Flow structure and axial velocity decay of supersonic free and impinging jets in reduced pressure environments

High pressure ratio supersonic jets expanding into low pressure environments are studied experimentally in this work, motivated by their relevance to space propulsion and Lunar and planetary landings. Key features are compared to available data and model relations, mostly focused on sonic jets discharging at ambient atmospheric conditions and moderate expansion ratios. Experiments are conducted in a vacuum chamber using a cold Mach 5 jet at varying expansion ratios and exit Reynolds numbers, which is achieved by parametric variations of stagnation and ambient pressures. Shock structure and flow velocities for free jets are characterized via Schlieren and Laser Doppler Velocimetry. We will discuss the flow structure of free jets and its dependence on pressure ratios, far field jet self-similarity, and present semi-empirical relations for axial velocity decay based on turbulent mixing models. In addition, Schlieren imaging of wall normal impinging jets for varying pressure ratios show their impact on shock stand-off distance and width.