Wavelet-based vortical structure detection and length scale estimate for laboratory spilling waves

Turbulent flow fields under spilling breaking waves are measured by particle image velocimetry and analyzed using the wavelet techniques in a laboratory surf zone. The turbulent vortical structures and corresponding length scales in the flow are detected through the eduction of the most excited mode with local intermittency measure that is found to correlate with the passage of the structure. Distributions and evolution of the educed vortical structures are presented and discussed. Packets of vortical structures with high intermittency is observed to stretch downward below the initially low-intermittency trough level, indicating these structures play a crucial role in turbulent mixing below the trough level. It is found that the probability density functions of the intermittent energy of the educed structures, vorticity and swirl strength display an exponential decay. Ensemble-averaged length scales of the educed vortical structures are found to be about 0.1 to 0.2 times the local water depth, close to the turbulent mixing length reported in the surf zone. The Kolmogorov microscale is evaluated and the turbulent mixing length is estimated using the k-ξ relation and mixing length hypothesis. The k-ξ relation may overestimate the mixing length scale for energetic descending eddies.