Visualization of Mach waves produced by a supersonic jet and theoretical explanations

Abstract

Supersonic jets emit shock waves to the outside called “Mach waves” which are long, straight and have about equal angles. Experiments done by Oertel Sen. clearly visualized these Mach waves and he showed, moreover, that such Mach waves also exist inside the jet. Measuring the Mach wave angles from many shock tube experiments revealed the existence of three wave families. They move with different speeds, denoted as w, w′ and w″, which only depend on the jet Mach number M i and the ratio a a /a i of the sound speed a a outside and a i inside the jet. For illuminating theoretically the experimental outcomes, a brand-new assumption is made which explains the three found speeds w, w′ and w″. Its central point is the production of Mach waves by two rows of vortices moving downstream inside of the jet mixing layer in two separate vortex trains. The inner vortex train creates the w′-Mach waves outside the jet and the outer vortex chain is accompanied by the w″-Mach waves towards inside the jet. Both chains form a contact surface which moves with speed w. For validating the results of the new theory the shock tube experiments are the only ones worldwide by which these three wave families have been visualized using differential interferometry: by frame series, by immobilization images and by streak records. They show density differences by light intensity variations on the image. By the immobilizations and the streak records the flow is observed through a slit on a rotating drum camera. Immobilizations use a wide opened slit and the camera turns with the speed of the Mach waves to be observed. With a narrow adjusted slit, streak records show the Mach wave traces clearly.