Abbas Yeganeh-Bakhtiary

A three-dimensional distinct element model for bed-load transport

A three-dimensional model is developed to simulate bed-load transport using Distinct Element Method (DEM). The model describes the bed-load transport as dynamically interdependent motions of individual sediment particles under the flow action. A steady logarithmic velocity distribution over the flow depth is considered to represent fluid motion, while turbulent intensities are also included within the closure model. The present model accounts for both fluid–particle and particle–particle interactions, which are the predominant micro-mechanisms of bed-load transport under low and high tractive forces, respectively. Characteristic features of bed-load transport, previously reported by experimental observations, are numerically reproduced. The concept of vertical momentum transfer is exploited to describe features of concurrently present of saltation and sheet-flow layers. It is concluded that the vertical motion of particles is not significantly active in sheet-flow layer and the particle momentum is preserved. Consequently the interparticle collisions is the predominant mechanism of bed-load transport at high fluid tractive forces.

Climate change impact on wave energy in the Persian Gulf

Excessive usage of fossil fuels and high emission of greenhouse gases have increased the earth’s temperature, and consequently have changed the patterns of natural phenomena such as wind speed, wave height, etc. Renewable energy resources are ideal alternatives to reduce the negative effects of increasing greenhouse gases emission and climate change. However, these energy sources are also sensitive to changing climate. In this study, the effect of climate change on wave energy in the Persian Gulf is investigated. For this purpose, future wind data obtained from CGCM3.1 model were downscaled using a hybrid approach and modification factors were computed based on local wind data (ECMWF) and applied to control and future CGCM3.1 wind data. Downscaled wind data was used to generate the wave characteristics in the future based on A2, B1, and A1B scenarios, while ECMWF wind field was used to generate the wave characteristics in the control period. The results of these two 30-yearly wave modelings using SWAN model showed that the average wave power changes slightly in the future. Assessment of wave power spatial distribution showed that the reduction of the average wave power is more in the middle parts of the Persian Gulf. Investigation of wave power distribution in two coastal stations (Boushehr and Assalouyeh ports) indicated that the annual wave energy will decrease in both stations while the wave power distribution for different intervals of significant wave height and peak period will also change in Assalouyeh according to all scenarios.

Euler–Lagrange Two-Phase Model for Simulating Live-Bed Scour Beneath Marine Pipelines

In this study, an Euler–Lagrange coupling two-phase flow model, namely movable bed simulator (MBS)-two-dimensional (2D) model was employed to explore the currentinduced live-bed scour beneath marine pipelines. The fluid phase characteristics, such as velocity and pressure, were obtained by the Reynolds-averaged Navier–Stokes (RANS) equations with a k-e turbulence closure model in a two-dimensional Eulerian grid, whereas the seabed beneath pipelines was traced as an assembly of discrete sand grains from the Lagrangian point of view. The live-bed scour was evolved as the motion of a granular media based on distinct element method (DEM) formulation, in which the frequent interparticle collision was described with a spring and dashpot system. The fluid flow was coupled to the sediment phase, considering the acting drag forces between. Comparison between the numerical result and experimental measurement confirms that the numerical model successfully estimates the bed profile and flow velocity field. It is evident that the fluid shear stress decreases with the increasing of gap ratio e/D. The numerical model provides a useful approach to improve mechanistic understanding of hydrodynamic and sediment transport in live-bed scour beneath a marine pipeline. [DOI: 10.1115/ 1.4023200]

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

ABSTRACT Bakhtyar, R., Razmi, A.M., Barry, D.A., Yeganeh-bakhtiary, A. Kees, C.E., and C.T. Miller, 2013. 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.

Cross-shore sediment transport estimation using fuzzy inference system in the swash zone

The interactions between fluid and sediment in the swash zone dominate the erosion or accretion of the beach, and they act as boundary conditions for nearshore hydrodynamic and morphodynamic models. Thus, the evaluation of sediment transport is of particular importance for many coastal processes and the design of coastal structures. In this paper, unlike conventional approaches, Fuzzy Inference System (FIS) and Adaptive-Network-Based Fuzzy Inference System (ANFIS) methods are used for the prediction and simulation of cross-shore sediment transport in the swash zone. The ANFIS and FIS are established using the free stream velocity time series, Shields parameter and antecedent cross-shore sediment transport of the available swash experimental data. Statistical measures were used to evaluate the performance of the models. Numerical experiments showed that the neuro-fuzzy approach provided satisfactory predictions in sediment transport modeling without incorporating different multipliers for uprush and backwash. Furthermore, the numerical results revealed that the ANFIS-based predictions are slightly superior to the FIS-based predictions.

