Free Surface Treatment Research Articles

CFD modeling of flow and local scour around submerged bridge decks

In this study, the level set method-based, multiphase hydro- and morphodynamic numerical model REEF3D is used for the simulation of the flow conditions and local scouring around submerged bridge decks. Presence of hydraulic jumps, bridge overtopping, pressurized jets and the resulting scouring make the selected test cases especially challenging from the numerical modelling point of view. The models are validated against data from laboratory experiments. The influence of the submergence ratio on the prevailing flow and scouring is investigated. The level set method showed robustness and good accuracy for the treatment of the complex free surface as well as for the tracking of the mobile bed. The submergence ratio showed no clear correlation with the prevailing erosion/deposition patterns. As the simulations offered insights into the prevailing hydrodynamics and thus the relevance of numerical modelling was emphasized for such complex sediment transport problems.

Grain-scale investigation of swash zone sediment transport on a gravel beach using DEM-MPS coupled scheme

ABSTRACT Sediment transport in the swash zone directly affects beach changes such as shoreline recession; thus, detailed understandings of sediment transport mechanisms are necessary to accurately estimate the short-time scales sediment transport rate. However, these detailed mechanisms under runup waves have not been elucidated because of the complex solid-gas-liquid multiphase turbulence flow. In this study, we attempt to numerically investigate the sediment grain-scale mechanism to overcome the shortcomings of experimental measurements and the free surface treatment in many numerical simulations. The gravel transport process on a sloped beach under regular waves was simulated using a 2D coupled model of the discrete element method (DEM) and a modified moving particle semi-implicit (MPS) method; a sub-model was built into the DEM-MPS model to improve fluid volume conservation. After validating the simulated performance by comparing it to a previous experiment, the gravel motions were investigated for turbulence and inner beach structure. The Shields number, estimated using the drag force distribution, revealed that significant turbulence contributed to onshore gravel transport near the rundown limit. The inter-gravel contact structure inside the beach explained the decrease in offshore sediment transport during backwash as increased resistance to gravel motions resulting from beach compaction.

Integrative Study on Flow Characteristics and Impact of Non-Submerged Double Spur Dikes on the River System.

The flow characteristics around non-submerged spur dikes continuously placed in the channel on the same side with orthogonal angle to the wall were investigated by numerical simulations and experimental measurements. Three-dimensional (3D) numerical simulations with the standard k-ε Model for incompressible viscous flow based on finite volume method and the rigid lid assumption for free surface treatment were conducted. A laboratory experiment was applied to verify the numerical simulation. The experimental data indicated that the developed mathematical model can effectively predict 3D flow around non-submerged double spur dikes (NDSDs). The flow structure and turbulent characteristics around them were analyzed and it was found that a distinct cumulative effect of turbulence exists between the dikes. By examining the interaction rules of NDSDs, the judgment criterion of spacing threshold was generalized as whether velocity distributions at the cross-sections of NDSDs along the main flow approximately coincided or not. It can be used to investigate the impact scale of the spur dike groups on the straight and prismatic channels and it is of great significance for artificial scientific river improvement and the assessment of river system health under human activities.

Prediction of Unstable Hydrodynamic Forces on Submerged Structures under the Water Surface Using a Data-Driven Modeling Approach

Catastrophic failures of partially or fully submerged structures, e.g., offshore platforms, hydrokinetic turbine blades, bridge decks, etc., due to the dynamic impact of free surface flows such as waves or floods have revealed the need to evaluate their reliability. In this respect, an accurate estimation of hydrodynamic forces and their relationship to instability in structures is required. The computational fluid dynamics (CFD) solver is known as a powerful tool to identify dynamic characteristics of flow; however, it commonly consumes a huge computational cost, especially in cases of re-simulations needed. In this paper, an efficient surrogate model based on the Gaussian process is developed to rapidly predict the nonlinear hydrodynamic pressure coefficients on submerged bodies near the water surface. For this purpose, a CFD model is first developed, which is based on a two-dimensional incompressible Navier–Stokes solver incorporating free surface treatment and turbulent flow models. Then, an experimental design is adopted to generate initial training samples considering the effect of the submerged body shape ratio and flow Re number. Surrogate models of hydrodynamic pressure coefficients and their instability based on Gaussian process modeling are established using the outcome from the CFD simulations, where optimal trend and correlation functions are also investigated. Once surrogate models are obtained, the mean and oscillation amplitudes of hydrodynamic pressure coefficients on a submerged rectangular body, which represents the shape of most civil structures, can be rapidly predicted without the attempt at re-simulation. The findings can be practically applied in rapidly assessing hydrodynamic forces and their instability of existing submerged civil structures or in designing new structures, where a suitable shape ratio should be adopted to avoid flow-induced instability of hydrodynamic forces.

