 1879, the 6th extended edition appeared first in 1932.
Milne-Thompson, L. M. (1968). Theoretical Hydrodynamics (5th ed.). Macmillan. Originally published in 1938.
Shinbrot, M. (1973). Lectures on Fluid Mechanics. Gordon and Breach. ISBN 0-677-01710-3.
Nazarenko, Sergey (2014), Fluid Dynamics via Examples and Solutions, CRC Press (Taylor & Francis group), ISBN 978-1-43-988882-7
Encyclopedia: Fluid dynamics Scholarpedia
External links
Wikimedia Commons has media related to Fluid dynamics.
Wikimedia Commons has media related to Fluid mechanics.
National Committee for Fluid Mechanics Films (NCFMF), containing films on several subjects in fluid dynamics (in RealMedia format)
List of Fluid Dynamics books
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Fluid Mechanics
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Branches of physics
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Heating, ventilation, and air conditioning

Revision as of 23:09, 20 February 2021

In physics and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids—liquids and gases. It has several subdisciplines, including aerodynamics (the study of air and other gases in motion) and hydrodynamics (the study of liquids in motion). Fluid dynamics has a wide range of applications, including calculating forces and moments on aircraft, determining the mass flow rate of petroleum through pipelines, predicting weather patterns, understanding nebulae in interstellar space and modelling fission weapon detonation.

Fluid dynamics offers a systematic structure—which underlies these practical disciplines—that embraces empirical and semi-empirical laws derived from flow measurement and used to solve practical problems. The solution to a fluid dynamics problem typically involves the calculation of various properties of the fluid, such as flow velocity, pressure, density, and temperature, as functions of space and time.

Before the twentieth century, hydrodynamics was synonymous with fluid dynamics. This is still reflected in names of some fluid dynamics topics, like magnetohydrodynamics and hydrodynamic stability, both of which can also be applied to gases.[1]

Contents 1 Equations 1.1 Conservation laws 1.2 Compressible versus incompressible flow 1.3 Newtonian versus non-Newtonian fluids 1.4 Inviscid versus viscous versus Stokes flow 1.5 Steady versus unsteady flow 1.6 Laminar versus turbulent flow 1.7 Subsonic versus transonic, supersonic and hypersonic flows 1.8 Reactive versus non-reactive flows 1.9 Magnetohydrodynamics 1.10 Relativistic fluid dynamics 1.11 Other approximations 2 Terminology 2.1 Terminology in incompressible fluid dynamics 2.2 Terminology in compressible fluid dynamics 3 See also 3.1 Fields of study 3.2 Mathematical equations and concepts 3.3 Types of fluid flow 3.4 Fluid properties 3.5 Fluid phenomena 3.6 Applications 3.7 Fluid dynamics journals 3.8 Miscellaneous 3.9 See also 4 References 5 Further reading 6 External links Equations The foundational axioms of fluid dynamics are the conservation laws, specifically, conservation of mass, conservation of linear momentum, and conservation of energy (also known as First Law of Thermodynamics). These are based on classical mechanics and are modified in quantum mechanics and general relativity. They are expressed using the Reynolds transport theorem.

In addition to the above, fluids are assumed to obey the continuum assumption. Fluids are composed of molecules that collide with one another and solid objects. However, the continuum assumption assumes that fluids are continuous, rather than discrete. Consequently, it is assumed that properties such as density, pressure, temperature, and flow velocity are well-defined at infinitesimally small points in space and vary continuously from one point to another. The fact that the fluid is made up of discrete molecules is ignored.

For fluids that are sufficiently dense to be a continuum, do not contain ionized species, and have flow velocities small in relation to the speed of light, the momentum equations for Newtonian fluids are the Navier–Stokes equations—which is a non-linear set of differential equations that describes the flow of a fluid whose stress depends linearly on flow velocity gradients and pressure. The unsimplified equations do not have a general closed-form solution, so they are primarily of use in computational fluid dynamics. The equations can be simplified in a number of ways, all of which make them easier to solve. Some of the simplifications allow some simple fluid dynamics problems to be solved in closed form.[citation needed]

In addition to the mass, momentum, and energy conservation equations, a thermodynamic equation of state that gives the pressure as a function of other thermodynamic variables is required to completely describe the problem. An example of this would be the perfect gas equation of state:

{\displaystyle p={\frac {\rho R_{u}T}{M}}}p={\frac {\rho R_{u}T}{M}} where p is pressure, ρ is density, T the absolute temperature, while Ru is the gas constant and M is molar mass for a particular gas.

