Simulation Of Free Surface Flows Research Articles

A Hybrid Parallel Numerical Model for Wave-Induced Free-Surface Flow

An advanced numerical model is presented for the simulation of wave-induced free-surface flow, utilizing an efficient hybrid parallel implementation. The model is based on the solution of the Navier–Stokes equations using large-eddy simulation of large-scale coastal free-surface flows. The three-dimensional immersed boundary method was used for the enforcement of the no-slip boundary condition on the bed surface. The water-air interface was tracked using the level-set method. The numerical model was effectively validated against laboratory measurements involving wave propagation over a flatbed with an elliptical shoal, whose presence induces combined wave refraction and diffraction phenomena. The parallel implementation of the model enabled the efficient simulation of depth-resolved, wave-induced, three-dimensional, free-surface flow; the model parallel efficiency and strong scaling are quantitatively demonstrated.

An improved smoothed particle hydrodynamics approach using new inverse kernel function

The main limitation of Smoothed Particle Hydrodynamics (SPH) method that resists the method's potential is its lack of providing stability and accuracy to the numerical technique. We improve the accuracy of the standard SPH technique, by formulating a new inverse logarithmic kernel function. This new kernel function is derived based on the underlying properties of kernel functions. The approximation technique used here is based on the Moving Least Squares based technique. The adequacy of the proposed kernel function is tested by simulation of 2D shock wave propagation and 3D dam-break free surface flow against a cuboidal obstacle. The method was validated against experimental data by Kleefsman et al., [1]. The numerical results reveal that our new SPH approach using inverse logarithmic kernel function outperforms existing ones in particle restoration, zero error, better accuracy and enhanced efficiency in kernel approximation. This new kernel function showed some improvement over existing kernels by showing very less error approximation value of 0.035h2. The results showed some improvements over standard technique by being capable of handling problems with large deformations accurately and precisely.

Further enhancement of the particle shifting technique: Towards better volume conservation and particle distribution in SPH simulations of violent free-surface flows

The non-conservation of total fluid volume, caused by the accumulating errors on the potential energy of the fluid, has been a serious numerical issue in a Weakly-Compressible SPH (WCSPH) simulation when transitional Particle Shifting Techniques (PSTs) are employed to prevent disordered particle distribution and the tensile instability from negative pressures. This paper is dedicated to, within the framework of WCSPH, developing an enhanced version that remedies the aforementioned deficiency of the traditional PSTs, and meanwhile improves the quality of the particle distribution in the vicinity of a free-surface region. To this end, a Corrective Cohesion Force (CCF) between a target particle and its interacting particles is introduced to provide adaptive compensation corresponding to the particle repositioning. Four classical benchmarks are implemented to validate the effectiveness and stability of the present PST. It is demonstrated that the new PST incorporating with the CCF shows satisfactory performance to improve the conservation of total fluid volume, and to obtain more uniform particle distribution in the proximity of the free-surface. In addition, the newly-developed PST also maintains the accuracy and stability inherited from the traditional versions, suggesting that it can be treated as an ideal alternative with regard to the traditional PSTs in a WCSPH simulation, especially for a long-term-duration case with violent free-surface evolutions.

Partitioned MPS-FEM method for free-surface flows interacting with deformable structures

In this paper, the partitioned MPS-FEM method is employed for two-dimensional and three-dimensional free surface flow interacting with deformable structures. The discretization of fluid and structure govern equations relies independently on the Moving Particle Semi-implicit method (MPS) and Finite Element Method (FEM). Two types of interpolation techniques, including the Shape Function Based Interpolation Technique (SFBI) and Kernel Function Based Interpolation Technique (KFBI) are proposed on the fluid-structure isomerous interface. The performance of MPS-FEM coupled method is validated by a series of two-dimensional Fluid-Structure Interaction (FSI) tests, including precision test and convergence validation. The results obtained show good agreements with published experimental result as well as referenced numerical results. At last, an extension of aforementioned coupled method to three-dimensional model is performed to investigate characteristics of three-dimensional FSI phenomena. It indicates that partitioned MPS-FEM method is effective and stable for the simulation of free-surface flow interacting with deformable structures.

