Extracting Turbulence under Breaking Waves in the Surf Zone

Abstract

Separating turbulence from orbital motion under breaking waves in the surf zone is essential to understanding wave-energy dissipation. In this study, velocity data under monochromatic and random waves in the large-scale sediment-transport facility were analyzed using ensemble averaging (EA), high-pass filtering (HPF) and moving averaging (MA) to extract breaking-induced turbulence. Moving averaging provides a simple method for extracting turbulence from velocity measurements under random breaking waves collected at a reasonably high frequency. Various MA time intervals were examined. Three approaches were used to evaluate the ability of MA to extract turbulence, including (1) testing the ability of MA in extracting turbulence from artificially synthesized signals, (2) comparing turbulence strength of monochromatic wave case using MA and EA, and (3) comparing turbulence strength obtained from MA and Butterworth HPF for the random-wave case. The results indicate that approximately 30–42° phase angle (relative to the peak wave period) MA allows reasonable extraction of turbulence. The turbulence extraction particularly at the wave crest and trough can be improved by adjusting the averaging interval. An adaptive MA with variable averaging time is developed. The MA method is further examined and verified using velocity measurements in the inner surf zone at two sites along the west–central Florida coast.