Rayleigh problem


In fluid dynamics, Rayleigh problem also known as Stokes first problem is a problem of determining the flow created by a sudden movement of an infinitely long plate from rest, named after Lord Rayleigh and Sir George Stokes. This is considered as one of the simplest unsteady problem that have exact solution for the Navier-Stokes equations. The impulse movement of semi-infinite plate was studied by Keith Stewartson.

Flow descriptionBatchelor, George Keith. An introduction to fluid dynamics. Cambridge university press, 2000.Lagerstrom, Paco Axel. Laminar flow theory. Princeton University Press, 1996.

Consider an infinitely long plate which suddenly made to move with constant velocity in the direction, which is located at in an infinite domain of fluid, which is at rest initially everywhere. The incompressible Navier-Stokes equations reduce to
where is the kinematic viscosity. The initial and the no-slip condition on the wall are
the last condition is due to the fact that the motion at is not felt at infinity. The flow is only due to the motion of the plate, there is no imposed pressure gradient.

Self-Similar solutionAcheson, David J. Elementary fluid dynamics. Oxford University Press, 1990.

The problem on the whole is similar to the one dimensional heat conduction problem. Hence a self-similar variable can be introduced
Substituting this the partial differential equation, reduces it to ordinary differential equation
with boundary conditions
The solution to the above problem can be written in terms of complementary error function
The force per unit area exerted on the plate is

Arbitrary wall motion

Instead of using a step boundary condition for the wall movement, the velocity of the wall can be prescribed as an arbitrary function of time, i.e.,. Then the solution is given by

Rayleigh's problem in cylindrical geometry

Rotating cylinder

Consider an infinitely long cylinder of radius starts rotating suddenly at time with an angular velocity. Then the velocity in the direction is given by
where is the modified Bessel function of the second kind. As, the solution approaches that of a rigid vortex. The force per unit area exerted on the cylinder is
where is the modified Bessel function of the first kind.

Sliding cylinder

Exact solution is also available when the cylinder starts to slide in the axial direction with constant velocity. If we consider the cylinder axis to be in direction, then the solution is given by