On the Dynamics of Linear Plug Nozzle Flows Under Various Operating Conditions

The plug, or aerospike, nozzle is a promising concept for future space launcher propulsion systems. With the capability of altitude adaptation, it is well known to outperform traditional bell-shaped nozzles in terms of the specific impulse it can generate. However, in contrast to ordinary nozzles, the flow physics of plug nozzles is less well understood. This work aims to contribute to understanding the jet flow of a linear plug nozzle in a broad range of overexpanded operating conditions with [Formula: see text]. Schlieren and PIV measurements are carried out for two different plug lengths (72% and 24% of an ideal contour) to analyze the topology and the dynamics of the jet plume. It is found that the aerodynamic modes and their frequency not only deviate from typical supersonic jet modes but also change considerably for the two plug lengths: At low [Formula: see text]s, the base region of the shorter plug (24%) is dominated by an asymmetric vortex shedding mode responsible for acoustic wave emission before undergoing a sudden switch to a screeching-mode-dominated flowfield. Flow expansion around the base shrouds leads to increased local velocity magnitudes throughout the jet’s shock cell system. Measurements of the long plug (72%) reveal no vortex shedding mode being present, but a sudden doubling of screeching frequency in-between [Formula: see text] and 5.5. The steady trailing shock from the spike tip is suspected to play a crucial role in the mode staging behavior. An empirical approach to predict the frequency of dominant jet instabilities based on [Formula: see text] is presented.