Video-based particle image velocimetry of laboratory rip currents.

摘要

Accurate field measurements of rip currents are difficult due to their evanescent characteristics. In this study, Particle Image Velocimetry (PIV) is presented to quantify the horizontal flow structure of laboratory rip currents. PIV is a whole-flow-field technique providing instantaneous velocity vector measurements in a flow cross section. It has been widely applied to hydrodynamic and aerodynamics flow field experiment visualization.; Time-domain and frequency-domain based cross-correlation algorithms are discussed and compared. A multi-grid iterative FFT-based algorithm is selected in favor of its time efficiency and robustness. Lens calibration to remove the edge distortion effect and image rectification to cast the pixel-based image into real world coordinate is conducted before the PIV processing of each image. Proposed post-processing intending to smooth out the spurious vectors is tested on synthetic images.; As the first attempt to apply PIV in laboratory generated rip currents, PIV is validated by ADV measurement using a bar-incised rip channel. The results indicate that the two measurements compare favorably. The rip current flow features can be captured by the image processing technique of PIV.; Finally, the variability of rip current dynamics is investigated by changing basin geometry and incoming wave conditions. On all three geometries with different level of rip channel incision, the rip current strength approximately increases and then decreases with increased incoming wave height (from 0.03 m to 0.05 m to 0.07m) as the wave field is the predominant driving force of rips. As the rip channel is incised from 1/3 to 2/3 and further down to full channel, the rip current feedback system roughly changes from positive to negative. These above complex trends are shown in the spatial distribution of both mean and maximum offshore velocity. Vorticity and shear rate field are also investigated taking advantage of PIV's dense measurements.