Mathematical modeling of motion of a fluid partially filling a rotating cylinder with radially aligned edges

A boundary-value problem of unsteady vortex motion of a viscous incompressible fluid with a free surface in a cavity rotating with a variable angular velocity and shaped as a straight circular cylinder with equidistant radial ribs is solved in a two-dimensional formulation by finite-difference methods. The drag coefficient of the rib is obtained as a function of its depth relative to the free surface.