A numerical study of wave-breaking turbulence beneath solitary waves using large eddy simulation
Date
2013
Authors
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Publisher
University of Delaware
Abstract
Solitary wave breaking was investigated using a three-dimensional NavierStokes equation solver implemented in the OpenFOAM open-source C++ library of solvers (OpenCFD Limited, [2011]). Surface tracking was accomplished using the volume of fluid method (VOF) (Hirt and Nichols, [1981]), which was validated with physical experiments for non-breaking and breaking wave conditions. Wave generation was accomplished using the groovyBC boundary condition, which allows the free surface elevation and velocity at every grid adjacent to the inlet to be specified with a user-defined function (Gschaider, [2009]). The solitary wave equation implemented was that of Lee, et al., [1982]. Numerical dissipation was also evaluated, and the model performance was satisfactory in all these respects. The performance of the large eddy simulation (LES) turbulence closure model was evaluated via comparison with the laboratory study of Ting, [2006]. Using the dynamic Smagorinsky subgrid closure scheme of Germano,[1991] and amended by Lilly, [1992], time series of turbulent velocity fluctuations, Reynolds stresses, and turbulent kinetic energy showed very good agreement with the experimental data. Free surface elevation of the laboratory breaking solitary wave was consistently overpredicted by the model. This was attributedto uncertainties in the numerical wave generation method, which was not able to generate a non-breaking solitary wave of very large wave height to water depth ratio (0.73). The generation and fate of turbulent coherent structures, especially oblique descending eddies, was investigated by calculating lambda2 criterion contours (Jeong and Hussain,[1995]) and cross-sectional vorticity, average TKE, and instantaneous TKE plots. Oblique descending eddies were seen to initiate in a highly turbulent and rotational region around surface rollers, and evolve into three-dimensional structures after detaching from the waveform and being subjected to the shear of the ambient flow. When eddies were generated in this manner, their strength was such that theyeventually impinged on the bed, inducing highly focused turbulent regions. Bed stress data were also calculated,and Shields parameter contours were plotted for typical sediment sizes. Strong correlation was observed between turbulent coherent structure presence at the bed and potential for sediment transport. Bottom shear resulting from these eddies can be about six times greater at their local peak value than that resulting from the wave motion. These events also have long residency times, implying a larger impact over the course of the wave passage.