On the Relevance of Convection Effects for a Laser-Generated Sound Source

Abstract

A laser-generated sound source is used to generate a nearly omni-directional sound field. This sound field is investigated experimentally to reveal its characteristics as a function of the surrounding medium mean velocity. It is found that the sound source approximately has monopole character when generated in a stagnant medium. Increas- ing the surrounding flow velocity leads to a positive Doppler-shift combined with a convective amplification for an observer downstream of the source while the reverse is observed upstream of the source. The laser sound source behaves therefore as a convected point heat source in a uniform flow. A solution of the convected wave equa- tion for a moving point heat source in a uniform subsonic flow is derived, providing a closed-form description of the source and appropriate scaling parameters for exper- imental results. The derived theoretical model enables the determination of the heat release function resulting from the laser energy deposition in the medium and associ- ated to the measured far-field pressure pulses. An implementation of the heat release function in CAA codes is straigthforward, making numerical replication of shielding tests with the laser sound source possible.