ROTATION OF A SPHERE IN BRINKMAN FLUIDS

The velocity field about a sphere of radius a rotating in a Brinkman f luid is derived and the hydrodynamic torque is calculated. The effect of the Darcy permeability (k) on the torque is O(a(2)/k), compared to O(a/k(1/2)) for the translational drag. The Brinkman effect on rotatio n is substantially weaker than on translational motion for any value o f a/k(1/2). Measurements of hydrodynamic torque and drag on spheres of different size could be used to determine the length scale at which t he Brinkman model fails, thus providing insight into the microstructur e of the fluid. (C) 1996 American Institute of Physics.