A Numerical Study on Hydrodynamics of Standing Waves in Front of Caisson Breakwaters with WCSPH Model

In this paper, a two-dimensional Lagrangian model based on the weakly compressible smoothed particle hydrodynamics (WCSPH) was developed to explore the hydrodynamics of standing waves impinge on a caisson breakwater. The developed model is validated against experimental data and applied then to analyze the wave horizontal velocity in front of a vertical caisson. The effect of wall steepness was investigated in terms of the steady streaming pattern due to generation of fully to partially standing waves. The numerical results indicated that the partially standing waves generated in front of the sloped caisson change the pattern of steady streaming. For the vertical caisson, the velocity component of recirculating cells increased in front of the vertical wall; whereas, for the sloped caisson it decreased from the sloped wall with reducing the wall steepness. In addition, near the milder sloped wall the intensity of velocity component is higher, which is an important parameter in scour process in front of caisson breakwater.

Numerical Study of the Effect of Submerged Vertical Breakwater Dimension on Wave Hydrodynamics and Vortex Generation

The effect of submerged vertical breakwater dimension on wave hydrodynamics and vortex generation around the breakwater is investigated with numerical modeling via two dimensionless parameters: the breakwater dimensionless submergence depth (a/H i; a-the breakwater depth of submergence) and the Keulegan-Carpenter number (KC = H i/L bw; H i-incident wave-height and Lbw-breakwater width). In the numerical model, Reynolds Averaged Navier-Stokes (RANS) equations with a standard k–ε turbulence closure model were implemented; the free surface was traced using the VOF method. A total of 10 different simulations with different KC number and breakwater submergence depth were conducted for this study. The results revealed that the transmission coefficient increases with increasing a/H i and KC number, but that the effect of the KC number is not linear like the relation to a/H i. For the waves modeled, the transmission coefficient increases dramatically with increasing the KC number until the KC number reaches a critical value, this critical value is observed when breakwater width is equal to a quarter of wavelength. This gives a hint in design of breakwater width. Turbulence intensity decreases with increasing a/H i and KC on the seaside of the breakwater while it increases especially near the bed on the leeside of the breakwater; this can increase scour risk on the leeside of the breakwater. The optimum a/H i for both, high-energy dissipation rate and low risk of scour tends to 3.5 for KC ≈ 1.0.

An Application of Evolutionary Optimization Algorithms for Determining Concentration and Velocity Profiles in Sheet Flows and Overlying Layers

The Particle Swarm Optimization (PSO) method and the Genetic Algorithm (GA) were used to derive formulas for determining the velocity and concentration profiles in sheet flows. Specifically, these evolutionary optimization algorithms were used in conjunction with experimental data to determine coefficients and identify parameters for pre-selected formulas. The objective function, defined as the sum-of-squared errors between observed and predicted values of sediment velocity and concentration, was minimized by adjusting the parameter values in the formulas. Two well-known empirical formulas were also applied to the same data. The bias, root mean square error and scatter index were used to evaluate the comparison between predictions and measurements. The results indicated that the errors based on the PSO and GA approaches to predicting sediment parameters were less than those of the existing empirical formulas. Overall, both evolutionary approaches provided formulas that were in good agreement with the experimental data, giving improved descriptions of the vertical distribution of velocity and sediment concentration in the sheet flow for practical purposes. These models also described well the behavior of the velocity and sediment concentration above the sheet flow layer, in contrast with most existing formulas that are applicable only to the sheet flow layer.

Aerodynamic Granular-Material Model of Wind-Blown Sand Layer

The solid-gas two-phase flow model is combined with the granular material model to simulate the detailed mechanics of sediment transport in wind-blown sand layer. The physical characteristics of the wind-blown sand layer is investigated from the viewpoint of computational mechanics of sediment transport by taking into account the sediment-flow interaction and the interparticle collision. Changes in the windvelocity profile and the Reynolds stress profile due to the load of sediment are calculated. The velocity and the existing probability density of moving particles, and the distribution of colliding frequency of particles are estimated to consider the physical structure of the wind-blown sand layer.