Structure of Three-Dimensional Separated Flow on Symmetric Bumps

: Surface mean pressures, oil flow visualization, and 3-velocity-component laser-Doppler velocimeter measurements are presented for a turbulent boundary layer (TBL) of momentum thickness Reynolds number, Re(theta) and TBL thickness delta over two axisymmetric and one symmetric bump. LDV data were obtained at one plane x/H = 3.26 for each case. Vortical separations on the leeside merge into large stream-wise mean vortices downstream. The near-wall flow (y < 90) is dominated by the wall. For the axisymmetric cases, the vortices in the outer region produce large turbulence levels near the centerline and appear to have low frequency motions that contribute to turbulent diffusion. For symmetrical bump there are sharper separation lines and lower turbulence intensities in the vortical downstream flow. Fine-spatial-resolution LDV measurements were also obtained on half of the leeside of the axisymmetric bump. Three-dimensional (3-D) separations occur on the leeside with one saddle separation on the centerline that is connected by a separation line to one focus separation on each side of the centerline. Downstream of the saddle point the mean backflow converges to the focal separation points in a thin region. 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 turbulent motions. By the mode-averaged analysis of bimodal histograms, highly unsteady flow structures are estimated and unsteady 3-D separations seem to be occurring over a wide region on the bump leeside. Because of the variation of the mean flow angle in the separation zones, turbulent flow from different directions is non-correlated, resulting in lower shearing stresses. Farther from the wall, large stream-wise vortices form from flow around the sides of the bump.