On the interaction of a linear plug nozzle flow with sub-, trans-, and supersonic outer flow

Abstract A plug/aerospike nozzle is a promising concept as a propulsion system for space launchers and space planes. The inherent ability to adapt the nozzle jet to the ambient pressure level improves the thrust performance under overexpanded operating conditions compared to conventional bell nozzles, which is of great interest for future single-stage-to-orbit vehicles. This experimental study investigates the topology and aerodynamics of a cold flow linear plug nozzle jet in an outer flow environment. PIV and high-speed schlieren measurements are utilized to understand the mutual aerodynamic interaction between each other. The jet flow is studied for a variety of nozzle pressure ratios in combination with an outer flow at sub-, trans-, and supersonic Mach numbers. The flow is examined for two plug lengths, which are 72% and 24% of an ideal contour. It is found that the combination of nozzle pressure ratio and outer Mach number strongly influences the flow pattern and local velocity magnitudes. Backflow regions are measured, mainly emerging through the integration of the nozzle in a bluff aft body. The strength and frequency of aerodynamic modes are found to be highly dependent on the operating conditions as well. The most relevant ones are jet screeching, alternating vortex shedding of the outer flow, and vortex shedding in the base wake of the plug with strong truncation. The latter causes strong fluctuations in the flow, which are transmitted to the shear layer and induce acoustic wave emission. In addition, the flow locally accelerating in the plug base region results in increased shock strength in the jet structure. At trans- and supersonic outer flow, however, the aerodynamic modes of the jet flow are strongly suppressed. The impact of plug truncation on the velocity field becomes less for higher nozzle pressure ratios and outer flow Mach numbers.