Studies of the Large-Scale Structure in Turbulent Boundary Layers Using Simultaneous Velocity-Wavefront Measurements

Simultaneous velocity field and optical distortions along the wall-normal direction were performed in a subsonic boundary layer. Comparison between wavefronts and the ones predicted using instantaneous Strong Reynolds Analogy had shown that instantaneous SRA is valid most of the time. Conditional velocity analysis had shown that positive OPD distortions are created by predominantly ejection-type Q2 events near the wall and overall uv-Reynolds shear stress is larger near the wall and smaller away from the wall. The negative distortions are mostly due to sweep-type Q4 events near the middle of the boundary layer, with significantly larger uv-Reynolds shear stress throughout the boundary layer. Conditional velocity fields during the events, where the instantaneous SRA failed to predict optical distortions, were extracted and analyzed and possible reasons for these differences are presented and discussed.