The Kutta–Joukowski theorem is a fundamental theorem of aerodynamics, for the calculation of the lift on a rotating cylinder. It is named after the German Martin Wilhelm Kutta and the Russian Nikolai Zhukovsky (or Joukowski) who first developed its key ideas in the early 20th century. The theorem relates the lift generated by a right cylinder to the speed of the cylinder through the fluid, the density of the fluid, and the circulation. The circulation is defined as the line integral, around a closed loop enclosing the cylinder or airfoil, of the component of the velocity of the fluid tangent to the loop. The magnitude and direction of the fluid velocity change along the path.
The flow of air in response to the presence of the airfoil can be treated as the superposition of a translational flow and a rotational flow, known as a "vortex". (It is, however, misleading to picture a vortex like a tornado encircling the cylinder or the wing of an airplane in flight. The vortex is defined by the integral's path that encircles the cylinder, and is defined by the mathematical value of the vorticity; not a vortex of air.) In descriptions of the Kutta–Joukowski theorem the airfoil is usually considered to be a circular cylinder or some other Joukowski airfoil.
The theorem refers to two-dimensional flow around a cylinder (or a cylinder of infinite span) and determines the lift generated by one unit of span. When the circulation is known, the lift per unit span (or ) of the cylinder can be calculated using the following equation:
where and are the fluid density and the fluid velocity far upstream of the cylinder, and is the (anticlockwise positive) circulation defined as the line integral,
around a closed contour enclosing the cylinder or airfoil and followed in the positive (anticlockwise) direction. This path must be in a region of potential flow and not in the boundary layer of the cylinder. The integrand is the component of the local fluid velocity in the direction tangent to the curve and is an infinitesimal length on the curve, . Equation (1) is a form of the Kutta–Joukowski theorem.
Kuethe and Schetzer state the Kutta–Joukowski theorem as follows:
- The force per unit length acting on a right cylinder of any cross section whatsoever is equal to , and is perpendicular to the direction of
For a rather heuristic argument, consider a thin airfoil of chord and infinite span, moving through air of density . Let the airfoil be inclined to the oncoming flow to produce an air speed on one side of the airfoil, and an air speed on the other side. The circulation is then
The difference in pressure between the two sides of the airfoil can be found by applying Bernoulli's equation:
so the lift force per unit span is
Formal derivation of Kutta–Joukowski theorem First of all, the force exerted on each unit length of a cylinder of arbitrary cross section is calculated. Let this force per unit length (from now on referred to simply as force) be . So then the total force is:
where C denotes the borderline of the cylinder, is the static pressure of the fluid, is the unit vector normal to the cylinder, and ds is the arc element of the borderline of the cross section. Now let be the angle between the normal vector and the vertical. Then the components of the above force are:
Now comes a crucial step: consider the used two-dimensional space as a complex plane. So every vector can be represented as a complex number, with its first component equal to the real part and its second component equal to the imaginary part of the complex number. Then, the force can be represented as:
The next step is to take the complex conjugate of the force and do some manipulation:
Surface segments ds are related to changes dz along them by:
Plugging this back into the integral, the result is:
Now the Bernoulli equation is used, in order to remove the pressure from the integral. Throughout the analysis it is assumed that there is no outer force field present. The mass density of the flow is Then pressure is related to velocity by:
With this the force becomes:
Only one step is left to do: introduce the complex potential of the flow. This is related to the velocity components as where the apostrophe denotes differentiation with respect to the complex variable z. The velocity is tangent to the borderline C, so this means that Therefore, and the desired expression for the force is obtained:
which is called the Blasius–Chaplygin formula.
To arrive at the Joukowski formula, this integral has to be evaluated. From complex analysis it is known that a holomorphic function can be presented as a Laurent series. From the physics of the problem it is deduced that the derivative of the complex potential will look thus:
The function does not contain higher order terms, since the velocity stays finite at infinity. So represents the derivative the complex potential at infinity: . The next task is to find out the meaning of . Using the residue theorem on the above series:
Now perform the above integration:
The first integral is recognized as the circulation denoted by The second integral can be evalutated after some manipulation:
Here is the stream function. Since the C border of the cylinder is a streamline itself, the stream function does not change on it . Hence the above integral is zero. As a result:
Take the square of the series:
Plugging this back into the Blasius–Chaplygin formula, and performing the integration using the residue theorem:
And so the Kutta–Joukowski formula is:
The lift predicted by Kutta Joukowski theorem within the framework of inviscid flow theory is quite accurate even for real viscous flow, provided the flow is steady and unseparated.
For an impulsively started flow such as obtained by suddenly accelerating an airfoil or setting an angle of attack, there is a vortex sheet continuously shed at the trailing edge and the lift force is unsteady. This is known as the Wagner problem  for which the initial lift is one half of the final lift given by the Kutta Joukowski formula.
When a source is present outside the body, a force correction due to this source can be expressed as the product of the strength of outside source and the induced velocity at this source by all the causes except this source. This is known as the Lagally theorem.
For free vortices and other bodies outside the body, a generalized Lagally theorem holds, with which the forces are expressed as the products of strength of inner singularities (image vortices, sources and doublets inside each body) and the induced velocity at these singularities by all causes except those inside this body. The contribution due to each inner singularity sums up to give the total force. The motion of outside singularities also contributes to forces, and the force component due to this contribution is proportional to the speed of the singularity.
For two-dimensional inviscid flow, the classical Kutta Joukowski theorem predicts a zero drag. When, however, there is vortex outside the body, there is a vortex induced drag, in a form similar to the induced lift.
When in addition to multiple free vortices and multiple bodies, there are bound vortices and vortex production on the body surface, the generalized Lagally theorem still holds, but a force due to vortex production exists. This vortex production force is proportional to the vortex production rate and the distance between the vortex pair in production. With this approach, an explicit and algebraic force formula, taking into account of all causes (inner singularities, outside vortices and bodies, motion of all singularities and bodies, and vortex production) holds individually for each body, with the role of other bodies represented by additional singularities. Hence a force decomposition according to bodies is possible.
For general three-dimensional, viscous and unsteady flow, force formulas are expressed in integral forms. The volume integration of certain flow quantities, such as vorticity moments, is related to forces. Various forms of integral approach are now available for unbounded domain and for artificially truncated domain.
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