The development of a turbulent boundary layer in high free-stream turbulence produced by a two-stream mixing layer

Experiments were conducted to document the response of a turbulent boundary layer to a free-stream flow with high and approximately streamwise-uniform levels of turbulence intensity. Measurements were made in a flat-plate turbulent boundary layer that developed beneath a free stream produced by a two-stream mixing layer. A flat test surface was positioned downstream of, and orthogonal to, the splitter wall used to form the mixing layer. The gradient direction of the mixing layer was parallel to the span of the boundary layer. The free-stream turbulence intensity, as defined by streamwise velocity fluctuations, was approximately uniform in the streamwise direction. Profiles of mean and fluctuating streamwise velocity were recorded using hot-wire anemometry. Far above the plate, the free-stream flow exhibited the self-similar behavior of fully turbulent mixing layers. Free-stream turbulence intensity levels of up to 16% were produced that were uniform along a mean streamline to within ±5% of that level for turbulence intensities above 10%. An excess in streamwise momentum was formed in and above the outer region of the boundary layer due to an interaction between the vorticity fields of the boundary layer and the mixing layer. The secondary flows responsible for this excess were identified. Skin friction increased by up to 73% compared to the expected values based on the streamwise development length of the boundary layer. When the results were scaled on the peak velocity in the region of excess momentum, the skin friction enhancements agreed well with data from the literature at similar turbulence intensities.