Spatio-temporal measurement of density distribution in oscillatory boundary layer using phase-shifting interferometer

Abstract

The objective of this study is to directly measure the acoustic density fluctuations of a gas confined between two parallel plates. For this purpose, we originally developed a phase-shifting interferometer, where a polarizing Mach–Zehnder interferometer was incorporated with a polarization camera. This enabled us to achieve a time resolution of 70 FPS in addition to the high-resolution measurement by the phase-shifting technique. The test beam passes through the two parallel plates of width 10 mm, where an acoustic field is generated using a loudspeaker that produces high acoustic pressures (up to 3000 Pa) at frequencies ranging from 0.5 to 5 Hz. In this work, we successfully measure the density fluctuations, which are otherwise difficult to obtain using conventional optical interferometers. Additionally, the experimental results are compared to the visco-thermal acoustic theory, where a good agreement was reached, affirming the effectiveness of this phase-shifting interferometer. Furthermore, using the proposed setup, we are able to measure and deduce the spatiotemporal variations of the pressure, density, and temperature without interfering with the acoustic field, which distinguishes it from measurement probes like thermocouples. This interferometer can be used to study more complex phenomena such as edge effects in thermoacoustic engines.