Mesh-free simulations of injection molding processes

In this paper, we introduce a mesh-free numerical framework using the finite pointset method for the modeling and simulation of injection molding processes. When compared to well-established mesh-based methods, which have been widely applied for these applications, our approach avoids the need for extensive preprocessing and enables accurate treatment of free surfaces and other associated phenomena. To accurately model the polymer injections, we consider a detailed material model, with temperature dependent viscosity and density, while also considering shear thinning behavior with a strain rate dependent viscosity. Our numerical investigations show that injection molding-specific problems such as the modeling of viscous flows and the fountain flow effect can be successfully implemented using our presented framework. For a thorough validation of our proposed model, we compare the simulated flow behavior with injection molding experiments, which are also performed in this work. The experimental setup considers the injection of a polymer melt into a spiral mold. The flow behavior is investigated experimentally at varying melt injection and wall temperature, with different threshold pressures. Our numerical simulations show a good comparison with these experimental results, both qualitatively and quantitatively. We also introduce a correction mechanism to ensure energy conservation, which has often been challenging in mesh-free approaches. This is the first time that the flow behavior in a mesh-free injection molding method has been experimentally validated and successfully applied to the simulation of an actual industrial vehicle component.

Stabilized LSMPS method for complex free-surface flow simulation

In this study, a new quasi-Lagrangian particle method is developed aiming at accurate and stable numerical simulation of complex free-surface flows. The main novelties of this development are the free-surface treatment using surface fitting and the new discretization schemes. By using linear surface fitting, the normal vector and the signed distance to the free surface are accurately and stably obtained. The existing consistent spatial discretization scheme, namely, the LSMPS scheme, is extended by adopting regularized least squares, diffuse derivative constraints, and constrained least squares. Based on this scheme, new formulations are proposed particularly for the pressure Poisson equation and the pressure gradient operator. By using a Taylor–Green vortex problem, 2nd-order accuracy in space is confirmed for the 2nd-order discretization scheme. Furthermore, highly accurate solutions are obtained for various test cases, including free-surface deformations of moderate to high complexity. Thus, the proposed method is indicated to have high accuracy and stability for complex free-surface flow simulations.

Propulsion simulation on fully appended ship model

This study presents the numerical investigation for the flow around the propeller of the ONR Tumblehome combatant in open water and for the flow around the same ship in the case of self-propulsion with actuator disk method. Computational Fluid Dynamics based on RANS-VOF solver have been used in order to analyse the flow. The free surface treatment is multi-phase flow approach, incompressible and nonmiscible flow phases are modelled through the use of conservation equations for each volume fraction of phase. Accuracy involves close attention to the physical modelling, particularly the effects of turbulence, as well as the numerical discretization.

A review of multi-scale and multi-physics simulations of metal additive manufacturing processes with focus on modeling strategies

Numerical simulations have recently shown their potential as a robust, cheap and reliable tool for predicting the quality of components produced by metal additive manufacturing (MAM) processes. Despite the advantages of the MAM processes over conventional manufacturing methods, there is still a lack of thorough understanding on how different defects can form and originate during MAM processing. In that respect, advanced numerical techniques recently developed have the ability to predict the occurrence of such defects. These techniques have paved the way to efficiently obtain the optimal processing window for targeted mechanical properties to satisfy the end-use design requirements. The aim of this review paper is hence to present and classify numerical simulations of MAM, not solely based on their length-scale as often seen, but also based on the involved physics, as well as the modeling strategies at both the meso-scale and part-scale. The paper is arranged in the following way: First, literature describing purely conduction-based heat transfer simulations at meso-scale are presented. This is followed by a review of fluid-based simulations of increasing complexity based on the treatment of free surface of the melt pool at meso-scale. Finally, contributions based on different part-scale modeling approaches with a focus on thermo-mechanical behavior are reviewed.

