Two-phase flow modeling of the influence of wave shapes and bed slope on nearshore hydrodynamics

An Eulerian two-phase flow model (air-water) was used to simulate nearshore hydrodynamic processes driven by wave motion. The flow field was computed with the Reynolds-Averaged Navier-Stokes equations in conjunction with the Volume-Of-Fluid method and the RNG turbulence-closure scheme. To study the effects of different wave shapes on surf-swash zone hydrodynamics, a set of numerical experiments was carried out. Predictions of three wave theories (Airy, 2nd-order Stokes and 5th-order Stokes) were compared, with a focus on the turbulence and flow fields. Model performance was assessed by comparing numerical results with laboratory experimental observations. Relationships between the water depth, undertow, TKE and wave characteristics are presented. The results indicate that the characteristics of turbulence and flow, for example the position of wave breaking and magnitude of TKE, are affected by different wave types. Numerical simulations showed that only high-order Stokes wave theory predicts the nonlinearity required for predicting hydrodynamic characteristics in agreement with existing understanding of nearshore processes. Numerical simulations were run for different hydrodynamic conditions, but with a focus on different bed slopes. The transformation of incoming waves as they reach shallow water occurs closer to the shoreline for steeper profiles. Consistently, the peaks in TKE and wave set-up are shifted onshore for steeper slopes. The numerical results showed that TKE and undertow velocity are smaller on dissipative beaches than on intermediate beaches.

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

Numerical simulation of surf-swash zone motions and turbulent flow

Roham Bakhtyar, David Andrew Barry

A two-dimensional numerical model was presented for the simulation of wave breaking, runup and turbulence in the surf and swash zones. The main components of the model are the Reynolds-Averaged Navier-Stokes equations describing the average motion of a turbulent flow, a k-epsilon turbulence closure model describing the transformation and dissipation processes of turbulence and a Volume-Of-Fluid technique for tracking the free surface motion. Nearshore wave evolution on a sloping bed, the velocity field and other wave characteristics were investigated. First, the results of the model were compared with experimental results for different surf zone hydrodynamic conditions. Spilling and plunging breakers were simulated and the numerical model investigated for different wave parameters. Besides the flow field, the turbulence field was also considered and the spatial and time-dependent variations of turbulence parameters were discussed. In the next stage of the study, the results of numerical experiments were compared against two sets of experimental data in the swash zone. Generally, there is good agreement between the results of the model and experimental data, except for turbulence predictions near the breaking point. On the other hand, turbulence predictions were found to be excellent for the swash zone. The model pro-vides a precise and efficient tool for the simulation of the flow field and wave transformations in the nearshore, especially in the swash zone. The numerical model can efficiently simulate the sur-face elevation of the vertical shoreline excursion on sloping beaches, while swash-swash interac-tions within the swash zone are accounted for.

2009

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