Beltrami flow

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In fluid dynamics, Beltrami flows are flows in which the vorticity vector and the velocity vector are parallel to each other. In other words, Beltrami flow is a flow where Lamb vector is zero. It is named after the Italian mathematician Eugenio Beltrami due to his derivation of the Beltrami vector field, while initial developments in fluid dynamics were done by the Russian scientist Ippolit S. Gromeka in 1881.[1][2]

Mathematical description[edit]

Since the vorticity vector and the velocity vector are parallel to each other, we can write

where is some scalar function. One immediate consequence of Beltrami flow is that it can never be a planar or axisymmetric flow because in those flows, vorticity is always perpendicular to the velocity field. The other important consequence will be realized by looking at the incompressible vorticity equation

where is an external body forces such as gravitational field, electric field etc., and is the kinematic viscosity. Since and are parallel, the non-linear terms in the above equation are identically zero . Thus Beltrami flows satisfies the linear equation

When , the components of vorticity satisfies a simple heat equation.

Trkalian flow[edit]

Viktor Trkal considered the Beltrami flows without any external forces in 1919[3] for the scalar function , i.e.,

Introduce the following separation of variables

then the equation satisfied by becomes

Berker's solution[edit]

Ratip Berker obtained the solution in Cartesian coordinates for in 1963,[4]

Generalized Beltrami flow[edit]

The generalized Beltrami flow satisfies the condition[5]

which is less restrictive than the Beltrami condition . Unlike the normal Beltrami flows, the generalized Beltrami flow can be studied for planar and axisymmetric flows.

Steady planar flows[edit]

For steady generalized Beltrami flow, we have and since it is also planar we have . Introduce the stream function

Integration of gives . So, complete solution is possible if it satisfies all the following three equations

A special case is considered when the flow field has uniform vorticity . Wang (1991)[6] gave the generalized solution as

assuming a linear function for . Substituting this into the vorticity equation and introducing the separation of variables with the separating constant results in

The solution obtained for different choices of can be interpreted differently, for example, represents a flow downstream a uniform grid, represents a flow created by a stretching plate, represents a flow into a corner, represents a Asymptotic suction profile etc.

Unsteady planar flows[edit]

Here,

.

Taylor's decaying vortices[edit]

G. I. Taylor gave the solution for a special case where , where is a constant in 1923.[7] He showed that the separation satisfies the equation and also

Taylor also considered an example, a decaying system of eddies rotating alternatively in opposite directions and arranged in a rectangular array

which satisfies the above equation with , where is the length of the square formed by an eddy. Therefore, this system of eddies decays as

Steady axisymmetric flows[edit]

Here we have . Integration of gives and the three equations are

Marris and Aswani (1977)[8] showed that the only possible solution is and the remaining equations reduce to

A simple set of solutions to the above equation is

represents a flow due to two opposing rotational stream on a parabolic surface, represents rotational flow on a plane wall, represents a flow ellipsoidal vortex (special case – Hill's spherical vortex), represents a type of toroidal vortex etc.

The homogeneous solution for as shown by Berker[9]

where are the Bessel function of the first kind and Bessel function of the second kind respectively. A special case of the above solution is Poiseuille flow for cylindrical geometry with transpiration velocities on the walls. Chia-Shun Yih found a solution in 1958 for Poiseuille flow into a sink when .[10]

See also[edit]

References[edit]

  1. ^ Gromeka, I. "Some cases of incompressible fluid motion." Scientific notes of the Kazan University (1881): 76–148.
  2. ^ Truesdell, Clifford. The kinematics of vorticity. Vol. 954. Bloomington: Indiana University Press, 1954.
  3. ^ Trkal, V. "A remark on the hydrodynamics of viscous fluids." Cas. Pst. Mat, Fys 48 (1919): 302–311.
  4. ^ Berker, R. "Integration des equations du movement d'un fluide visqueux incompressible. Handbuch der Physik." (1963).
  5. ^ Drazin, Philip G., and Norman Riley. The Navier–Stokes equations: a classification of flows and exact solutions. No. 334. Cambridge University Press, 2006.
  6. ^ Wang, C. Y. 1991 Exact solutions of the steady-state Navier–Stokes equations, Annu. Rev. Fluid Mech. 23, 159–177.
  7. ^ Taylor, G. I. "LXXV. On the decay of vortices in a viscous fluid." The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science 46.274 (1923): 671–674.
  8. ^ Marris, A. W., and M. G. Aswani. "On the general impossibility of controllable axi-symmetric Navier–Stokes motions." Archive for Rational Mechanics and Analysis 63.2 (1977): 107–153.
  9. ^ Berker, R. "Integration des equations du movement d'un fluide visqueux incompressible. Handbuch der Physik." (1963).
  10. ^ Yih, C. S. (1959). Two solutions for inviscid rotational flow with corner eddies. Journal of Fluid Mechanics, 5(1), 36-40.