Digital Design of Batch Cooling Crystallization Processes: Computational Fluid Dynamics Methodology for Modeling Free-Surface Hydrodynamics in Agitated Crystallizers

A framework for the digital design of batch cooling crystallization processes is presented comprising three stages, which are based on different levels of process complexity, integrating crystallizer hydrodynamics with crystallization kinetics and consequently with expected crystal size distribution. In the first stage of the framework, a computational fluid dynamics methodology is developed to accurately assess hydrodynamics in a typical batch crystallizer configuration, comprising a 20 L scale dish-bottom vessel with a single beavertail baffle agitated by a retreat curve impeller, used in the pharmaceutical as well as in the fine chemical industries. The hydrodynamics of crystallizers with such configurations is characterized by vortex formation on the free liquid surface. It is therefore important to model the free surface using the Volume-of-Fluid (VoF) method. Comparison of the predicted mean velocity components with experimental measurements using laser Doppler anemometry reveals that improved predictions are obtained using a differential Reynolds-stress transport model for turbulence coupled with the VoF for modeling the gas-liquid interface compared with those using the Shear-stress transport model and with a flat liquid surface. This study demonstrates that an accurate treatment of the liquid free surface for capturing vortex formation is essential for reliable predictions of the crystallizer’s flow field. While the vortex depth is predicted to increase with increasing impeller Reynolds number, the dependence of hydrodynamic macroparameters, including power number, impeller flow number, and secondary circulation flow number, on Reynolds number reveals that they are essentially constant within the turbulent regime but fluctuate when the flow is in the transitional and laminar regimes as fluid viscosity increases.

The particle finite element method for transient granular material flow: modelling and validation

The prediction of transient granular material flow is of fundamental industrial importance. The potential of using numerical methods in system design for increasing the operating efficiency of industrial processes involving granular material flow is huge. In the present study, a numerical tool for modelling dense transient granular material flow is presented and validated against experiments. The granular materials are modelled as continuous materials using two different constitutive models. The choice of constitutive models is made with the aim to predict the mechanical behaviour of a granular material during the transition from stationary to flowing and back to stationary state. The particle finite element method (PFEM) is employed as a numerical tool to simulate the transient granular material flow. Use of the PFEM enables a robust treatment of large deformations and free surfaces. The fundamental problem of collapsing rectangular columns of granular material is studied experimentally employing a novel approach for in-plane velocity measurements by digital image correlation. The proposed numerical model is used to simulate the experimentally studied column collapses. The model prediction of the in-plane velocity field during the collapse agrees well with experiments.

Numerical Analysis of Full-Scale Ship Self-Propulsion Performance with Direct Comparison to Statistical Sea Trail Results

Accurate prediction of the self-propulsion performance is one of the most important factors for the energy-efficient design of a ship. In general, the hydrodynamic performance of a full-scale ship could be achieved by model-scale simulation or towing tank tests with extrapolations. With the development of CFD methods and computing power, directly predict ship performance with full-scale CFD is an important approach. In this article, a numerical study on the full-scale self-propulsion performance with propeller operating behind ship at model- and full-scale is presented. The study includes numerical simulations using the RANS method with a double-model and VOF (Volume-of-Fluid) model respectively and scale effect analysis based on overall performance, local flow fields and detailed vortex identification. The verification study on grid convergence is also performed for full-scale simulation with global and local mesh refinements. A series of sea trail tests were performed to collect reliable data for the validation of CFD predictions. The analysis of scale effect on hull-propeller interaction shows that the difference of hull boundary layer and flow separation is the main source of scale effect on ship wake. The results of the fluctuations of propeller thrust and torque along with circulation distribution and local flow field show that the propeller’s loading is significantly higher for model-scale ship. It is suggested that the difference of vortex evolution and interaction is more pronounced and has larger effects on the ship’s powering performance at model-scale than full-scale according to the simulation results. From the study on self-propulsion prediction, it could be concluded that the simplification on free surface treatment does not only affect the wave-making resistance for bare hull but also the propeller performance and propeller induced ship resistance which can be produced up to 5% uncertainty to the power prediction. Roughness is another important factor in full-scale simulation because it has up to an approximately 7% effect on the delivery power. As a result of the validation study, the numerical simulations of full-scale ship self-propulsion shows good agreement with the sea trail data especially for cases that have considered both roughness and free surface effects. This result will largely enhance our confidence to apply full-scale simulation in the prediction of ship’s self-propulsion performance in the future ship designs.

