Propagation characteristics of laser-induced acoustic sources in hybrid anechoic wind tunnels

Abstract

The propagation characteristics of an acoustic point source generated using laser-induced plasma (LIP) were investigated experimentally. Experiments were performed in a Kevlar-walled hybrid anechoic wind tunnel (HAWT) where the sound of the LIP was measured using a 251-element microphone array, while the flow speed in the empty test section was varied. The time instant of the LIP formation was also captured. The far field sound pressure was assessed through arrival times (source to microphones) and pressure correction levels, and these quantities were compared against a commonly used shear layer refraction model. A detailed uncertainty assessment is presented on the arrival times and pressure levels. It was found that the time domain analysis was limited by the sampling rate of the analog-to-digital converter regardless of the flow speed. The uncertainty of the pressure levels was limited by the uncertainty of the microphones at low flow speeds, while they increased with flow speed at shallow observer angles. The high-speed Schlieren imaging of the LIP was performed, which revealed that the sound of the LIP reaches the far field microphones over a shorter time duration than modeled because the wave speed was initially supersonic. The discrepancy was found to be comparable to the temporal resolution of the aeroacoustic experiments. The discrepancy between the experimental and theoretical arrival times was found to increase with flow speed, and they were nearly independent of the azimuth angles. The discrepancy between the experimental and theoretical pressure correction ratio was found to be uniform for most observer locations. With an increase in flow speed, the discrepancy became positive at large, and negative at low polar angles. The sound refraction at the Kevlar wall did not change the frequency content of the sound over the investigated range of frequencies (1–10 kHz).