Wall shear stress modulation in a turbulent flow subjected to imposed unsteadiness with adverse pressure gradient

The modulation characteristics of the turbulent wall shear stress measured in a plane diffuser subjected to imposed velocity oscillations are presented. The measurements reported in this paper pertain to nearly 150 different flows: imposed oscillations with three amplitudes and six frequencies, two different diverging channels and four streamwise positions. Only the most significant data are analyzed and discussed. The imposed unsteadiness affects the time-mean flow under the effect of the adverse pressure gradient (APG), in contrast to the canonical unsteady turbulent wall layers. The laminar viscous solution that adequately describes the amplitude and phase of the wall shear stress in channel flows in the high-imposed frequency regime is no longer valid in unsteady turbulent boundary layers subjected to APG. The latter modifies also the behavior of the phase shifts of the modulations in the turbulent quantities. The time lag of the wall shear stress turbulent intensity decreases as the pressure gradient increases. Some of these structural modifications are explained by the effect of the eddy viscosity that plays a key role in the vorticity diffusion process as the APG increases.