3-D Numerical Study of a Bottom Ramp Fish Passage Using Smoothed Particle Hydrodynamics

Worldwide, the overwhelming number of man-made barriers in fluvial systems has been identified as one of the major causes of the reported staggering average declines of migratory fish. Fish passages have been shown to help mitigate such problems. Close-to-nature types of fish passages, such as bottom ramps, bypass channels, and fish ramps can be used to minimize the impact of artificial steep drops (e.g., weirs) on the migration of aquatic fauna, especially in cases of low-head barriers. This study focuses on the characterization of the flow pattern in a bottom ramp. A 3-D numerical model based on the meshless smoothed particle hydrodynamics (SPH) method was successfully validated and then employed for the simulation of turbulent free-surface flow in a straight channel with complex geometry. The effects of bed roughness, channel slope, and flow rate were quantified in terms of flow depth, velocity fields, and area‒velocity ratios. During the study, several new tools were developed, leading to new functionalities in pre-processing, solver, and post-processing which increase the applicability of DualSPHysics in the field of eco-hydraulics.

Experimental validation of numerical simulations of free-surface flow within casting mould cavities

PurposeThe paper targets on providing new experimental data for validation of the well-established mathematical models within the framework of the lattice Boltzmann method (LBM), which are applied to problems of casting processes in complex mould cavities.Design/methodology/approachAn experimental campaign aiming at the free-surface flow within a system of narrow channels is designed and executed under well-controlled laboratory conditions. An in-house lattice Boltzmann solver is implemented. Its algorithm is described in detail and its performance is tested thoroughly using both the newly recorded experimental data and well-known analytical benchmark tests.FindingsThe benchmark tests prove the ability of the implemented algorithm to provide a reliable solution when the surface tension effects become dominant. The convergence of the implemented method is assessed. The two new experimentally studied problems are resolved well by simulations using a coarse computational grid.Originality/valueA detailed set of original experimental data for validation of computational schemes for simulations of free-surface gravity-driven flow within a system of narrow channels is presented.

Large-eddy simulation of turbulent free surface flow over a gravel bed

In this paper, a large-eddy simulation study of turbulent free surface flow over a natural rough bed is presented. A three-dimensional multiphase flow model is employed to study the roughness effects on the turbulence properties and free surface dynamics. The governing equations have been discretized using the finite volume method, with the Cartesian cut-cell method being implemented to deal with the precise scanned gravel bed topography and the deformable free surface being captured by a volume of fluid method. The predicted mean flow velocities and turbulence statistics have been compared with experimental data. A close agreement has been obtained between the two sets of results, providing confirmation that this complementary approach to experimental investigations gives further insight into the turbulent free surface flow dynamics over rough beds. It is found that small waves are generated on the free surface due to the roughness effect for the relative low submergence case.

Eulerian–Lagrangian and Eulerian–Eulerian approaches for the simulation of particle-laden free surface flows using the lattice Boltzmann method

This paper studies the Eulerian–Lagrangian and Eulerian–Eulerian approaches for the simulation of interaction between free surface flow and particles. The dynamics of the fluid as well as the transport of the particles in the Eulerian description is solved using the lattice Boltzmann method. To minimize artificial diffusion in particle transport the lattice Boltzmann method with direction-dependent stabilization is proposed. The interaction between the fluid and particles is ensured using the particulate immersed boundary method, where the two-ways coupling algorithm is considered. The developed Eulerian–Lagrangian and Eulerian–Eulerian schemes are compared with each other and validated against results from literature. Both schemes are applied to free surfaces flow problems with complex geometries similar to real gravity casting problems with fillers.

