Visualizations and measurements of axisymmetric base flows altered by surface disturbances

The effects that sub-boundary layer disturbances have on the near-wake turbulence structure in a reattaching, compressible, axisymmetric flow are examined. Both delta-shaped and axisymmetric-strip disturbances are examined. The effect that the number and thickness of delta-shaped tabs and the placement of axisymmetric strip tabs have on the base pressure is quantified. These pressure data show that delta-shaped disturbances can generate a base pressure decrease of up to 10%, while strip disturbances can generate a base pressure increase of up to 3%. Planar visualizations in both the side and end views were obtained for key tab arrangements using Rayleigh/Mie scattering of condensed ethanol droplets. These visualizations show that each delta-tab generates a streamwise counter-rotating vortex pair, which is easily identifiable in average images, through the mean reattachment point. The strip-tab configuration generates increased symmetric shear layer motion and large, circumferentially eccentric mean end-view structures near the base. Both of these effects are quickly damped because of the high convective Mach number in the initial portion of the shear layer.