THE INTERACTION OF AN ISOTROPIC FIELD OF ACOUSTIC-WAVES WITH A SHOCK-WAVE

Moore's (1954) inviscid linear analysis of the interaction of a shock wave with a plane acoustic wave is evaluated by comparison to computat ion. The analysis is then extended to study the interaction of an isot ropic held of acoustic waves with a normal shock wave. The evolution o f fluctuating kinetic energy, sound level and thermodynamic fluctuatio ns across the shock wave are examined in detail. The interaction of ac oustic fluctuations with the shock is notably different from that of v ortical fluctuations. The kinetic energy of the acoustic fluctuations decreases across the shock wave for Mach numbers between 1.25 and 1.8. For Mach numbers exceeding 3, the kinetic energy amplifies by levels that significantly exceed those found in the interaction of vortical f luctuations with the shock. Upon interacting with the shock wave, the acoustic waves generate vortical fluctuations whose contribution to th e far-field kinetic energy increases with increasing Mach number. The level of sound increases across the shock wave. The rise in the sound pressure level across the shock varies from 5 to 20 dB for Mach number varying from 1.5 to 5. The fluctuations behind the shock wave are nea rly isentropic for Mach number less than 1.5, beyond which the generat ion of entropy fluctuations becomes significant.