Comparison of Large Scale Features in Zero and Adverse Pressure Gradient Turbulent Boundary Layers

Abstract

This paper presents a detailed description of a scanning 3D flow visualization technique which was used to compare and contrast the large scale features in a zero pressure gradient (ZPG) turbulent boundary layer with those in two adverse pressure gradient (APG) turbulent boundary layers. Ramps were installed in a wind tunnel to produce an adverse pressure gradient on the wind tunnel wall. The boundary layer flow was seeded with smoke from a reservoir that was drawn into the tunnel. A high-repetition-rate laser formed into a light sheet was scanned in the wall-normal direction through the boundary layer. The 2D images were captured using a high-speed camera and reconstructed into a nearly instantaneous 3D flow visualization volume of the intensity of the smoke in the turbulent boundary layer. Static pressure measurements and 2D particle image velocimetry (PIV) were taken to characterize the flow with and without the ramps. Observations about the boundary layer structures were made by investigating the flow visualization volumes, the power spectral density of the images, the features from binary image slice, and conditionally averaged volumes around a structure. In general, the ZPG structures were larger and spaced farther apart compared to the APG cases. The structures in the APG cases were more elongated in the streamwise direction and occurred at a steeper angle from horizontal compared to the ZPG case.