Conservation laws Three conservation laws are used to solve fluid dynamics problems, and may be written in integral or differential form. The conservation laws may be applied to a region of the flow called a control volume. A control volume is a discrete volume in space through which fluid is assumed to flow. The integral formulations of the conservation laws are used to describe the change of mass, momentum, or energy within the control volume. Differential formulations of the conservation laws apply Stokes' theorem to yield an expression which may be interpreted as the integral form of the law applied to an infinitesimally small volume (at a point) within the flow.

Mass continuity (conservation of mass) The rate of change of fluid mass inside a control volume must be equal to the net rate of fluid flow into the volume. Physically, this statement requires that mass is neither created nor destroyed in the control volume,[2] and can be translated into the integral form of the continuity equation: {\displaystyle {\frac {\partial }{\partial t}}\iiint _{V}\rho \,dV=-\,{}}{\displaystyle {\frac {\partial }{\partial t}}\iiint _{V}\rho \,dV=-\,{}} \oiint{\displaystyle {\scriptstyle S}}{\scriptstyle S} {\displaystyle {}\,\rho \mathbf {u} \cdot d\mathbf {S} }{}\,\rho \mathbf {u} \cdot d\mathbf {S} Above, ρ is the fluid density, u is the flow velocity vector, and t is time. The left-hand side of the above expression is the rate of increase of mass within the volume and contains a triple integral over the control volume, whereas the right-hand side contains an integration over the surface of the control volume of mass convected into the system. Mass flow into the system is accounted as positive, and since the normal vector to the surface is opposite the sense of flow into the system the term is negated. The differential form of the continuity equation is, by the divergence theorem: {\displaystyle \ {\frac {\partial \rho }{\partial t}}+\nabla \cdot (\rho \mathbf {u} )=0}{\displaystyle \ {\frac {\partial \rho }{\partial t}}+\nabla \cdot (\rho \mathbf {u} )=0} Conservation of momentum Newton's second law of motion applied to a control volume, is a statement that any change in momentum of the fluid within that control volume will be due to the net flow of momentum into the volume and the action of external forces acting on the fluid within the volume. {\displaystyle {\frac {\partial }{\partial t}}\iiint _{\scriptstyle V}\rho \mathbf {u} \,dV=-\,{}}{\frac {\partial }{\partial t}}\iiint _{\scriptstyle V}\rho \mathbf {u} \,dV=-\,{} \oiint{\displaystyle _{\scriptstyle S}}_{\scriptstyle S} {\displaystyle (\rho \mathbf {u} \cdot d\mathbf {S} )\mathbf {u} -{}}(\rho \mathbf {u} \cdot d\mathbf {S} )\mathbf {u} -{} \oiint{\displaystyle {\scriptstyle S}}{\scriptstyle S} {\displaystyle {}\,p\,d\mathbf {S} }{}\,p\,d\mathbf {S} {\displaystyle \displaystyle {}+\iiint _{\scriptstyle V}\rho \mathbf {f} _{\text{body}}\,dV+\mathbf {F} _{\text{surf}}}\displaystyle {}+\iiint _{\scriptstyle V}\rho \mathbf {f} _{\text{body}}\,dV+\mathbf {F} _{\text{surf}} In the above integral formulation of this equation, the term on the left is the net change of momentum within the volume. The first term on the right is the net rate at which momentum is convected into the volume. The second term on the right is the force due to pressure on the volume's surfaces. The first two terms on the right are negated since momentum entering the system is accounted as positive, and the normal is opposite the direction of the velocity u and pressure forces. The third term on the right is the net acceleration of the mass within the volume due to any body forces (here represented by fbody). Surface forces, such as viscous forces, are represented by Fsurf, the net force due to shear forces acting on the volume surface. The momentum balance can also be written for a moving control volume.[3] The following is the differential form of the momentum conservation equation. Here, the volume is reduced to an infinitesimally small point, and both surface and body forces are accounted for in one total force, F. For example, F may be expanded into an expression for the frictional and gravitational forces acting at a point in a flow. {\displaystyle \ {\frac {D\mathbf {u} }{Dt}}=\mathbf {F} -{\frac {\nabla p}{\rho }}}{\displaystyle \ {\frac {D\mathbf {u} }{Dt}}=\mathbf {F} -{\frac {\nabla p}{\rho }}} In aerodynamics, air is assumed to be a Newtonian fluid, which posits a linear relationship between the shear stress (due to internal friction forces) and the rate of strain of the fluid. The equation above is a vector equation in a three-dimensional flow, but it can be expressed as three scalar equations in three coordinate directions. The conservation of momentum equations for the compressible, viscous flow case are called the Navier–Stokes equations.[2] Conservation of energy Although energy can be converted from one form to another, the total energy in a closed system remains constant. {\displaystyle \ \rho {\frac {Dh}{Dt}}={\frac {Dp}{Dt}}+\nabla \cdot \left(k\nabla T\right)+\Phi }{\displaystyle \ \rho {\frac {Dh}{Dt}}={\frac {Dp}{Dt}}+\nabla \cdot \left(k\nabla T\right)+\Phi } Above, h is the specific enthalpy, k is the thermal conductivity of the fluid, T is temperature, and Φ is the viscous dissipation function. The viscous dissipation function governs the rate at which mechanical energy of the flow is converted to heat. The second law of thermodynamics requires that the dissipation term is always positive: viscosity cannot create energy within the control volume.[4] The expression on the left side is a material derivative. Compressible versus incompressible flow All fluids are compressible to an extent; that is, changes in pressure or temperature cause changes in density. However, in many situations the changes in pressure and temperature are sufficiently small that the changes in density are negligible. In this case the flow can be modelled as an incompressible flow. Otherwise the more general compressible flow equations must be used.

