VORTEX-INDUCED DISTURBANCE FIELD IN A COMPRESSIBLE SHEAR-LAYER

The disturbance field induced by a small isolated vortex in a compress ible shear layer is studied using direct simulation in a convected fra me. The convective Mach number, M(c), is varied from 0.1 to 1.25. The vorticity perturbation is rapidly sheared by the mean velocity gradien t. The resulting disturbance pressure field is observed to decrease bo th in magnitude and extent with increasing M(c), becoming a narrow tra nsverse zone for M(c) > 0.8. A similar trend is seen for the perturbat ion velocity magnitude and for the Reynolds shear stress. By varying t he vortex size, it was verified that the decrease in perturbation leve ls is due to the mean-flow Mach number and not the Mach number across the vortex. At high M(c), the vortex still communicates with the edges of the shear layer, although communication in the mean-flow direction is strongly inhibited. The growth rate of perturbation kinetic energy declines with M(c) primarily due to the reduction in shear stress. Fo r M(c) greater-than-or-equal-to 0.6, the pressure dilatation also cont ributes to the decrease of growth rates. Calculation of the perturbati on field induced by a vortex doublet revealed the same trends as in th e single-vortex case, illustrating the insensitivity of the Mach-numbe r effect to the specific form of initial conditions.