THE DEVELOPMENT OF A TURBULENT BOUNDARY-LAYER IN HIGH FREE-STREAM TURBULENCE PRODUCED BY A 2-STREAM MIXING LAYER

Experiments were conducted to document the response of a turbulent bou ndary layer to a Gee-stream flow with high and approximately streamwis e-uniform levels of turbulence intensity. Measurements were made in a flat-plate turbulent boundary layer that developed beneath a free stre am produced by a two-stream mixing layer. A flat test surface was posi tioned downstream of, and orthogonal to, the splitter wall used to for m the mixing layer. The gradient direction of the mixing layer was par allel to the span of the boundary layer. The free-stream turbulence in tensity, as defined by streamwise velocity fluctuations, was approxima tely uniform in the streamwise direction. Profiles of mean and fluctua ting streamwise velocity were recorded using hot-wire anemometry. Far above the plate, the free-stream flow exhibited the self-similar behav ior of fully turbulent mixing layers. Free-stream turbulence intensity levels of up to 16% were produced that were uniform along a mean stre amline to within +/-5% of that level for turbulence intensities above 10%. An excess in streamwise momentum was formed in and above the oute r region of the boundary layer due to an interaction between the vorti city fields of the boundary layer and the mixing layer. The secondary flows responsible for this excess were identified. Skin friction incre ased by up to 73% compared to the expected values based on the streamw ise development length of the boundary layer. When the results were sc aled on the peak velocity in the region of excess momentum, the skin f riction enhancements agreed well with data from the literature at simi lar turbulence intensities.