Mach waves produced in the supersonic jet mixing layer by shock/vortex interaction

Abstract

The noise emission of free jets has been extensively investigated for many decades. At subsonic jet velocities, coherent structures of the mixing layer move at subsonic speed and emit sound waves. Free jets blowing at supersonic speeds, however, can emit weak shock waves, called Mach waves. At supersonic speeds, two cases must be distinguished: the structures move either subsonically or supersonically relative to the inside and/or outside speed of sound. In the case of supersonic movement, the Mach waves exist inside as well as outside the jet. At subsonic speeds, no Mach waves appear. Although numerous theories have been established to find the origin of the Mach waves, to the authors’ best knowledge, the mechanism of the Mach wave formation has not yet been clearly explained. Recently another theory of Mach waves in supersonic jets was developed, as described herein, which outlines the causes for the Mach wave production and stability as well as their dynamics. The theory’s principle is that the Mach waves are initiated by vortices which move downstream at three speeds w, $${w}'$$ and $${w}''$$ inside of the mixing layer. These three types of vortices and Mach waves are described in a comprehensive manner by the theory and are called the “w-, $${w}'$$ - and $${w}''$$ -vortices” and “w-, $${w}'$$ - and $${w}''$$ -Mach waves,” respectively.