Thermodynamic scaling of supersonic retropropulsion flowfields

The influences of gas composition and temperature on supersonic retropropulsion (SRP) flowfields are experimentally explored. It is revealed that the standoff distance of the bow shock produced by SRP can be scaled to account for changes in thrust, mass flow rate, forebody size, gas composition and temperature within the high-thrust, steady flow regime. These parameters were systematically varied for Mach 2 and 3 heated jets at zero angle of attack, employing nitrogen, helium and argon within a Mach 2 nitrogen or carbon dioxide freestream. Comparisons are also made with higher freestream Mach number data from the literature for similar geometries. These datasets are used to provide new insights into multi-gas, multi-temperature SRP and are more widely transferable to flight conditions than the unheated nitrogen or air interactions that have been more widely studied. We find that the momentum ratio can successfully account for changes in gas composition and temperature. Similarly, the mass flow rate ratio can account for these variables, down to a function of Mach numbers when multiplied by functions of gas molecular weight, temperature and the ratio of specific heats that are derived from mass conservation control volume analysis. Despite the collapse of shock standoff data in this study, a small dependence on the jet Mach number is seen to remain. Some additional scatter might also be expected at low thrust conditions as the bow shock transitions between one fully dominated by the freestream and forebody geometry to one dominated by the jet exhaust. Shock radius was seen to be more variable at lower thrust levels, potentially indicative of these effects. This work greatly aids the extrapolation of SRP between experiments, simulations and flight conditions.