Particle Image Velocimetry with an Acoustic Camera*

We performed PIV analysis on images acquired with an acoustic camera (AC) to measure the flow in the wake of a circular cylinder at two flowrates.  The AC observes micro-bubbles suspended in the flow which were used as tracer particles ("bubble image velocimetry").  The size distribution of the bubbles is measured with an acoustic bubble spectrometer (ABS).  The AC PIV measurements of the wake agree well with the comparison measurements made by cross-stream arrays of acoustic Doppler velocimeters (ADVs) for the lower flowrate.  The AC PIV measurements of cross-stream profiles of the mean downstream velocities and the profiles of characteristics of the velocity fluctuations all matched the ADV measurements well.  However, the technique is sensitive to bubble size distribution, as the higher flowrate AC PIV measurements struggled to replicate the ADV measurements, particularly the velocity fluctuations.  The higher flowrate experiment has an order of magnitude more bubbles with size exceeding 15 μm present in the flow than the lower flowrate experiment. The increased bubble density caused a “flickering” effect, with bubbles further away from the camera being intermittently obscured by those closer to the camera.