Improvement of Pressure Calculations in the Moving Particle Semi-Implicit Method for Free-Surface Flows

In this paper, numerical improvements are implemented for solving for the pressure in the moving particle semi-implicit (MPS) method for free-surface flow simulations. The tensile instability problem is solved using a dynamic stabilization (DS) algorithm. The low numerical diffusion of this algorithm is shown through numerical tests. A free-surface treatment that includes an accurate free-surface particle detection algorithm and the implicit application of a free-surface boundary condition is used. The solution of the Navier–Stokes equation is improved using a particle shifting (PS) algorithm. The proposed MPS method for free-surface flow simulations is successfully applied in several benchmark tests and two- and three-dimensional dam break problems. The numerical simulation results agree well with the analytical and empirical ones. It is shown that the proposed MPS method effectively improves the stability and accuracy of simulations of free-surface flows.

Improving the hydrodynamics performance of the catamaran passenger

Interest in medium and high-speed marine transportation has acquired a wide variety of research on catamaran hydrodynamics. The demand for high-speed small ship has increased in last period, especially for the passenger ferry boat. The paper considers the assessment of the free surface flow around the hull of the catamaran passenger ship in order to improve its hydrodynamics performance. To accomplish the objective, Computational Fluid Dynamics based on RANS-VOF solver have been used. The free surface treatment is multi-phase flow approach, incompressible and non-miscible flow phases are modelled through the use of conservation equations for each volume fraction of phase. To evaluate the effect of the central bulb positioned between the hull bodies on the ship resistance, nine different sets simulations were carried out for eleven speeds corresponding to Froude numbers between 0.12 and 0.80. Nine different configurations of the bulb considering three different depth and three position along the hull (bow, midship and stern) have been considered. The calculations revealed that wave resistance can be reduced by using a central bulb between the catamaran hulls by up to 8%. This happens when the bulb is positioned at the bow and the stern and for Froude numbers 0.3-0.6.

Free‐surface implementation in a mesh‐free finite‐difference method for elastic wave propagation in the frequency domain

ABSTRACTWe present an original implementation of the free‐surface boundary condition in a mesh‐free finite‐difference method for simulating elastic wave propagation in the frequency domain. For elastic wave modelling in the frequency domain, the treatment of free surfaces is a key issue which requires special consideration. In the present study, the free‐surface boundary condition is directly implemented at node positions located on the free‐surface. Flexible nature of the mesh‐free method for nodal distribution enables us to introduce topography into numerical models in an efficient manner. We investigate the accuracy of the proposed implementation by comparing numerical results with an analytical solution. The results show that the proposed method can calculate surface wave propagation even for an inclined free surface with substantial accuracy. Next, we calculate surface wave propagation in a model with a topographic surface using our method, and compare the numerical result with that using the finite‐element method. The comparison shows the excellent agreement with each other. Finally, we apply our method to the SEG foothill model to investigate the effectiveness of the proposed method. Since the mesh‐free method has high flexibility of nodal distribution, the proposed implementation would deal with models of topographic surface with sufficient accuracy and efficiency.

A conservative and consistent Lagrangian gradient smoothing method for simulating free surface flows in hydrodynamics

A novel particle method, Lagrangian gradient smoothing method (L-GSM), has been proposed in our earlier work to avoid the tensile instability problem inherently existed in SPH, through replacing the SPH gradient operator with a robust GSM gradient operator. However, the nominal area of each L-GSM particle determined by the relative location of particles is always inconsistent with the real representative area of it in simulation, especially in large-deformation problems. This is why the earlier L-GSM model has to be limited to the solid-like flow simulations where the deformation is not very serious. In this work, a conservative and consistent Lagrangian gradient smoothing method (CCL-GSM) is developed for handling large-deformation problems in hydrodynamics with an arbitrarily changing free surface profile. This is achieved by deriving a conservative and consistent form for the discretized Navier–Stokes governing equations in L-GSM, which even holds in the neighbor-updating or ‘remeshing’ process. Special techniques are also devised for free surface treatment, which is important to restore the conservation and consistency manner of CCL-GSM simulation on free surface boundary. The effectiveness of the proposed CCL-GSM framework is evaluated with a number of benchmarking examples, including dam break, wall impacts of breaking dam, water discharge and water splash. It shows that the CCL-GSM model can handle the incompressible flows with complicated free surfaces effectively and easily. The results comparison with experiments and SPH solutions demonstrates that the CCL-GSM can give a desirable result for all these examples.

