A family of adverse pressure gradient turbulent boundary layers with upstream favorable pressure gradients

Abstract

A flat plate turbulent boundary layer (TBL) is experimentally subjected to a family of 22 favorable-adverse pressure gradients (FAPGs) by using a ceiling panel of variable convex curvature. We define a FAPG as a sequence of streamwise pressure gradients in the order of favorable followed by adverse, similar to the pressure gradient sequence over the suction side of an airfoil. The adverse pressure gradient region of this configuration is studied using particle image velocimetry (PIV) in the streamwise--wall-normal plane. The streamwise mean and Reynolds stresses are presented and discussed in this work. Although favorable and adverse pressure gradients are known to evoke opposing responses from a TBL, the current adverse pressure gradient (APG) that followed a favorable pressure gradient (FPG) was not found to 'reverse' the effect of the FPG. Instead, a complex response that is neither characteristic of a ZPG nor an APG TBL was observed. The Reynolds stresses showed a bimodal structure, with the outer-scaled stresses strengthening along the streamwise direction in $y < ~ 0.2 \delta$ and weakening in $y > ~ 0.2\delta$, where $y$ is the wall-normal coordinate, and $\delta$, the local boundary layer thickness. These were signatures of a new internal layer, previously observed in bump/hill flows, formed due to the pressure gradient sign change from favorable to adverse. The non-typical structure of and trends in the statistics were more significant in the stronger pressure gradient cases, where the stronger upstream FPG seemed to have exerted a stronger influence on the downstream APG. The current dataset with 22 cases provided a well-resolved picture of the breakdown of equilibrium of the ZPG TBL under an FAPG configuration. The statistics for all the cases are included under the ancillary file section.