Observation of the coherent turbulent structure under breaking wave

Previous studies qualitatively sketched that relative larger coherent turbulent motions generated in the crest region would stretch downward into the trough level during propagation of breaking wave in the surf zone. This event further suggests that an instantaneous burst of turbulence intensity will occur locally in the path of the penetrating turbulent motion. To observe such a spatial depended turbulent structure, the internal flow field of spilling wave propagating on an inclined bottom slope of 1/20 was measured quantitatively by Particle Image Velocimetry. The ensemble average method is employed to decompose the mean flow and turbulent fluctuations by repeating the same experiment for twenty-five times in each Field of View (FOV). Seven FOVs were integrated for representing the global results, which covers the breaking wave from the outer to inner surf zones. The temporal and spatial evolutions of the coherent turbulent structure were examined and discussed. Results manifest that a relative large turbulent motion is initially organized in the wave crest of the bore-established region. As the wave crest passed, this turbulent structure will then stretch downward to the interior region of the water column. Two important dynamic turbulence transport terms, the turbulent convection and diffusion, were estimated to investigate the underline mechanism of the turbulence transport. It is found that the convection term is much significant to spread the turbulent structure from the wave crest to trough region.