Optical visualization of sound source of edge tone using parallel phase-shifting interferometry

In order to reduce aerodynamic noise, understanding the nature of aerodynamic sound sources is important. Generally, aerodynamic sound is measured by using microphones. However, microphones should be installed far from aerodynamic sound sources, which makes difficult to understand the nature of aerodynamic sound sources. As non-contact measurement methods, optical measurement methods of sound have been proposed. Among those, parallel phase-shifting interferometry (PPSI) can capture time-varying phenomena. Recently, simultaneous visualization of flow and sound using PPSI has been proposed. This method enables to capture the propagation of sound inside a flow. In this paper, as an application of aerodynamic sound visualization using PPSI, results of visualization of sound sources of edge tones are shown. The nozzle-edge distance and the flow rate, which are parameters of changing frequency of an edge tone, were adjusted from 5 mm to 13 mm at intervals of 1 mm and from 15 L/min to 45 L/min at intervals of 5 L/min, respectively. The frame rate of the high-speed camera in PPSI was set to 20,000 frames per second and the size of the visualization area was 77 mm by 56 mm. From the visualized images, the characteristics of spatial spread of edge tones were observed.