Flow visualizations and measurements of a three-dimensional supersonic separated flow

The flow along the afterbody and in the base region of a circular cylinder with a length-to-radius ratio of 3.0 aligned at a 10-deg angle of attack to a nominal Mach 2.5 freestream has been investigated experimentally. The objective is to better understand the mechanisms that control base flow for supersonic bodies with a nonzero-angle-of-attack orientation. Laser Doppler velocimetry measurements were conducted in the incoming boundary layer to quantify the initial conditions at the onset of three-dimensional behavior. Schlieren and Mie scattering visualizations were obtained to discern governing flow features and to image the large-scale turbulent structures of this separated flow. Surface oil-streak visualizations were obtained to determine the three dimensionality of the afterbody surface flow and to deduce the base surface flowfield. Pressure-sensitive paint measurements were completed to determine the spatial evolution of surface pressure along the cylindrical body at angle of attack and to determine the change in base pressure caused by inclination of the body. Results provide evidence of expected mean-flow features, including base-corner expansions, separated shear layer development, recompression shocks, and a turbulent wake. No evidence of lee-side flow separation was detected along the afterbody. However, a strong secondary circumferential flow, which develops along the afterbody due to pressure gradients on its surface, results in the entrainment of fluid into the base region from the leeward portion of the flow. The average base pressure ratio measured for the angle of attack case is 48.4% lower than that measured for zero angle of attack, resulting in a significant increase in base drag for cylindrical objects inclined at angle of attack.