In the science of fluid flow, Stokes' paradox is the phenomenon that there can be no creeping flow of a fluid around a disk in two dimensions; or, equivalently, the fact there is no non-trivial steady-state solution for the Stokes equations around an infinitely long cylinder. This is opposed to the 3-dimensional case, where Stokes' method provides a solution to the problem of flow around a sphere.
As the stream function in a Stokes flow problem, satisfies the biharmonic equation. Since the plane may be regarded to as the complex plane, the problem may be dealt with using methods of complex analysis. In this approach, is either the real or imaginary part of
Here , where is the imaginary unit, , and are holomorphic functions outside of the disk. We will take the real part without loss of generality. Now the function , defined by is introduced. can be written as , or (using the Wirtinger derivatives). This is calculated to be equal to
The boundary conditions are:
the first condition implies for all .
Using the polar form of results in . After deriving the series form of u, substituting this into it along with , and changing some indices, the second boundary condition translates to
Since the complex trigonometric functions compose a linearly independent set, it follows that all coefficients in the series are zero. Examining these conditions for every after taking into account the condition at infinity shows that and are necessarily of the form
where is an imaginary number (opposite to its own complex conjugate), and and are complex numbers. Substituting this into gives the result that globally, compelling both and to be zero. Therefore, there can be no motion – the only solution is that the cylinder is at rest relative to all points of the fluid.
The paradox is caused by the limited validity of Stokes' approximation, as explained in Oseen's criticism: the validity of Stokes' equations relies on Reynolds number being small, and this condition cannot hold for arbitrarily large distances .
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