Bubble image velocimetry with a field-deployable acoustic camera

Abstract

This study explores performing bubble image velocimetry (BIV) analysis on images acquired from an acoustic camera (AC) to obtain measurements of the flow in the wake of a circular cylinder at two flowrates. The AC observes the returns from air micro-bubbles suspended in the flow which are used as tracer particles in PIV-style analysis. The size distribution of the micro-bubbles is measured with an acoustic bubble spectrometer (ABS). The AC BIV measurements of the wake agree well with the comparison measurements made by cross-stream arrays of acoustic Doppler velocimeters (ADVs) for the lower of the two measured flowrates. The AC BIV measurements of cross-stream profiles of the mean downstream velocities and the profiles of characteristics of the velocity fluctuations, such as the standard deviation of the downstream and cross-stream velocity fluctuations, the covariance of the horizontal velocity fluctuations, and the kinetic energy of the horizontal velocity fluctuations, all matched the ADV measurements well. However, the measurement technique is sensitive to bubble size distribution, as the higher flowrate AC BIV measurements struggled to replicate the ADV measurements, particularly the measurements of the velocity fluctuations. The higher flowrate experiment is revealed to have an order of magnitude more bubbles with size exceeding 15 present in the flow than the lower flowrate experiment. The increased bubble density caused increased noise in the AC images and a ‘flickering’ effect, with bubbles further away from the camera being intermittently obscured by those closer to the camera. The ability to make PIV-style measurements with an AC has ramifications for flow measurements in the field. ACs operate in turbid environments, do not require a laser and extensive optics, and do not require calibration with a grid plate to convert pixel space to physical space.

A large-scale community storm processes field experiment : the During Nearshore Event Experiment (DUNEX) overview reference report

“…Observing the motion of suspended particles in a quantitative way allowed the images to be processed with particle-image and particle-tracking velocimetry algorithms. This technique was previously successfully tested in a laboratory setting by Young et al (2018).…”

Capabilities of an Acoustic Camera to Inform Fish Collision Risk with Current Energy Converter Turbines

“…For instance, increased volume scattering from microbubbles and particles are documented as limiting the smallest detectable target and accuracy of target tracking when using 200 and 420 kHz sidelooking echosounders in two Alaskan rivers [51]. Microbubbles are also a part of volume scattering in a volume of ensonified water with ACs [52] as they are with sonars. In fact, microbubbles' strong resonance at the frequencies of most ACs (1-3 MHz) makes them candidates for flow field measurements such as those of [52], who explored AC data at 3 MHz for bubble image velocimetry methods.…”

“…Microbubbles are also a part of volume scattering in a volume of ensonified water with ACs [52] as they are with sonars. In fact, microbubbles' strong resonance at the frequencies of most ACs (1-3 MHz) makes them candidates for flow field measurements such as those of [52], who explored AC data at 3 MHz for bubble image velocimetry methods. Our research did not quantify the suspended microbubbles and thus the attenuation affects, though likely, are not known.…”