Numerical simulation of two-phase flow for sediment transport in the inner-surf and swash zones

A two-dimensional two-phase flow framework for fluid- sediment flow simulation in the surf and swash zones was described. Propagation, breaking, uprush and backwash of waves on sloping beaches were studied numerically with an emphasis on fluid hydrodynamics and sediment transport characteristics. The model includes interactive fluid- solid forces and intergranular stresses in the moving sediment layer. In the Euler-Euler approach, two phases were defined using the Navier-Stokes equations with interphase coupling for momentum conservation. The k-ε closure model and Volume- Of-Fluid approach were used to describe the turbulence and tracking of the free surface, respectively. Numerical simulations explored incident wave conditions, specifically spilling and plunging breakers, on both dissipative and intermediate beaches. It was found that the spatial variation of sediment concentration in the swash zone is asymmetric, while the temporal behavior is characterized by maximum sediment concentrations at the start and end of the swash cycle. The numerical results also indicated that the maximum turbulent kinetic energy and sediment flux occurs near the wave breaking point. These predictions are in general agreement with previous observations, while the model describes the fluid and sediment phase characteristics in much more detail than existing measurements. With direct quantifications of velocity, turbulent kinetic energy, sediment concentration and flux, the model provides a useful approach to improve mechanistic understanding of hydrodynamic and sediment transport in the nearshore zone.

Numerical Simulation of Two-Phase Flow for Nearshore Hydrodynamics under Wave-Current Interactions

Roham Bakhtyar, David Andrew Barry, Amir Mehdi Razmi

Surf and swash zone hydrodynamics are complex, being composed of a highly energetic water-air mixture under periodically unsteady motion. Wave-current interactions have a major effect on the results of hydrodynamic simulations, and consequently on predictions of mixing and sediment transport processes. An Euler-Euler air-water two-phase flow model was used to simulate hydrodynamic processes under waves and currents in the nearshore zone. The flow field was computed with the Reynolds-Averaged Navier-Stokes equations in conjunction with the Volume-Of-Fluid method and RNG turbulence closure scheme. Different flow fields were simulated and compared (wave, current, wave+current). The results indicated that the characteristics of oscillatory flow were affected by the combined current and wave motions, especially near the bed, which is important for sediment transport. This study demonstrates that currents modify the hydrodynamics of wave-driven motion in the surf and swash zones. The kinetic energy magnitude was much improved in the wave breaking process and the breaking point occurred further offshore than for purely wave-driven motion. The effects of currents on the flow field were found to be different depending on their magnitudes. Vertical variations of the velocity distribution (undertow), turbulence characteristics, void fractions, and non-hydrostatic pressure were used to assess the hydrodynamics of the nearshore zone and wave-breaking processes under different combinations of wave and current. These numerical studies simulate the different types of wave breaking and accompanying nearshore zone hydrodynamics that affect the physical processes in surf and swash zones.

Coastal Education and Research Foundation2011

Process-based model for nearshore hydrodynamics, sediment transport and morphological evolution in the surf and swash zones

Roham Bakhtyar, David Andrew Barry

Nearshore hydrodynamics and sediment transport in the nearshore zone were modeled numerically taking into account turbulent unsteady flow. The flow field was computed using the Reynolds Av-eraged Navier-Stokes equations with a k-ε turbulence closure model, while the free surface was tracked using the Volume-Of-Fluid technique. This hydrodynamical model was supplemented with a cross-shore sediment transport formula to calculate profile changes and sediment transport in the surf and swash zones. Based on the numerical solutions, flow characteristics and the effects of breaking waves on sediment transport were studied. The main characteristic of breaking waves, i.e., the instantaneous sediment transport rate, was investigated numerically, as was the spatial distribu-tion of time- averaged sediment transport rates for different grain sizes. The analysis included an evaluation of different values of the wave friction factor and an empirical constant characterizing the uprush and backwash. It was found that the uprush induces a larger instantaneous sediment transport rate than the backwash, indicating that the uprush is more important for sediment transport than the backwash. The results of the present model are in reasonable agreement with other numeri-cal and physical models of nearshore hydrodynamics. The model was found to predict well cross- shore sediment transport and thus it provides a tool for predicting beach morphology change.

2009

Air-water two-phase flow modeling of turbulent surf and swash zone wave motions

Roham Bakhtyar, David Andrew Barry, Amir Mehdi Razmi

Wave breaking and wave runup/rundown have a major influence on nearshore hydrodynamics, morphodynamics and beach evolution. In the case of wave breaking, there is significant mixing of air and water at the wave crest, along with relatively high kinetic energy, so prediction of the free surface is complicated. Most hydrodynamic studies of surf and swash zone are derived from single-phase flow, in which the role of air is ignored. Two-phase flow modeling, consisting of both phases of water and air, may be a good alternative numerical modeling approach for simulating nearshore hydrodynamics and, consequently, sediment transport. A two-phase flow tool can compute more realistically the shape of the free surface, while the effects of air are accounted for. This paper used models based on two-dimensional, two-phase Reynolds-Averaged Navier-Stokes equations, the Volume-Of-Fluid surface capturing technique and different turbulence closure models, i.e., k-ε, k-ω and Re-Normalized Group (RNG). Our numerical results were compared with the available experimental data. Comparison of the employed method with a model not utilizing a two-phase flow modeling demonstrates that including the air phase leads to improvement in simulation of wave characteristics, especially in the vicinity of the breaking point. The numerical results revealed that the RNG turbulence model yielded better predictions of nearshore zone hydrodynamics, although the k-ε model also gave satisfactory predictions. The model provides new insights for the wave, turbulence and means flow structure in the surf and swash zones.

Elsevier2010