Non-equilibrium development in turbulent boundary layers with changing pressure gradients

Abstract

Turbulence measurements were made in smooth-wall boundary layers subject to changing pressure gradients. Cases were documented over a range of Reynolds numbers and acceleration parameters. In all cases the boundary layer was subject to an initial zero pressure gradient (ZPG) development, followed by a favourable pressure gradient (FPG), a ZPG recovery and an adverse pressure gradient (APG). In the non-ZPG regions, the acceleration parameter, , was held constant. Two component velocity profiles were acquired at multiple streamwise locations to document the response to the changing pressure gradient of the mean velocity, Reynolds stresses and triple products of the fluctuating velocity components. Velocity field measurements were made to document the turbulence structure using two point correlations. In general, turbulence was suppressed by the FPG while structures became larger in streamwise and spanwise extent relative to the boundary layer thickness, particularly near the wall. In the recovery region, the return to canonical ZPG conditions was rapid. Changes in the structure in the APG region were less pronounced. The changes in the turbulence statistics and correlations relative to the ZPG baseline were quantified and presented as functions of streamwise location. When the streamwise location is scaled using the acceleration parameter, the results from all cases (including all statistical moments, and the size and inclination angles of turbulence structures), collapse in each region of the flow, showing a common non-equilibrium response to changes in the pressure gradient. These are new results which apply to the present flows and those with similar types of pressure gradients, but are not necessarily applicable to all flows with arbitrary pressure gradients.