Combined irregular roughness and favorable-pressure-gradient effects in a turbulent boundary layer

An experimental study of the combined impact of highly irregular surface roughness and moderate favorable-pressure-gradient (FPG) conditions on the structure of a turbulent boundary layer was assessed using two-dimensional particle image velocimetry (PIV) measurements in the streamwise-wall-normal plane and stereo PIV measurements in the wall-normal-spanwise plane. The roughness under consideration was replicated from a turbine blade damaged by deposition of foreign materials and contains a broad range of topographical scales. The two-dimensional PIV measurements were compared to measurements of smooth-wall ow under both identical FPG conditions as well as zeropressure-gradient (ZPG) conditions in order to reveal the impact of roughness and FPG conditions on the underlying structure of the flow. The suppression of boundary layer thickness and Reynolds normal and shear stresses was observed in the smooth-wall FPG case. However, with the addition of surface roughness to the identical FPG condition, enhanced momentum deficit and Reynolds normal and shear stresses were found in the combined FPG and roughness condition. The result of quadrant analysis revealed the significant dominance of ejections over sweeps under FPG condition regardless of the surface conditions, while the comparable impact of sweeps and ejections was observed under ZPG conditions. Similar impacts of FPG and surface roughness were observed in the cross-plane stereo PIV measurements. Of interest, smooth-wall results displayed homogeneity in the spanwise direction, while strong inhomogeneity was observed in the FPG rough-wall case due to roughness protrusions along the spanwise direction.;In terms of ow structural modifications, inspection of instantaneous velocity fields in the 2D PIV measurements revealed vortex organization consistent with ZPG smooth-wall ow, though focused closer to the wall with a shallower inclination angle under smooth-wall FPG conditions. In contrast, the combined FPG and surface roughness effect promoted vortical structure penetration much further away from the wall and enhanced an momentum deficit, indicating that roughness mitigates the FPG-induced focusing of these structural attributes toward the wall. Results from the two-point velocity correlations support these instantaneous observations. Instantaneous velocity fields in stereo PIV measurement revealed alternating, low- and high-momentum regions (LMRs and HMRs) in the spanwise direction that embody a significant fraction of the Reynolds shear stress. Consistent with the 2D PIV measurements, reduced wall-normal extent and less intense LMRs and HMRs were observed under FPG conditions, while these characteristics were mitigated due to the presence of surface roughness. To examine the average spatial structure of the flow, two-point velocity correlations were computed. While two-point correlations of velocity re ected the basic signature of spanwise-alternating LMRs and HMRs, correlations of velocity fields embodying only the largest spatial scales revealed an even higher degree of spanwise coherence of these patterns. However, the shortening of correlation, especially in spanwise direction was observed in FPG-Rough case.