Acoustic velocimetry imaging

• Published in: The Journal of the Acoustical Society of America 2020-10, Vol.148 (4), p.2534-2534
• Main Authors: Murray, Nathan E., Tinney, Charles E., Lyons, Gregory W.
• Format: Article
• Language: English
• ISSN: 0001-4966; 1520-8524
• DOI: 10.1121/1.5147037

We review a technique that we developed that combines shadowgraph or schlieren imaging with image processing tools to study the sound produced by turbulent shear flows. These flows are challenging acoustically because they are characterized by a distributed cloud of correlated sources that produce sound through multiple mechanisms. In the far field, 100 s of characteristic lengths from the source region, the pressure waves are purely acoustic, the apparent the source location shrinks to a point, and the common assumption is that the propagation behavior is spherical. In the near field, pressure signals from both acoustic and hydrodynamic (or evanescent) signatures are difficult to separate, especially when the convective Mach number of the sound-producing events is supersonic. Single-point measurements such as condenser-type microphones are blind to the direction of the approaching/departing pressure waves unless they are combined in arrays. Non-intrusive optical-based methods offer some benefit in this scenario and can be tailored in new and different ways. Examples include acoustic laser Doppler velocimetry and quantitative schlieren methods. Our technique combines modern digital camera technology (with high-resolution, high-speed capabilities) with tomographical image processing techniques images to yield a “picture” of the sound field that can provide quantitative assessment of the source distribution and source velocities. This optical technique is a strong complement for single-point or array microphone measurements.