Statistics of intermittent surf zone turbulence and observations of large eddies using PIV

The large-scale structure of intermittent turbulence below trough level produced by depth-limited wave breaking was investigated in a laboratory wave flume using point velocity measurements and particle image velocimetry. A conditional sampling technique was ap plied to the instantaneous turbulent kinetic energy per unit mass, k/rho, estimated from the point velocity measurements where the large scale turbulent fluctuations were separated from the orbital wave motion by phase-averaging. The analysis shows that large, intermittent events exceeding a threshold equal to the mean and standard deviation of k/rho occur for only 7% of the record but account for approximately 400/0 of the total turbulent kinetic energy below trough level. The PIV technique was used to quantify the size and vorticity of the large eddies associated with wave breaking. The PIV technique was applied such that the light sheet was in the horizontal plane to quantify the eddy size and vorticity. Observations show that the nominal diameter of the eddy was 0.05 m with a maximum vorticity of 30 l/s for laboratory scale wave breaking.