Acoustic particle-image velocimetry : development and applications
Particle Image Velocimetry (PIV) is a non-intrusive technique for simultaneously measuring the velocities at many points in a fluid flow. The fluid is seeded with tracer particles and the region under investigation is illuminated. An image of the illuminated region is captured and then, a short time period later, a second image is taken. Suitable analysis of these images yields an instantaneous velocity vector map. Until recently, restrictions in the rate at which images could be captured have limited the PIV technique to the analysis of slow flows. However, advances in camera technology have now opened up the possibility of using PIV in the analysis of faster flows. Indeed, image capture rates are now fast enough to enable two images to be captured during a fraction of an acoustic cycle, indicating the potential for using PIV to analyse sound fields. In this thesis, after some aspects of sound field theory have been outlined and following a discussion of the theory of PIV, the development of experimental PIV apparatus for measuring sound fields is described. Measurements of the temporal variation in the velocities of particles within some common sound fields are presented. In particular, the passage of an acoustic pulse is monitored and the sinusoidal motion of particles in a resonating tube is recorded yielding the corresponding standing wave pattern. Finally, the main limitations of the PIV technique when applied to acoustic fields are discussed.