Numerical simulations of shock-vortex interactions in supersonic jet screech

Abstract

Supersonic jet screech is a form of jet noise which adversely impacts both the environment and the life of aircraft structures. A basic understanding of the mechanisms which generate the screech tone and determine its amplitude is therefore important. In the present study we perform direct numerical simulations (DNS) of the interaction of an oblique shock with instability waves of a finite thickness supersonic shear layer. We thereby retain the basic elements of an isolated jet screech source. The simulations are carried out in two dimensions using a high order accurate spatial scheme with nonreflecting boundary conditions. The unsteady shock motion is resolved with the essentially non-oscillatory (ENO) discontinuity capturing scheme. The shear layer (M = 1.2, Re = 1000 based on initial vorticity thickness) is forced at the most unstable frequency such that the instability waves develop into fully formed vortices upstream of the shock. The interaction of the vortices with the shock causes streamwise oscillations in the shock near its tip. On the subsonic side of the shear layer, the acoustic wave is released as the shock tip deflects upstream through the braid region between vortices. This acoustic field is approximately cylindrical and its directivity is nearly uniform. The acoustic waveform is comprised of a sharp compression followed by a longer expansion. On the supersonic side of the shear layer, a complex wave field is observed. Cases in which the incident shock is replaced by a nearly isentropic compression wave produce qualitatively similar behavior. Acoustic pressure amplitude is found to Copyright ©1998 Ted A. Manning and Sanjiva K. Lele. Published by the American Institute of Aeronautics and Astronautics with permission. talso affiliated with the Department of Mechanical Engineering, Stanford University. scale with compression wave strength. Acoustic wave form is insensitive to compression wave amplitude and profile width for cases examined. This research is directed toward modeling the screech generation process.