Supersonic starting jet impingements upon surface-mounted hemispheres

An OpenFOAM-based large-eddy simulation study was conducted to inspect the flow and shock structures associated with a starting supersonic jet impinging upon differently sized surface-mounted hemispheres and compare them to flat wall- and concavity-based impingements studied previously. The underexpanded supersonic jet has a nozzle pressure ratio of 4 and an exit Mach number of 1.45, while diameter-ratios of D/d=0.5, 1, and 2 located at h/d=1.5 away were used, where D, d, and h are the hemisphere, jet diameters, and separation distance, respectively. Numerical schlieren and Mach number distributions document how the present impingement behavior differs from those of flat walls and concavities, as well as how diameter ratio affects it. Interquartile range and fast Fourier transform analyses also show hemispheres producing significantly different standoff shock positions and unsteadiness levels as compared to concavities. Binning distributions show hemispheres produce much wider position fluctuations than concavities, regardless of diameter ratio. In addition, shock formation and behavior assessed along the jet centerline are generally insensitive toward the hemisphere size. The study demonstrates not only differences in the impingement behavior but also significant discrepancies in the standoff shock unsteadiness between hemispheres and concavities.