The use of optical methods for measuring weak acoustic shock waves in homogeneous air and close to reflecting boundaries

Abstract

Measuring blast waves in laboratory-scale experiments using acoustical methods is a challenge. Optical methods provide an attractive possibility for recording pressure signatures of weak acoustic shocks. In this presentation, recent experiments for reconstructing pressure waveforms of spark-generated spherically divergent blast waves (1.8 kPa amplitude and 50 μs duration at 15 cm distance from the source) both in homogeneous air and close to reflecting surfaces are overviewed. Three methods were employed: shadowgraphy, schlieren, and interferometry. It was shown that shadowgraphy method allowed for measuring shock thickness and its amplitude. Schlieren technique provided reconstruction of the pressure waveforms in homogeneous air. In the reconstruction process the front geometry was assumed to be spherical or cylindrical. The exposure time of the high-speed camera was a limiting factor for the time resolution. Mach-Zehnder interferometer method was the most relevant for laboratory-scale measurements. The method reached 0.4 μs of time resolution, which was more than 6 times higher than that of 1/8-in condenser microphones. Moreover, the Mach-Zehnder interferometry allowed quantitative reconstruction of the pressure waveform without additional calibrations. The method was successfully applied for measuring waveforms in homogeneous air and reflected waves from rigid smooth and rough surfaces. [Work supported by ANR-10-LABX-0060/ANR-16-IDEX-0005.]