Effects of pressure gradient sequences on wall shear stress

The variation of wall shear stress within non equilibrium turbulent boundary layers exposed to favorable-to-adverse pressure gradient sequences is quantified using an array of three, one-direction capacitive wall shear stress sensors. These are flush mounted with uniform spacing on a flat plate in the adverse pressure gradient region, just downstream of the favorable pressure gradient. We validate the skin friction results in a zero pressure gradient condition against literature and mean skin friction coefficients extracted from laser Doppler velocimetry across a range of friction Reynolds numbers from 1090 to 2578 acquired in the same experimental facility. We then apply sequences of pressure gradients of different signs, characterized by non-constant Clauser pressure gradient parameter (dβ/dx≠const), through a false convex deformable ceiling above the flat plate. Results reveal strong flow history, with non-monotonic increases of skin friction coefficients and reduction of skin friction fluctuations as the strength of the pressure gradient sequence increases. We present skin friction statistics together with coherence spectrum and cross-correlation results.