Experimental study of turbulence and water levels in shoaling and breaking waves using digital image processing techniques

In this article we present results of laboratory experiments undertaken to study the structure of turbulence generated by mild plunging waves breaking on a sloping beach bed. Measurements of water levels and instantaneous fluid velocities were conducted on a 1:20 sloping beach fitted inside a glass flume. The instantaneous water levels were measured using calibrated capacitive wave gauges, while the instantaneous velocity flow fields were measured using video techniques together with digital correlation techniques. A digital camera was employed to capture images of the breaking waves at 20 phases. For each phase, 100 image pairs were captured from which 100 instantaneous velocity fields were derived using a digital correlation image velocimetry technique. The 100 instantaneous velocity flow fields were averaged to provide information on the mean flow. Further analysis of the acquired data yielded turbulence quantities such as turbulence intensity, turbulent kinetic energy and vorticity at each phase of the flow. Results are presented for four phases where turbulence is predominant. Contour plots are used to provide a temporal and spatial distribution of the evolution of the turbulence characteristics. Vertical profiles of these quantities at different phases are also presented. These results may provide guidance on the approximations that can be expected in computational fluid model studies.