Amplitude modulated near-wall cycle in a turbulent boundary layer under an adverse pressure gradient

THIS PAPER PRESENTS the effect of amplitude modulation of small-scale turbulence by large-scale structures in a turbulent boundary layer subjected to an adverse pressure gradient. The results have been compared with the literature data for high Reynolds number, zero-pressure gradient cases (MATHIS et al. [1]). It was observed that for relatively low-Reynolds-number turbulent boundary layer (Re-tau approximate to 1000) subjected to the adverse pressure gradient, apart from the inner peak of streamwise velocity fluctuations u, a second maximum located in the outer zone of turbulent boundary layer appears. It was found that the large-scale motions are much more energetic for adverse pressure gradients compared with the zero gradient case and the pressure gradient affects mostly the outer region of the flow. Using the method proposed by MATHIS et al. [1], based on the correlation function and Hilbert transform, we obtained a clear evidence of a strong correlation between small inner-layer and large log-layer structures, which is similar to that observed for high Reynolds number zero-pressure gradient cases. A simple model of modulation process occurring in adverse pressure gradient boundary layer has been proposed.