SPH Open Boundary Simulation of Free-Surface Flow Over Ogee-crested Spillway

This study was conducted to compare water surface profiles with standard ogee-crested spillways. Different methods were used, such as (experimental models, numerical models, and design nomographs for the United States Army Corps of Engineers, USACE). In accordance with the USACE specifications, three different models were constructed from rigid foam and then installed in a testing flume. The water surface profile has been recorded for these models with different design heads. For modeling the experimental model configurations, a numerical model based on the smoothed particle hydrodynamics (SPH) technique was used and is developed to simulate the water surface profile of the flow over the ogee-crested spillway. A 2D SPHysics open-source software has been used in this study, using the SPH formulation to model fluid flow, developing the SPH boundary procedure to handle open-boundary simulations, and modifying the open-source SPHysics code for this purpose. The maximum absolute difference between the measured and computed results of the water surface profile for all head ratios of (H/Hd), does not exceed 4.63% at the crest region, the numerical results for the water surface profile showed good agreement with the physical model results. The results obtained experimentally and numerically by SPH are compared with the CFD results in order to be more reassuring from the results. Additional comparisons were made using interpolated data from USACE, Waterways Experiment Station (WES), and design nomographs. The SPH technique is considered very promising and effective for free surface flow applications.

Free surface flow over square bars at different Reynolds numbers

Large-eddy simulations of free surface flow over bed-mounted square bars are performed for laminar, transitional and turbulent flows at constant Froude number. Two different bar spacings are selected corresponding to transitional and k-type (reattaching flow) roughness, respectively. The turbulent flow simulations are validated with experimental data and convincing agreement between simulation and measurement is obtained in terms of water surface elevations and streamwise velocity profiles. The water surface deforms in response to the underlying bed roughness ranging from mild undulation for transitional roughness to distinct standing waves for k-type roughness. The instantaneous water surface deformations increase with an increase in Reynolds number. Contours of the mean streamwise and wall-normal velocities, the total shear stress and the streamfunction reveal the presence and extension of recirculation zones in the trough between two consecutive bars. The flow is governed by strong local velocity gradients as a result of the rough bed and the deformed water surface. The local Froude number at the free surface increases for low Reynolds number in the flow over transitional roughness and decreases for low Reynolds number in the flow over k-type roughness. The transitional and turbulent flows exhibit a very similar distribution of the pressure coefficient Cp in both cases, whilst Cp is generally lower for the laminar flow. Regarding the friction coefficient, Cf, it is significantly lower in the turbulent case than in the transitional and laminar cases. The bar spacing does not affect significantly the relative contribution of friction and pressure forces to the total force, neither does the Reynolds number. The friction factor is greater for transitional roughness and decreases with increasing Reynolds number.

Large-scale flow simulations using lattice Boltzmann method with AMR following free-surface on multiple GPUs

Free-surface flow simulations require high-resolution grids to capture phenomena at the interface as well as a long computational time. In this paper, we propose a numerical method for realizing large-scale free-surface flow simulations using the lattice Boltzmann method and multiple GPUs. By introducing the adaptive mesh refinement (AMR) method, which adapts high-resolution grids to free surfaces, to the lattice Boltzmann method, the number of lattice points can be greatly reduced. In the calculation of the AMR method, the spatial distribution of a computational load changes with time; therefore, the number of lattice points assigned to each GPU is kept equal by dynamic domain partitioning using a space-filling curve. We measured the weak scalability of our AMR code on the TSUBAME3.0 supercomputer at the Tokyo Institute of Technology. By hiding GPU–GPU communication overheads by the overlapping method, the performance increased 1.29 times that of the naïve implementation, and we achieved the fairly high performance of 14,570 MLUPS using 256 GPUs. We demonstrate large-scale simulations for the dam breaking problem and show a reduction in computational cost with the AMR method.

Experimental and numerical simulation of free surface flow over an obstacle on a sloped channel

In the present study, both experimental and numerical were conducted on a free surface flow over an obstacle. Numerical simulations were performed using the Renormalization Group (RNG-k-ɛ) based Reynolds-Averaged Navier–Stokes (RANS) turbulence model coupled with the Volume OF Fluid (VOF) method in FLUENT Software to investigate the effect of the channel slope on the flow pattern upstream, above and downstream the obstacle. Respectively, 5%, 7%, 8%, 10%, 20% and 50% channel slopes were considered. Numerical simulation has showed a good agreement compared against experimental results. Effect of the slope on the flow is observed particularly upstream of the obstacle where the flow takes the vertical direction after hitting the upstream wall. The more the slope becomes steeper, the higher the level of the water is. Recirculation zones in the case of a horizontal channel are elongated downstream the weir, whereas in the case of a sloped channel, they are localized just at the foot of the downstream wall.