Mathematically, incompressibility is expressed by saying that the density ρ of a fluid parcel does not change as it moves in the flow field, that is,

{\displaystyle {\frac {\mathrm {D} \rho }{\mathrm {D} t}}=0\,,}{\frac {\mathrm {D} \rho }{\mathrm {D} t}}=0\,, where D / Dt

is the material derivative, which is the sum of local and convective derivatives. This additional constraint simplifies the governing equations, especially in the case when the fluid has a uniform density.

For flow of gases, to determine whether to use compressible or incompressible fluid dynamics, the Mach number of the flow is evaluated. As a rough guide, compressible effects can be ignored at Mach numbers below approximately 0.3. For liquids, whether the incompressible assumption is valid depends on the fluid properties (specifically the critical pressure and temperature of the fluid) and the flow conditions (how close to the critical pressure the actual flow pressure becomes). Acoustic problems always require allowing compressibility, since sound waves are compression waves involving changes in pressure and density of the medium through which they propagate.

Newtonian versus non-Newtonian fluids

Flow around an airfoil All fluids are viscous, meaning that they exert some resistance to deformation: neighbouring parcels of fluid moving at different velocities exert viscous forces on each other. The velocity gradient is referred to as a strain rate; it has dimensions T−1. Isaac Newton showed that for many familiar fluids such as water and air, the stress due to these viscous forces is linearly related to the strain rate. Such fluids are called Newtonian fluids. The coefficient of proportionality is called the fluid's viscosity; for Newtonian fluids, it is a fluid property that is independent of the strain rate.

Non-Newtonian fluids have a more complicated, non-linear stress-strain behaviour. The sub-discipline of rheology describes the stress-strain behaviours of such fluids, which include emulsions and slurries, some viscoelastic materials such as blood and some polymers, and sticky liquids such as latex, honey and lubricants.[5]

Inviscid versus viscous versus Stokes flow The dynamic of fluid parcels is described with the help of Newton's second law. An accelerating parcel of fluid is subject to inertial effects.

