An experimental investigation of the effects of base bleed in axisymmetric supersonic flow

Base bleed is a technique wherein a small quantity of fluid is injected into the base region of a projectile to reduce the base drag. The effects of base bleed on the near-wake flowfield of a cylindrical afterbody in a Mach 2.5 flow have been investigated in the present study. This experimental study is aimed at better understanding the complex fluid dynamic interactions occurring in the near-wake due to base bleed and is motivated by the lack of detailed velocity and turbulence data in this flowfield. The experimental techniques used include static pressure measurements, schlieren and shadowgraph photography, and two-component laser Doppler velocimetry (LDV). The comprehensive LDV mean velocity and turbulence measurements obtained during this study provide valuable insight into the physics of the base bleed mechanism and constitute a benchmark data set to aid analytical and computational efforts in modeling and predicting supersonic base flows.Static pressure measurements show that with increasing bleed flow rate, the average base pressure increases initially, attains a peak value near an injection parameter of I = 0.0148, and then decreases with further increase in I. The peak base pressure ratio at the optimum bleed condition is 18.5% higher than the blunt base case and 5.7% higher than for a 5 degree boattailed afterbody. Axial LDV traverses show peaks in turbulent kinetic energy along the centerline at the forward and rear stagnation point locations. Centerline measurements also indicate the near-disappearance of the primary recirculation region near the optimum bleed condition. Detailed mean velocity and turbulence data were obtained in the entire near-wake flowfield for three different bleed cases corresponding to low bleed, slightly pre-optimal bleed, and slightly post-optimal bleed conditions. These measurements indicate that the bleed flow provides at least some of the fluid required for shear layer entrainment and shields the base annulus from the outer shear layer and the primary recirculation region, leading to an increase in base pressure. There is an overall reduction in turbulence levels throughout the base bleed flowfields relative to the near-wake flowfields of blunt-based and boattailed 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. At all bleed conditions, the normal stress distribution is highly anisotropic with the axial component dominating the near-wake turbulence field. The net benefits of base bleed are maximized at the optimum bleed condition, which corresponds to the highest base pressure, the disappearance of the primary recirculation region, and the lowest turbulence and entrainment levels in the near-wake flowfield.U of I OnlyETDs are only available to UIUC Users without author permissio