Flow Reversal in Turbulent Boundary Layers with Varying Pressure Gradients

Abstract

This contribution describes detailed PIV measurements obtained in turbulent boundary layers in order to capture rarely occurring flow reversals within the viscous sublayer, which previously have been observed in both experiments and direct numerical simulations. Due to their confinement to the viscous sublayer along with their rare occurrence on the order of 10^-4, any statistical investigation requires very large data sets exceeding 10^5 samples. In the present investigation, this was achieved by capturing long PIV records using image high magnification near unity. To investigate the influence of Reynolds number and pressure gradient, the measurements were performed in turbulent boundary layers in both zero pressure Gradient (ZPG) and adverse pressure gradient (APG) conditions. The measurements were conducted in the TBL wind tunnel of Lille which features a test section length of 20 m and cross-section of 2x1 m^2. A ramp model placed inside the tunnel introduced APG conditions. Both visual inspection of the raw data and a dedicated processing scheme to retrieve the unsteady wall shear stress were used to quantify the flow reversal events. The occurrence of the self-similar flow reversals was found to weakly depend on the Reynolds numbers in ZPG and roughly doubled in frequency in the APG condition.