Editorial: Underexpanded and Overexpanded Supersonic Jets

Abstract

INTRODUCTION Supersonic jets exhausting from convergent–divergent nozzles are inherently present in all propulsive systems of flight vehicles. The aerodynamic study of such flow configurations represents one of the most challenging issues in aerospace and military applications (rockets, missiles, supersonic aircrafts, etc). Various physical phenomena involved in this basic fluid-dynamics problem, such as shock formation and shock/turbulent boundary layer interaction inside the nozzle, are directly linked to the performance of jet engines. Particularly, when the flow exiting the nozzle encounters a pressure difference (either positive or negative) due to the ambient atmosphere, it leads to the formation of complicated shock-wave structures (direct or inverse Mach reflections, reflected shock, triple point and associated sliplines, which grow to become slip zones as a result of the Kelvin-Helmholtz instability). Generally, as the chamber pressure increases, the flow gradually adapts to the ambient conditions as it passes through the system of expansion and compression waves, forming different types of jet structures (ranging from underto overexpanded regimes and passing by the adapted nearly shock-free supersonic case). Figure 1, which is based on experimental photography illustrations, highlights the different types of jet structures as function of the nozzle pressure ratio. In addition to the above mentioned phenomena that are mainly encountered in classical compressible aerodynamics, high-speed jets are also of interest in many other applications. Examples are the needle-free powdered drug and vaccine delivery using supersonic jets for medical purposes [13], oxyand thermal jet plasma for metal cutting process [4,5], transient jet ejecta in astrophysical such extra-galactic jets [6] and the explosion safety associated with accidental puncture of high-pressure vessels or lines designed for the storage of gas fuels, such as hydrogen [6]. Although over several decades of extensive researches (which largely relies on experimental measurements and analytical predictions) into supersonic flows with particular interest in jet structures [7-17], the subject is quite complex and not yet clearly understood. These analyses and measurements provide a useful but sometimes incomplete indication of important flow aspects, such as the starting process, and the jet separation inside the nozzle. In fact, the experimental analysis of such a situation is quite difficult and rather expensive, because it requires flow visualizations and measurements inside the divergent section in the few seconds of the run (or milliseconds for the crucial part of the transient). Since the main finding is revealing the unsteady nature of the jet flow, the quantitative CFD data, if previously well validated through appropriate benchmark calculations, should in principle help to understand and explain the complex jet behavior.