REFLECTION OF ACOUSTIC PULSES FROM STABLE AND INSTABLE INTERFACES BETWEEN MOVING FLUIDS

Cagniard's method is used to find the transient solution for the reflection of line-source generated waves from an interface between two fluid half-spaces in relative motion, but with the same density and sound speed. The resulting solution for the reflected wave in the (normally motionless) lower half-space may be conveniently considered as the sum of a specularly reflected wave, a refracted arrival wave, a neutral stability wave comprising one or more resonance waves, and an instability wave. Critical angles limiting the appropriate domains of reception are derived for each wave type as a function of the Mach number M of the fluid motion in the upper half-space. The instability wave appears in the solution when M < 2 √2. This feature may possibly represent the prediction of the spatial extent of a turbulent region arising from an inherent instability of the ambient medium which has been triggered by the arrival of the incident pulse at the interface.