Numerical analysis of the nonlinear free surface flow around an advancing ship using moving particle semi-implicit method

This paper presents a methodology to simulate the nonlinear free surface flow around an advancing ship. This methodology is based on the Moving Particle Semi-implicit (MPS) method, which is widely used to simulate the incompressible fluid flow. In this paper, the original MPS method is modified in order to analyze effectively the nonlinear free surface flow around an advancing ship. Several treatment techniques are newly proposed, and the original MPS method is modified by combining these techniques with other previous research studies. The proposed techniques include the Laplacian compensation model in the region near the body boundary, the collision model between fluid particles and wall particles, and some treatments for the opened boundary. The accuracy of the modified MPS method is verified through the numerical simulations on two benchmark problems for the performance evaluation of the particle methods and on the flows around both the wedged body and model ship. The numerical results demonstrate that the modified MPS method can be effectively applied to the simulation of nonlinear free surface flow around an advancing ship in marine engineering.

A coupled level set/volume of fluid method for simulation of two-phase flow on unstructured grids

We present a fully coupled level set and volume of fluid method for free surface flow simulations on unstructured grids in two- and three-dimensions. The evolution of the level set function and the volume fraction are updated at each time step. The level set advection equation is solved by a least squares weighted residual method while the volume fraction advection is solved using an unsplit Eulerian-Lagrangian scheme. The reconstruction of the interface in the volume of fluid method is performed using the volume fraction information together with the normal vector obtained from the level set function. The reconstructed interface is then used to reinitialize the level set function. Thanks to the fully coupling of the two methods, the interface reconstruction is carried out efficiently and the mass conservation is preserved exactly. The proposed method is validated against several benchmarks and is coupled with the incompressible Navier-Stokes solver to solve two-phase flow problems. Numerical results show that the method is capable of resolving complex interface changes efficiently and accurately.

A Backwards-Tracking Lagrangian-Eulerian Method for Viscoelastic Two-Fluid Flows

A new Lagrangian–Eulerian method for the simulation of viscoelastic free surface flow is proposed. The approach is developed from a method in which the constitutive equation for viscoelastic stress is solved at Lagrangian nodes, which are convected by the flow, and interpolated to the Eulerian grid with radial basis functions. In the new method, a backwards-tracking methodology is employed, allowing for fixed locations for the Lagrangian nodes to be chosen a priori. The proposed method is also extended to the simulation of viscoelastic free surface flow with the volume of fluid method. No unstructured interpolation or node redistribution is required with the new approach. Furthermore, the total amount of Lagrangian nodes is significantly reduced when compared to the original Lagrangian–Eulerian method. Consequently, the method is more computationally efficient and robust. No additional stabilization technique, such as both-sides diffusion or reformulation of the constitutive equation, is necessary. A validation is performed with the analytic solution for transient and steady planar Poiseuille flow, with excellent results. Furthermore, the proposed method agrees well with numerical data from the literature for the viscoelastic die swell flow of an Oldroyd-B model. The capabilities to simulate viscoelastic free surface flow are also demonstrated through the simulation of a jet buckling case.

Simulation of complex free surface flows

We study the Serre-Green-Naghdi system under a non-hydrostatic formulation, modelling incompressible free surface flows in shallow water regimes. This system, unlike the well-known (nonlinear) Saint-Venant equations, takes into account the effects of the non-hydrostatic pressure term as well as dispersive phenomena. Two numerical schemes are designed, based on a finite volume - finite difference type splitting scheme and iterative correction algorithms. The methods are compared by means of simulations concerning the propagation of solitary wave solutions. The model is also assessed with experimental data concerning the Favre secondary wave experiments [12].

