VELOCITY AND TURBULENCE MEASUREMENTS IN A SUPERSONIC BASE-FLOW WITH MASS BLEED

Two-component laser Doppler velocimetry was used to obtain detailed me an velocity and turbulence measurements in the near wake of a cylindri cal afterbody with base bleed in a Mach 2.5 flow, The bleed flow provi des at least some of the fluid required for shear layer entrainment an d shields the base annulus from the enter shear layer and the primary recirculation region, leading to an increase in base pressure, There i s an overall reduction in turbulence levels throughout the base bleed flowfields relative to the near-wake flowfields of blunt-based and boa ttailed afterbodies. With increasing bleed, the formation of a strong bleed jet shear layer and secondary recirculation region near the base annulus offsets the benefits of base bleed, leading to a drop in the base pressure. The net benefits of base bleed are maximized at the opt imum bleed condition, which corresponds to the highest base pressure, the disappearance of the primary recirculation region, and the lowest turbulence levels in the near-wake flowfield. Increased benefits from base bleed could be achieved by injecting the bleed fluid at the lowes t possible velocity through the use of larger bleed orifices, porous b ases, or bleed orifices located along the outer base annulus.