The Reynolds number is a dimensionless quantity which characterises the magnitude of inertial effects compared to the magnitude of viscous effects. A low Reynolds number (Re ≪ 1) indicates that viscous forces are very strong compared to inertial forces. In such cases, inertial forces are sometimes neglected; this flow regime is called Stokes or creeping flow.

In contrast, high Reynolds numbers (Re ≫ 1) indicate that the inertial effects have more effect on the velocity field than the viscous (friction) effects. In high Reynolds number flows, the flow is often modeled as an inviscid flow, an approximation in which viscosity is completely neglected. Eliminating viscosity allows the Navier–Stokes equations to be simplified into the Euler equations. The integration of the Euler equations along a streamline in an inviscid flow yields Bernoulli's equation. When, in addition to being inviscid, the flow is irrotational everywhere, Bernoulli's equation can completely describe the flow everywhere. Such flows are called potential flows, because the velocity field may be expressed as the gradient of a potential energy expression.

This idea can work fairly well when the Reynolds number is high. However, problems such as those involving solid boundaries may require that the viscosity be included. Viscosity cannot be neglected near solid boundaries because the no-slip condition generates a thin region of large strain rate, the boundary layer, in which viscosity effects dominate and which thus generates vorticity. Therefore, to calculate net forces on bodies (such as wings), viscous flow equations must be used: inviscid flow theory fails to predict drag forces, a limitation known as the d'Alembert's paradox.

A commonly used[citation needed] model, especially in computational fluid dynamics, is to use two flow models: the Euler equations away from the body, and boundary layer equations in a region close to the body. The two solutions can then be matched with each other, using the method of matched asymptotic expansions.

Steady versus unsteady flow

Hydrodynamics simulation of the Rayleigh–Taylor instability [6] A flow that is not a function of time is called steady flow. Steady-state flow refers to the condition where the fluid properties at a point in the system do not change over time. Time dependent flow is known as unsteady (also called transient[7]). Whether a particular flow is steady or unsteady, can depend on the chosen frame of reference. For instance, laminar flow over a sphere is steady in the frame of reference that is stationary with respect to the sphere. In a frame of reference that is stationary with respect to a background flow, the flow is unsteady.

Turbulent flows are unsteady by definition. A turbulent flow can, however, be statistically stationary. The random velocity field U(x, t) is statistically stationary if all statistics are invariant under a shift in time.[8]:75 This roughly means that all statistical properties are constant in time. Often, the mean field is the object of interest, and this is constant too in a statistically stationary flow.

Steady flows are often more tractable than otherwise similar unsteady flows. The governing equations of a steady problem have one dimension fewer (time) than the governing equations of the same problem without taking advantage of the steadiness of the flow field.

Laminar versus turbulent flow Turbulence is flow characterized by recirculation, eddies, and apparent randomness. Flow in which turbulence is not exhibited is called laminar. The presence of eddies or recirculation alone does not necessarily indicate turbulent flow—these phenomena may be present in laminar flow as well. Mathematically, turbulent flow is often represented via a Reynolds decomposition, in which the flow is broken down into the sum of an average component and a perturbation component.

It is believed that turbulent flows can be described well through the use of the Navier–Stokes equations. Direct numerical simulation (DNS), based on the Navier–Stokes equations, makes it possible to simulate turbulent flows at moderate Reynolds numbers. Restrictions depend on the power of the computer used and the efficiency of the solution algorithm. The results of DNS have been found to agree well with experimental data for some flows.[9]

Most flows of interest have Reynolds numbers much too high for DNS to be a viable option,[8]:344 given the state of computational power for the next few decades. Any flight vehicle large enough to carry a human (L > 3 m), moving faster than 20 m/s (72 km/h; 45 mph) is well beyond the limit of DNS simulation (Re = 4 million). Transport aircraft wings (such as on an Airbus A300 or Boeing 747) have Reynolds numbers of 40 million (based on the wing chord dimension). Solving these real-life flow problems requires turbulence models for the foreseeable future. Reynolds-averaged Navier–Stokes equations (RANS) combined with turbulence modelling provides a model of the effects of the turbulent flow. Such a modelling mainly provides the additional momentum transfer by the Reynolds stresses, although the turbulence also enhances the heat and mass transfer. Another promising methodology is large eddy simulation (LES), especially in the guise of detached eddy simulation (DES)—which is a combination of RANS turbulence modelling and large eddy simulation.

