The Formation of Mach Waves from Supersonic Jets with Outer Co-flowing

Optical studies of regular Mach waves inside and outside of supersonic jets, published by Oertel [1] in 1978, have shown that their angles, shock distances, and shock compressions depend quite simply on the jet data. At that time, it had long been known that a bypass reduces the noise of jet engines [2], i.e., that a subsonic jet cowling changes the angle of the Mach waves outside of the jet and, at well-defined jets’ conditions, can completely prevent the occurrence of the periodic Mach waves in the flow’s environment. Especially Oertel [3] and Oertel and Patz [4] conducted extensive experiments with supersonic free jets surrounded by a subsonic envelope. These experiments showed that by reasonably adapted secondary jet conditions, the primary Mach waves can be largely suppressed. Figure 1 shows shock tube visualizations [4] of a supersonic air jet at Mach number M _i  = 2, a single jet in Fig. 1a, and with surrounding air moving at Mach number M _a  = 0.79 in Fig. 1b. The Mach waves are almost suppressed in Fig. 1b, as predicted by the theory presented in this paper.