COBRAS (Cornell Breaking Waves and Structure) is a two-dimensional numerical model that solves the Reynolds Averaged Navier-Stokes Equations (RANS) for the mean flow field with a modified k-epsilon turbulence closure based on the nonlinear eddy viscosity assumption. The volume of fluid (VOF) method is employed to track the free surface movement. The model has been tested to be robust in the simulation of wave propagation in the surf zone, wave breaking and wave-structure interactions.
Recently, dynamic processes of bore propagation over a uniform slope are studied numerically using Cobras [Zhang and Liu 2008 Coast. Eng. doi:10.1016/j.coastaleng.2008.04.010]. The dam-break mechanism is used to generate bores in a constant depth region. Present numerical results for the ensemble-averaged flow field are compared with existing experimental data as well as theoretical and numerical results based on non-linear shallow water (NSW) equations. Reasonable agreement between the present numerical solutions and experimental data is observed. Using the numerical results, small-scale bore behaviors and flow features, such as the bore collapse process near the still-water shoreline, the 'mini-collapse' during the runup phase and the 'back-wash bore' in the down-rush phase, are described. In the case of a strong bore, the evolution of the averaged Turbulence Kinetic Energy (TKE) over the swash zone consists of two phases: in the region near the still-water shoreline, the production and the dissipation of TKE are roughly in balance; in the region farther landwards of the still-water shoreline, the TKE decay rate is very close to that of homogeneous grid turbulence. On the other hand, in the case of a weak bore, the bore collapse generated turbulence is confined near the bottom boundary layer and the TKE decays at a much slower rate.
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