Structure of Three-Dimensional Separated Flow on an Axisymmetric Bump

Fine-spatial-resolution laser Doppler velocimeter measurements were obtained on one-half of the leeside of an axisymmetric bump in a turbulent boundary layer. The ratio of bump height H and boundary-layer thickness 6 is HIS = 2. Three-dimensional separations occur on the leeside with one saddle separation on the centerline that is connected with a separation line to one focus separation on each side. Downstream of the saddle point the mean backflow converges to the focal separation points in a region confined within about 0.156 from the local bump surface. The mean backflow zone is supplied by the intermittent large eddies as well as by the near surface flow from the side of the bump. The separated flow has a higher turbulent kinetic energy and shows bimodal histograms in local U and W, which appear to be due to highly unsteady low-frequency meandering motions. Because of the variation of the mean flow angle in the separation zones, the turbulent flow from different directions is decorrelated, resulting in lower Reynolds shearing stresses. Farther from the wall, large streamwise vortices form from flow around the sides of the bump.