Subsonic versus transonic, supersonic and hypersonic flows While many flows (such as flow of water through a pipe) occur at low Mach numbers, many flows of practical interest in aerodynamics or in turbomachines occur at high fractions of M = 1 (transonic flows) or in excess of it (supersonic or even hypersonic flows). New phenomena occur at these regimes such as instabilities in transonic flow, shock waves for supersonic flow, or non-equilibrium chemical behaviour due to ionization in hypersonic flows. In practice, each of those flow regimes is treated separately.

Reactive versus non-reactive flows Reactive flows are flows that are chemically reactive, which finds its applications in many areas, including combustion (IC engine), propulsion devices (rockets, jet engines, and so on), detonations, fire and safety hazards, and astrophysics. In addition to conservation of mass, momentum and energy, conservation of individual species (for example, mass fraction of methane in methane combustion) need to be derived, where the production/depletion rate of any species are obtained by simultaneously solving the equations of chemical kinetics.

Magnetohydrodynamics Main article: Magnetohydrodynamics Magnetohydrodynamics is the multidisciplinary study of the flow of electrically conducting fluids in electromagnetic fields. Examples of such fluids include plasmas, liquid metals, and salt water. The fluid flow equations are solved simultaneously with Maxwell's equations of electromagnetism.

Relativistic fluid dynamics Relativistic fluid dynamics studies the macroscopic and microscopic fluid motion at large velocities comparable to the velocity of light.[10] This branch of fluid dynamics accounts for the relativistic effects both from the special theory of relativity and the general theory of relativity. The governing equations are derived in Riemannian geometry for Minkowski spacetime.

Other approximations There are a large number of other possible approximations to fluid dynamic problems. Some of the more commonly used are listed below.

The Boussinesq approximation neglects variations in density except to calculate buoyancy forces. It is often used in free convection problems where density changes are small. Lubrication theory and Hele–Shaw flow exploits the large aspect ratio of the domain to show that certain terms in the equations are small and so can be neglected. Slender-body theory is a methodology used in Stokes flow problems to estimate the force on, or flow field around, a long slender object in a viscous fluid. The shallow-water equations can be used to describe a layer of relatively inviscid fluid with a free surface, in which surface gradients are small. Darcy's law is used for flow in porous media, and works with variables averaged over several pore-widths. In rotating systems, the quasi-geostrophic equations assume an almost perfect balance between pressure gradients and the Coriolis force. It is useful in the study of atmospheric dynamics. Terminology The concept of pressure is central to the study of both fluid statics and fluid dynamics. A pressure can be identified for every point in a body of fluid, regardless of whether the fluid is in motion or not. Pressure can be measured using an aneroid, Bourdon tube, mercury column, or various other methods.

Some of the terminology that is necessary in the study of fluid dynamics is not found in other similar areas of study. In particular, some of the terminology used in fluid dynamics is not used in fluid statics.

Terminology in incompressible fluid dynamics The concepts of total pressure and dynamic pressure arise from Bernoulli's equation and are significant in the study of all fluid flows. (These two pressures are not pressures in the usual sense—they cannot be measured using an aneroid, Bourdon tube or mercury column.) To avoid potential ambiguity when referring to pressure in fluid dynamics, many authors use the term static pressure to distinguish it from total pressure and dynamic pressure. Static pressure is identical to pressure and can be identified for every point in a fluid flow field.

A point in a fluid flow where the flow has come to rest (that is to say, speed is equal to zero adjacent to some solid body immersed in the fluid flow) is of special significance. It is of such importance that it is given a special name—a stagnation point. The static pressure at the stagnation point is of special significance and is given its own name—stagnation pressure. In incompressible flows, the stagnation pressure at a stagnation point is equal to the total pressure throughout the flow field.

