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Optical distortion caused by changes in the refractive index of fluid flow is a common issue in flow visualization using techniques, such as particle image velocimetry (PIV). In thermally driven convection, this distortion can severely interfere with PIV results due to the ubiquitous density and, therefore, refractive index heterogeneity in the fluid. The distortion also varies spatially and temporally, adding to the challenge. We propose a composite filter, the shadow-affected PIV region filter, which combines a series of conventional image filters to address this issue, focusing on optical distortion of thermal plumes in laminar flow. We verify the effectiveness of the filter using both synthetic particle images created from ray tracing and real particle images from the laboratory. For the first time, we effectively mitigate the optical distortion from plumes while preserving the in-plane plume velocity and overall flow pattern, with the PIV data alone. Our filter is efficient and does not require additional measurements, expensive ray tracing, or a large dataset to begin with. It can be extended to separate the flow field and the effect of optical distortion in other fluid experiments when the two components are visually distinct. Additionally, this filter can serve as a baseline algorithm for comparison when developing more advanced methods like neural networks.
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Development and laboratory assessment of a subsea particle image velocimetry system for bubble and turbulence measurements in marine seeps
Abstract We present the development and laboratory evaluation of RPiPIV, an underwater particle image velocimetry (PIV) system controlled by a Raspberry Pi. Designed specifically to measure bubble characteristics and bubble‐induced flow in natural hydrocarbon seeps, RPiPIV comprises three primary pressure enclosures, housing a consumer‐grade laser for particle illumination, a Gig‐E camera for image capture, a Raspberry Pi for system control, and essential supporting electronics for voltage conversion, battery management, and remote connection. Operating on 24–36 V DC power, the RPiPIV system can be deployed tethered onto a remotely operated vehicle or self‐contained for extended duration measurements. Comparing the RPiPIV and a laboratory high‐speed camera system, we conducted assessments of bubble imaging in a bubble stream and PIV measurements in a water jet, bubble‐chain flow, and single‐orifice bubble plume. Laboratory assessments revealed that bubble diameter estimates differed by approximately 5%. In PIV measurements, mean axial velocities exhibited differences of approximately 5%, while turbulent normal and shear stresses showed variances within 10–30%. Dissipation rates of turbulence kinetic energy differed by approximately 60%. These findings underscore the system's potential for reliably quantifying complex multiphase flow characteristics in deep‐sea environments.
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- PAR ID:
- 10576650
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Limnology and Oceanography: Methods
- Volume:
- 23
- Issue:
- 3
- ISSN:
- 1541-5856
- Format(s):
- Medium: X Size: p. 139-154
- Size(s):
- p. 139-154
- Sponsoring Org:
- National Science Foundation
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