Viscous simulation of shock reflection hysteresis in ideal and tapered overexpanded planar nozzles

While a CFD simulation of the flow in overexpanded planar nozzles shows, inside an ideal nozzle, the existence of a hysteresis process in the transition from regular to Mach and from Mach to regular reflections such a process does not appear in tapered nozzles. Previous simulations have dealt only with the flow outside the nozzle and thus concluded that the hysteresis phenomenon takes place outside the nozzle even when viscous effects were introduced. When including the geometry of the nozzle in the simulation it becomes evident that flow separation will occur before transition from regular to Mach reflection for all relevant flow Mach numbers. The simulation reveals complex changes in the flow structure as the ratio between the ambient and the stagnation pressures is increased and decreased. The pressure along the nozzle wall downstream of the separation point was found to be less than the ambient pressure with the effect being more pronounced in the case of the ideal nozzle. The present study complements a previous study that dealt only with flow separation in an ideal nozzle.