Terminology in compressible fluid dynamics In a compressible fluid, it is convenient to define the total conditions (also called stagnation conditions) for all thermodynamic state properties (such as total temperature, total enthalpy, total speed of sound). These total flow conditions are a function of the fluid velocity and have different values in frames of reference with different motion.

To avoid potential ambiguity when referring to the properties of the fluid associated with the state of the fluid rather than its motion, the prefix "static" is commonly used (such as static temperature and static enthalpy). Where there is no prefix, the fluid property is the static condition (so "density" and "static density" mean the same thing). The static conditions are independent of the frame of reference.

Because the total flow conditions are defined by isentropically bringing the fluid to rest, there is no need to distinguish between total entropy and static entropy as they are always equal by definition. As such, entropy is most commonly referred to as simply "entropy".

See also Fields of study Acoustic theory Aerodynamics Aeroelasticity Aeronautics Computational fluid dynamics Flow measurement Geophysical fluid dynamics Hemodynamics Hydraulics Hydrology Hydrostatics Electrohydrodynamics Magnetohydrodynamics Metafluid dynamics Quantum hydrodynamics Mathematical equations and concepts Airy wave theory Benjamin–Bona–Mahony equation Boussinesq approximation (water waves) Different types of boundary conditions in fluid dynamics Helmholtz's theorems Kirchhoff equations Knudsen equation Manning equation Mild-slope equation Morison equation Navier–Stokes equations Oseen flow Poiseuille's law Pressure head Relativistic Euler equations Stokes stream function Stream function Streamlines, streaklines and pathlines Torricelli's Law Types of fluid flow Aerodynamic force Cavitation Compressible flow Couette flow Effusive limit Free molecular flow Incompressible flow Inviscid flow Isothermal flow Open channel flow Pipe flow Secondary flow Stream thrust averaging Superfluidity Transient flow Two-phase flow Fluid properties List of hydrodynamic instabilities Newtonian fluid Non-Newtonian fluid Surface tension Vapour pressure Fluid phenomena Balanced flow Boundary layer Coanda effect Convection cell Convergence/Bifurcation Darwin drift Drag (force) Droplet vaporization Hydrodynamic stability Kaye effect Lift (force) Magnus effect Ocean current Ocean surface waves Rossby wave Shock wave Soliton Stokes drift Thread breakup Turbulent jet breakup Upstream contamination Venturi effect Vortex Water hammer Wave drag Wind Applications Acoustics Aerodynamics Cryosphere science Fluidics Fluid power Geodynamics Hydraulic machinery Meteorology Naval architecture Oceanography Plasma physics Pneumatics 3D computer graphics Fluid dynamics journals Annual Review of Fluid Mechanics Journal of Fluid Mechanics Physics of Fluids Experiments in Fluids European Journal of Mechanics B: Fluids Theoretical and Computational Fluid Dynamics Computers and Fluids International Journal for Numerical Methods in Fluids Flow, Turbulence and Combustion Miscellaneous Important publications in fluid dynamics Isosurface Keulegan–Carpenter number Rotating tank Sound barrier Beta plane Immersed boundary method Bridge scour Finite volume method for unsteady flow See also Aileron – Aircraft control surface used to induce roll Airplane – A powered, flying vehicle with wings Angle of attack Banked turn – Inclination of road or surface other than flat Bernoulli's principle – principle relating to fluid dynamics Bilgeboard Boomerang – Thrown tool and weapon Centerboard Chord (aircraft) Circulation control wing – Aircraft high-lift device Currentology – A science that studies the internal movements of water masses Diving plane Downforce Drag coefficient – Dimensionless parameter to quantify fluid resistance Fin – Flight control surface Flipper (anatomy) – Flattened limb adapted for propulsion and maneuvering in water Flow separation Foil (fluid mechanics) Fluid coupling Gas kinetics Hydrofoil – A type of fast watercraft and the name of the technology it uses Keel – Lower centreline structural element of a ship or boat hull (hydrodynamic) Küssner effect Kutta condition Kutta–Joukowski theorem Lift coefficient Lift-induced drag Lift-to-drag ratio Lifting-line theory – Mathematical model to quantify lift NACA airfoil Newton's third law Propeller – Device that transmits rotational power into linear thrust on a fluid Pump – Device that imparts energy to the fluids by mechanical action Rudder – Control surface for fluid-dynamic steering in the yaw axis Sail – Fabric or other surface supported by a mast to allow wind propulsion (aerodynamics) Skeg – Extension of a boat's keel at the back, also a surfboard's fin Spoiler (automotive) Stall (flight) Surfboard fin Surface science – Study of physical and chemical phenomena that occur at the interface of two phases Torque converter Trim tab – Small surfaces connected to the trailing edge of a larger control surface on a boat or aircraft, used to control the trim of the controls Wing – Surface used for flight, for example by insects, birds, bats and airplanes Wingtip vortices References

