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

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.

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

Roham Bakhtyar, David Andrew Barry

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.

Elsevier2010

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

Wave-induced watertable fluctuations, sediment transport and beach profile change: Modeling and comparison with large-scale laboratory experiments

Roham Bakhtyar, David Andrew Barry, Alessandro Brovelli

Coastal groundwater systems can have a considerable impact on sediment transport and foreshore evolution in the surf and swash zones. Process-based modeling of wave motion on a permeable beach taking into account wave-aquifer interactions was conducted to investigate the effects of the unconfined coastal aquifer on beach profile evolution, and wave shoaling on the water table. The simulation first dealt with wave breaking and wave runup/rundown in the surf and swash zones. Nearshore hydrodynamics and wave propagation in the cross-shore direction were simulated by solving numerically the two-dimensional Navier-Stokes equations with a k-ε turbulence closure model and the Volume-Of-Fluid technique. The hydrodynamic model was coupled to a groundwater flow model based on SEAWAT-2000, the latter describing groundwater flow in the unconfined coastal aquifer. The combined model enables the simulation of wave-induced water table fluctuations and the effects of infiltration/exfiltration on nearshore sediment transport. Numerical results of the coupled ocean/aquifer simulations were found to compare well with experimental measurements. Wave breaking and infiltration/exfiltration increase the hydraulic gradient across the beachface and enhance groundwater circulation inside the porous medium. The large hydraulic head gradient in the surf zone leads to infiltration across the beachface before the breaking point, with exfiltration taking place below the breaking point. In the swash zone, infiltration occurs at the upper part of the beach and exfiltration at the lower part. The simulations confirm that beaches with a low water table tend to be accreted while those with a high water table tend to be eroded.

Elsevier2011