Turbulent Boundary Layers With Unsteady Injection-Suction

The time dependent turbulent boundary-layer equations with a two-layer eddy viscosity model are integrated numerically over a porous flat plate for a steady outer flow and a sinusoidally distributed injection-suction that varies harmonically with time. The dependence of the unsteady wall shear on the frequency and strength of injection-suction is investigated. The present results indicate that the time-averaged skin friction is always larger than the skin friction observed over a nonporous flat plate. Comparison with previous theoretical results indicates that the time-averaged skin friction is always smaller than the skin friction observed over a porous flat plate with steady, simultaneous injection-suction of the same strength. The extension of eddy viscosity models, developed for steady flows, to unsteady flows is discussed.