New Explanation of Noise Production by Supersonic Jets with Gas Dredging

Summary

Supersonic jets emit Mach waves producing noise in their environment. Various experimental techniques have been used in the past for investigating the behaviour of structures present in the supersonic jet boundary layer. These structures are accompanied by long, almost straight and nearly parallel Mach waves. These Mach waves appear both outside and inside the jet. It was experimentally found that these Mach waves move at three preferred speeds: w, w’ and w”. They depend simply and exclusively on the jet Mach number M_i and the speed of sound ratio between inside (a_i) and outside (a_a) of the jet. Quite simple empiric formulae for w, w’ and w” have been established by Oertel sen. but no satisfying explanation could be found at that time. Recently, however Oertel sen. proposed a new idea based on the growth of long living pairs of vortices formed inside of the jet boundary layer. Vortices move with the velocities w’ and w” respectively, whereas the centre of the pair moves with w. The w’- and w”-vortices move supersonically with respect to a_a, therefore produce the so-called w’ -Mach waves outside of the jet. The w” - vortices move with u_i – w” > a_i , therefore produce the so-called w”-Mach waves inside of the jet. This theory suggests a new method of jet noise reduction.