Eckert, Michael (2006). The Dawn of Fluid Dynamics: A Discipline Between Science and Technology. Wiley. p. ix. ISBN 3-527-40513-5.
Anderson, J. D. (2007). Fundamentals of Aerodynamics (4th ed.). London: McGraw–Hill. ISBN 978-0-07-125408-3.
Nangia, Nishant; Johansen, Hans; Patankar, Neelesh A.; Bhalla, Amneet Pal S. (2017). "A moving control volume approach to computing hydrodynamic forces and torques on immersed bodies". Journal of Computational Physics. 347: 437–462. arXiv:1704.00239. Bibcode:2017JCoPh.347..437N. doi:10.1016/j.jcp.2017.06.047. S2CID 37560541.
White, F. M. (1974). Viscous Fluid Flow. New York: McGraw–Hill. ISBN 0-07-069710-8.
Wilson, DI (February 2018). "What is Rheology?". Eye. 32 (2): 179–183. doi:10.1038/eye.2017.267. PMC 5811736. PMID 29271417.
Shengtai Li, Hui Li "Parallel AMR Code for Compressible MHD or HD Equations" (Los Alamos National Laboratory) [1] Archived 2016-03-03 at the Wayback Machine
"Transient state or unsteady state? -- CFD Online Discussion Forums". www.cfd-online.com.
Pope, Stephen B. (2000). Turbulent Flows. Cambridge University Press. ISBN 0-521-59886-9.
See, for example, Schlatter et al, Phys. Fluids 21, 051702 (2009); doi:10.1063/1.3139294
Landau, Lev Davidovich; Lifshitz, Evgenii Mikhailovich (1987). Fluid Mechanics. London: Pergamon. ISBN 0-08-033933-6.

Further reading Acheson, D. J. (1990). Elementary Fluid Dynamics. Clarendon Press. ISBN 0-19-859679-0. Batchelor, G. K. (1967). An Introduction to Fluid Dynamics. Cambridge University Press. ISBN 0-521-66396-2. Chanson, H. (2009). Applied Hydrodynamics: An Introduction to Ideal and Real Fluid Flows. CRC Press, Taylor & Francis Group, Leiden, The Netherlands, 478 pages. ISBN 978-0-415-49271-3. Clancy, L. J. (1975). Aerodynamics. London: Pitman Publishing Limited. ISBN 0-273-01120-0. Lamb, Horace (1994). Hydrodynamics (6th ed.). Cambridge University Press. ISBN 0-521-45868-4. Originally published in 1879, the 6th extended edition appeared first in 1932. Milne-Thompson, L. M. (1968). Theoretical Hydrodynamics (5th ed.). Macmillan. Originally published in 1938. Shinbrot, M. (1973). Lectures on Fluid Mechanics. Gordon and Breach. ISBN 0-677-01710-3. Nazarenko, Sergey (2014), Fluid Dynamics via Examples and Solutions, CRC Press (Taylor & Francis group), ISBN 978-1-43-988882-7 Encyclopedia: Fluid dynamics Scholarpedia External links Wikimedia Commons has media related to Fluid dynamics. Wikimedia Commons has media related to Fluid mechanics. National Committee for Fluid Mechanics Films (NCFMF), containing films on several subjects in fluid dynamics (in RealMedia format) List of Fluid Dynamics books vte Fluid Mechanics vte Branches of physics vte Heating, ventilation, and air conditioning