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

Large-scale features in a zero-pressure-gradient turbulent boundary layer are compared with those in two adverse-pressure-gradient turbulent boundary layers using a scanning three-dimensional flow-visualization technique. The boundary-layer flow was formed on the wall of a wind tunnel and seeded with smoke introduced through a slit. A ramp was mounted on the opposite wall to create an adverse pressure gradient ( and 9.7). A high-repetition-rate laser formed into a light sheet was scanned in the wall-normal direction, and the two-dimensional images were reconstructed into a nearly instantaneous three-dimensional flow-visualization volume. Observations about the boundary-layer structures for Reynolds numbers between 7400 and 8400 are made by investigating the instantaneous flow-visualization volumes, the features from binary-image slices, and conditional averaged volumes around a structure. In general, structures in a zero-pressure-gradient boundary layer are larger and spaced farther apart compared to the adverse-pressure-gradient cases. Flow features identified through conditional averaging extend higher into the freestream flow for the adverse-pressure-gradient case, and are more distinct and separated from the wall than in the zero-pressure-gradient case. There were no noticeable differences in the spanwise direction for the three cases.