Evolution of a turbulent boundary layer over a series of pressure gradients

This study investigated the behavior of a turbulent boundary layer over a downward-facing ramp downstream of a flat plate, which imposed spatially varying pressure gradients on the turbulent boundary layer, through time-resolved particle image velocimetry and complementary RANS simulations. The ramp generated a mild favorable pressure gradient (FPG) from the leading edge of the flat plate, a strong FPG directly upstream of the ramp and a strong adverse pressure gradient on the ramp’s surface. The streamwise evolution of the turbulent boundary layer was evaluated using the PIV data, and comparisons between the PIV data and RANS simulations were performed at three different Reynolds numbers. The mean streamwise velocity contours showed a slight acceleration of the boundary layer due to the strong FPG before the ramp. On the ramp’s surface, the boundary layer decelerated significantly under the strong adverse pressure gradient, causing substantial reverse flow. The turbulent boundary layer detached at all Reynolds numbers, with higher Reynolds numbers detaching further downstream. Reynolds stresses increased in magnitude from the first to the fourth streamwise location, then decreased, with wall-normal peak positions shifting away from the wall before recovering beyond x/L = 0.5. Although the patterns were similar across Reynolds numbers, the overall Reynolds stresses decreased with higher Reynolds numbers. Comparisons between experimental data and RANS simulations revealed slight discrepancies in the detachment and reattachment points and the wall-normal extent of mean reverse flow.