Lamb-type solution and properties of unsteady Stokes equations.

In the present paper we derive the general solution of the unsteady Stokes equations in an unbounded fluid in spherical polar coordinates. The solution is an expansion in vector spherical harmonics and given as a sum of a particular solution, proportional to pressure gradient exhibiting power-law dependence, and a solution of vector Helmholtz equation decaying exponentially fast at infinity. The proposed decomposition resembles the classical Lamb's solution for the steady Stokes equations: the series coefficients are projections of radial component, divergence and curl of the boundary flow on scalar spherical harmonics. The proposed solution provides an explicit form of the potential far from an oscillating body (``generalized Darcy's law") and high- and low-frequency expansions. The leading order of the high-frequency expansion yields the well-known ideal (inviscid) flow approximation. Continuation of the proposed solution to imaginary frequency provides general solution of the Brinkman equations describing viscous flow in porous medium.