مجله هیدرولیک
سال یازدهم شماره 3 (پاییز 1395)

Knick points are generated, naturally or artificially, across the river beds in the form of sequential steps. In the present work, the experimental and numerical migration of two successive Knick points in a sandy bed of a rectangular channel is studied for different discharges. At the start of flow, the two Knick points became unstable in a short period of time. Erosion caused the migration of the first Knick point backward upstream and deposition, down the second Knick point, moved it forward to the downstream channel. At the same time a side bar was formed down the first Knick point and extended upward. Downstream of this bar and before the second Knick point the canal was meandered. Between the two Knick points, the channel developed a steep slope bed. Increase in discharge augmented the rate of movement of the two Knick points. Consequently, migration not only changed the channel elevation but also destabilized its alignment in plan. The flow and sediment transport equations were integrated by HEC-RAS (V4.1.0) for different sediment transport functions. In comparison with the available experimental data, Larsen and Ackers-White equations proved a better estimation for the prediction of bed level variations.

A 2D vertical numerical model for solving unsteady Navier-Stokes equations with capability of calculating dynamic pressures in free surface flows is used in the present study. In this model, the projection method is applied to solve the equations in non-orthogonal curvilinear coordinate system with collocated grid arrangement. The velocities (fluxes) on cell faces are calculated using a “linear interpolation” method and some “momentum interpolation” methods. These methods are evaluated using some test cases including flow in a trench, flow over a sill and gradually varied flow (M2 profile). The results show that for mild free surface slope, all the momentum interpolation methods implemented in the model have the same accuracy. Also, the linear interpolation of velocity results in an acceptable accuracy. In other words, the checker board pressure fluctuation does not occur in flow domain. For sharp free surface slope, the linear interpolation of velocity causes nonphysical pressure fluctuation and results in divergence. Therefore, the momentum interpolation methods are inevitable. Comparing the required computational time of the methods shows that momentum interpolation methods needs less time in comparison with velocity linear interpolation method. The tests show that evaluating the cell face velocities are more important in longitudinal direction than the vertical ones. Therefore, if the momentum interpolation method is used for east and west faces of cells and the linear interpolation of velocity is used for top and bottom faces, the model runs in less time without reducing the accuracy.

In this research, the wave propagation and periodic wave breaking process over an impermeable submerged breakwater is studied experimentally and numerically. Laboratory experiments were conducted in the hydraulics laboratory, school of engineering, Griffith University Gold Coast Campus. For the current study, deployed a space-averaged Navier–Stokes approach and laboratory experiments to investigate the time-dependent wave breaking processes are. The developed model is based on the smoothed particle hydrodynamic (SPH) method which is a pure Lagrangian approach and can handle large deformations of the free surface with high accuracy. Then, a weakly compressible version of the smoothed particle hydrodynamics (WCSPH) method together with a large eddy simulation (LES) approach are used to simulate the wave propagation and periodic wave breaking process over an impermeable submerged breakwater. The results of numerical simulations were compared qualitatively with those of laboratory experiments. Overall, good agreement was found between them. Also, the results of present numerical model were compared with the numerical model of Rambabo and Mani (2005). The results show, that WCSPH computations produce better results than those of Rambabo and Mani (2005), with respect to the experimental data. The results of this study also show that WCSPH method provides a useful tool to investigate the wave propagation over submerged breakwaters.

In this paper, an effective approach using Analytic Element Method (AEM) is presented for large-scale groundwater flow modelling in two dimensional steady state condition. Hydrologic characteristics are exerted in the AEM by using Laplace equations and linear functions. In this algorithm, the potential discharge of each hydrologic characteristic is obtained and combined using the superposition principle and then added to the aquifer potential discharge. In order to compare AEM results, Point Collocation Method (PCM) has been used which is a free-mesh approach based on the finite element method. In PCM approach, some points are distributed on the computational domain and the shape functions are created in order to solve the governing equation. Proposed algorithm evaluation was done by Neyshabur plain data. The particle swarm optimization algorithm has been employed for model calibration. The Python object-oriented programming language was used in implementation of modelling. There is no need for interpolation in AEM. The AEM approach is not a complicated method and uses simple superposition principle. The comparison of results indicate that the results of AEM are more accurate than the results of PCM.

Lattice Boltzmann method is a powerful numerical method for the simulation of fluid flow. There are two approaches for the description of groundwater flow in porous media. In the first approach, Navier-Stokes equations are used, while diffusion equation is applied to describe groundwater flow in the second approach. In this research, groundwater flow is modeled using the second approach. Furthermore, the governing equation of contaminant transport is called advection-dispersion equation. Here for the first time, the coupled solution of groundwater flow and contaminant transport using Lattice Boltzmann method is performed. The results indicated that Lattice Boltzmann method is capable of solving groundwater and contaminant transport equations simultaneously with high precision. Moreover, it was found that although the accuracy of Lattice Boltzmann and Crank-Nicolson method are the same, the speed of Lattice Boltzmann method is much higher than finite difference methods. In addition, Lattice Boltzmann method has higher range of stability and consistency in comparison with explicit finite difference method. Regarding this issue, grid Peclet number smaller than 7 is recommended for D1Q2 scheme of LBM.

Most of the rivers have meandering shape in plan view. By embedding bridge piers in the river, a series of vortices establish in the flow. These vortices are the main cause of pier scour, which gradually led to formation of scour hole around the piers. Todays, the local scour is known as the main cause of bridge destruction around the world. Since the meandering rivers are the most common type of rivers in the plan, in this paper the scour processes are studied around a group of three divergent-vertical and convergent-vertical piers located at the 180 degree bend, in both cases of streamwise and transverse layouts. The experiments were conducted in a 180 degree sharp bend channel (with a ratio of central curvature radius to width of channel equal to 2) in initial sedimentary bed conditions with particle mean diameter of 1.5 mm. The results showed that in both cases of three convergent-vertical and three divergent-vertical groups-piers embedded in the flow direction, the scour around the first inclined pier and the second vertical pier reached to same value. Furthermore, the bed changes in transverse direction was more than the flow direction. In the transverse layout, it was observed that the maximum depth of scour hole near the convergent-vertical group piers was 20% more than the divergent-vertical group piers.