Phase-resolved velocity flow fields induced by laboratory-generated plunging water waves

Abstract

Digital images of regular plunging water waves were captured by a camera and analyzed to examine the spatial and temporal evolution of phase-resolved instantaneous and ensemble-averaged velocity fields induced by the waves as they propagate and break in the surf zone of a laboratory flume. A computer-driven electronic measurement system was designed, developed and employed to capture images of the breaking waves as they propagate along a glass-walled laboratory flume and break on 1:20 plane slope. An 8-bit monochromatic, progressive scan digital camera was connected to the computer and mounted on the side of the flume to capture images of the breaking waves. Through the use of a trigger pulse from the wave generator, the computer synchronizes image acquisition by the camera at the instance the computer drives the strobe lights to illuminate the field of view. A full wave cycle of the breaking wave was too large to be imaged in one video frame, so the cycle was sub-divided into 20 overlapping phases. For a particular phase, 100 sequential image pairs were captured and saved on a computer. A digital correlation image velocimetry technique was later employed to calculate the spatial cross correlation of the gray scale image data by means of computing the cross-power spectrum of the Fourier transformed image samples. Displacements were measured to subpixel accuracy by interpolating the position of the cross-correlation peak.