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

turbulent boundary layer;favorable pressure gradient turbulent boundary layer;surface roughness;irregular roughness;combined effects of surface roughness and favorable pressure gradient;large scale motions in wall turbulence

Min, Dahhea

关键词: turbulent boundary layer;    favorable pressure gradient turbulent boundary layer;    surface roughness;    irregular roughness;    combined effects of surface roughness and favorable pressure gradient;    large scale motions in wall turbulence;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/44256/Dahhea_Min.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

An experimental study of the combined impact of highly irregular surface roughness and moderatefavorable-pressure-gradient (FPG) conditions on the structure of a turbulent boundary layer wasassessed 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 roughnessunder consideration was replicated from a turbine blade damaged by deposition of foreign materialsand contains a broad range of topographical scales. The two-dimensional PIV measurements werecompared 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 conditionson the underlying structure of the ow. The suppression of boundary layer thickness and Reynoldsnormal and shear stresses was observed in the smooth-wall FPG case. However, with the additionof surface roughness to the identical FPG condition, enhanced momentum de cit and Reynoldsnormal and shear stresses were found in the combined FPG and roughness condition. The result ofquadrant analysis revealed the signi cant dominance of ejections over sweeps under FPG conditionregardless of the surface conditions, while the comparable impact of sweeps and ejections wasobserved under ZPG conditions. Similar impacts of FPG and surface roughness were observed inthe cross-plane stereo PIV measurements. Of interest, smooth-wall results displayed homogeneityin the spanwise direction, while strong inhomogeneity was observed in the FPG rough-wall casedue to roughness protrusions along the spanwise direction.In terms of ow structural modi cations, inspection of instantaneous velocityelds in the 2DPIV measurements revealed vortex organization consistent with ZPG smooth-wall ow, thoughfocused closer to the wall with a shallower inclination angle under smooth-wall FPG conditions. Incontrast, the combined FPG and surface roughness e ect promoted vortical structure penetrationmuch further away from the wall and enhanced an momentum de cit, indicating that roughness mitigates the FPG-induced focusing of these structural attributes toward the wall. Results from thetwo-point velocity correlations support these instantaneous observations. Instantaneous velocity elds in stereo PIV measurement revealed alternating, low- and high-momentum regions (LMRsand HMRs) in the spanwise direction that embody a signi cant fraction of the Reynolds shear stress.Consistent with the 2D PIV measurements, reduced wall-normal extent and less intense LMRs andHMRs were observed under FPG conditions, while these characteristics were mitigated due to thepresence of surface roughness. To examine the average spatial structure of the ow, two-pointvelocity correlations were computed. While two-point correlations of velocity reected the basicsignature of spanwise-alternating LMRs and HMRs, correlations of velocityelds embodying onlythe 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-Roughcase.