ANALYSIS OF FREE SURFACE FLOW IN PELTON TURBINE USING LATTICE BOLTZMANN METHOD

Computational fluid dynamics (CFD) has become an indispensable tool in the development and optimization of various machines and devices in the last decade. In most cases, researchers deal with single-phase flows and internal flows where all externally wetted walls are defined. In some cases, it is also necessary to analyze multiphase flow and free-surface flow. Using classical numerical methods such as the finite element method (FEM) or the finite volume method (FVM), the results are highly dependent on the size and quality of the computational grids. Particular attention should be paid to the conditions at the boundary between the individual phases of the fluid. In most cases, it is necessary to consider non-stationary conditions and the necessity of using non-stationary numerical methods. All the above properties of multiphase flows and their consequences in performing numerical analyses lead us to very long computational times and consequently the use of very powerful computers. Industry often needs results in a relatively fast time therefore many simplifications need to be considered. Recently a new approach for numerical simulations of multiphase and free-surface flows based on the Lattice Boltzmann method (LBM) started to be used. The method is very useful especially for development of small-scale hydraulic turbines, where the expense for development process is usually very limited. In this paper, some examples of the use of LBM in the calculation of free-surface flows and compare the results using classical CFD methods have been presented.

ECONOMY AND POWER TUNING AN IC ENGINE FOR LOWER CO2 FOOTPRINT

Computational fluid dynamics (CFD) has become an indispensable tool in the development and optimization of various machines and devices in the last decade. In most cases, researchers deal with single-phase flows and internal flows where all externally wetted walls are defined. In some cases, it is also necessary to analyze multiphase flow and free-surface flow. Using classical numerical methods such as the finite element method (FEM) or the finite volume method (FVM), the results are highly dependent on the size and quality of the computational grids. Particular attention should be paid to the conditions at the boundary between the individual phases of the fluid. In most cases, it is necessary to consider non-stationary conditions and the necessity of using non-stationary numerical methods. All the above properties of multiphase flows and their consequences in performing numerical analyses lead us to very long computational times and consequently the use of very powerful computers. Industry often needs results in a relatively fast time therefore many simplifications need to be considered. Recently a new approach for numerical simulations of multiphase and free-surface flows based on the Lattice Boltzmann method (LBM) started to be used. The method is very useful especially for development of small-scale hydraulic turbines, where the expense for development process is usually very limited. In this paper, some examples of the use of LBM in the calculation of free-surface flows and compare the results using classical CFD methods have been presented.

Numerical Simulation of Free Surface Flow on Spillways and Channel Chutes with Wall and Step Abutments by Coupling Turbulence and Air Entrainment Models

Spillways and channel chutes are widely used in hydraulic works. Two kinds of abutment—walls and steps—are usually constructed to dissipate energy; however, they may also cause cavitation at the abutment position. In this study, we used Flow 3D with the Reynolds-averaged Navier–Stokes (RANS) and large eddy simulation (LES) turbulent models which included air entrainment to simulate the free surface flow through the spillway, channel chute and stilling basin of the Ngan Truoi construction to optimize the configuration of walls and dams. We measured the water level, velocity and pressure to estimate the influence of grid size and the turbulent model type used. Our results highlight the need to include air entrainment in the model simulating rapid flow over a hydraulic construction. With adjustments for energy loss, this study shows that walls provide the best results and the optimal distance between two walls is 2.8 m.

Verification of the LOGOS Software Package for Tsunami Simulations

Verification results for the LOGOS software package as applied to numerical simulations of tsunami waves are reported. The module of the LOGOS software package that is used for tsunami simulations is based on the numerical solution of three-dimensional Navier–Stokes equations. The verification included two steps. The first step involved the verification of LOGOS free-surface flow simulations on the test cases of a collapsing water column and gravity water sloshing in a tank and the known test cases of wave generation by objects falling into water or lifted out of it. The verification of LOGOS specifically for tsunami simulations was performed using a reference set of international benchmarks including the propagation and run-up of a single wave onto a flat slope and a vertical wall, the sliding of a wedge-shaped body down a slope, flow around an island and wave run-up over an obstacle. The results of the verification simulations demonstrate that LOGOS provides sufficient accuracy in numerical simulations of tsunami waves, namely, their generation, propagation and run-up.