Three dimensional SPH numerical modeling of a bar/rip channel system and turbulent vortex structures under broken water waves in the surf zone region
Jalali Farahani, Rozita
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A Lagrangian numerical method called Smoothed Particle Hydrodynamics (SPH) is used to analyze two different coastal problems. The first problem is a simplified bar/rip channel system on a beach. Prior studies have shown that SPH models propagating water waves well, including breaking waves; here we show that the SPH also models the mean wave-induced near-shore circulation created by breaking waves. Model predictions are compared to the previous laboratory measurements and show good agreement, including mean velocity profiles, mean surface elevation, and cross-shore velocity components over the rip channel. The alongshore variation of different components of radiation stress and the resulting alongshore force that acts as a feeder for the rip current are obtained from the numerical results. The second problem concerns numerical modeling of water waves in the surf zone and related three dimensional turbulent vortical structures generated under the broken waves. The 3D SPH method is used to model solitary waves both spilling and plunging and the numerical model predicts water surface evolution as well as horizontal velocity very well in comparison with the experimental results. In case of spilling solitary wave, the numerical results show organized coherent structures characterized as reversed horseshoe (hairpin) vortices, traveling downward and they appear to be the previously found obliquely descending eddies. The reversed horseshoe structures are associated with the turbulence motion of sweep events (downwelling motion). These reversed horseshoe coherent structures transport momentum and turbulent kinetic energy downward into the water column and likely have a significant role in bed and beach erosion. The mechanism of the generation of large-scale reversed horseshoe structures from the spanwise roller and the vortex stretching are studied. Vortex stretching and vortex bending play an important role on the generation and evolution of reversed horseshoe structures. In case of a plunging solitary wave, the vortex structures are generated under the broken wave and are carried towards the shore by the run-up but no reversed horseshoe structure is observed at the back of the broken wave. Periodic plunging waves are also numerically modeled using 3D SPH method. Three types of vortex structures are detected under broken periodic plunging waves: (1) horizontal rollers with axis of rotation parallel to the waves; (2) vertical counter-rotating vortex structures at the toe of the plunging jets (Known as downbursts); (3) reversed horseshoe structures at the back of the broken waves.