ZONES OF INFLUENCE IN THE COMPRESSIBLE SHEAR-LAYER

The effect of Mach number on communication between regions of a shear layer is analyzed in the limit of geometric acoustics. Communication i s quantified in terms of the acoustic intensity emitted by a point sou rce and received by an observer moving with the flow. The generalized Snell's law is used to trace the sound rays which are shown to be high ly distorted because of the Mach-number gradient. Sound-intensity calc ulations reveal that the influence of the source on the surrounding me dium becomes confined to a smaller and smaller area as the Mach number increases. The zones of influence on the Lagrangian observer are funn el-shaped and become narrower with increasing Mach number. Communicati on is inhibited primarily in the axial direction, both upstream and do wnstream, and to a lesser extent in the transverse direction. Hindranc e of communication may be the fundamental reason for enhanced stabilit y at high Mach numbers.