Convection Velocity in Turbulent Boundary Layers under Adverse Pressure Gradient

The convection velocity ($U_C$) of turbulent structures has been studied in adverse-pressure-gradient (APG) turbulent boundary layers (TBLs) for a wide range of Reynolds numbers $Re_\tau = 1400 - 4000$. The study is based on estimation of the convection velocity using decomposed streamwise skewness factor introduced in (Drozdz A., Elsner W., Int. J. of Heat and Fluid Flow. 63 (2017) 67\-74) and verified by means of two-point correlation method. It was shown that in the overlapping region of APG flows, the convection velocity profiles (when scaled in viscous units) reassemble the universal logarithmic law characteristic for the ZPG flows up to Clauser-Rota pressure gradient parameter $\beta \lesssim 19$ for the considered range of Reynolds number, what means that in the inner region of TBL the friction velocity in APG is not proportional to $U$ (as in ZPG) but to $U_C$ instead. The physical mechanism that explains the impact of increased convection velocity on the mean flow is proposed. The difference between $U_C$ and $U$ increases as a function of APG, which causes the stronger sweeping that enhances momentum transfer to the wall and compensates the weaker mean shear profile that is created by lower vorticity near the wall. This effect is a result of an enhancement of amplitude modulation of the small scales by large scale motion. The process becomes more pronounced as eddy density grows, so with increasing Re. The proposed model addresses a number of literature observations found in adverse pressure gradient flows which have been so far left without a well-founded explanation.