Improvements in highly viscous fluid simulation using a fully implicit SPH method

This study describes the application of two main improvements in highly viscous fluid simulations using the smoothed particle hydrodynamics (SPH) method: an implicit time integration scheme to overcome the problem of impractically small time step restriction and the introduction of air ghost particles to fix problems regarding the free surface treatment. This study adopts the incompressible SPH as a basis for the implementation of these improvements, which guarantees a stable and accurate pressure distribution. We verified the proposed implicit time integration scheme with simulations of pipe flow and the free surface treatment with a simple hydrostatic problem. As a result, the free surface of the hydrostatic problem became very smooth and stable. In addition, we conducted a variety of dam-break simulations to validate this proposed SPH method, as well as to analyze the density and divergence error. Finally, we demonstrate the potential of this method with the highly viscous vertical jet flow over a horizontal plate test, which features a complex viscous coiling behavior.

Flat vs. curved rigid-lid LES computations of an open-channel confluence

Abstract This paper describes the application of four Large Eddy Simulations (LES) to an open-channel confluence flow, making use of a frictionless rigid-lid to treat the free-surface. Three simulations are conducted with a flat rigid-lid, at different elevations. A fourth simulation is carried out with a curved rigid-lid which is a closer approximation to the real free-surface of the flow. The curved rigid-lid is obtained from the time-averaged pressure field on the flat rigid-lid from one of the initial three simulations. The aim is to investigate the limitations of the free-surface treatment by means of a rigid-lid in the simulation of an asymmetric confluence, showing the differences that both approaches produce in terms of mean flow, secondary flow and turbulence. After validation with experimental data, the predictions are used to understand the differences between adopting a flat and a curved rigid-lid onto the confluence hydrodynamics. For the present flow case, although it was characterized by a moderately low downstream Froude number (Fr ≈ 0.37), it was found that an oversimplification of the numerical treatment of the free-surface leads to a decreased accuracy of the predictions of the secondary flow and turbulent kinetic energy.

Dimension reduction in heterogeneous parametric spaces with application to naval engineering shape design problems

We present the results of the first application in the naval architecture field of a methodology based on active subspaces properties for parameter space reduction. The physical problem considered is the one of the simulation of the hydrodynamic flow past the hull of a ship advancing in calm water. Such problem is extremely relevant at the preliminary stages of the ship design, when several flow simulations are typically carried out by the engineers to assess the dependence of the hull total resistance on the geometrical parameters of the hull, and others related with flows and hull properties. Given the high number of geometric and physical parameters which might affect the total ship drag, the main idea of this work is to employ the active subspaces properties to identify possible lower dimensional structures in the parameter space. Thus, a fully automated procedure has been implemented to produce several small shape perturbations of an original hull CAD geometry, in order to exploit the resulting shapes and to run high fidelity flow simulations with different structural and physical parameters as well, and then collect data for the active subspaces analysis. The free form deformation procedure used to morph the hull shapes, the high fidelity solver based on potential flow theory with fully nonlinear free surface treatment, and the active subspaces analysis tool employed in this work have all been developed and integrated within SISSA mathLab as open source tools. The contribution will also discuss several details of the implementation of such tools, as well as the results of their application to the selected target engineering problem.

Transient Simulation of Cooling-Lubricant Flow for Deep-Hole Drilling-Processes

Today, the process of single-lip deep-hole drilling becomes more and more important for many industrial applications, e.g. in aerospace engineering, automotive industry or for medical products. The cooling lubricant is not only most important for the quality of the resulting bore, but also for the evacuation of chips and for increasing the tool life. For that reason, a coupled particle approach for the simulation of cooling lubricant in the context of deep-hole drilling was developed. The cooling lubricant in the presented simulations is described by the Smoothed Particle Hydrodynamics method and the drill chips are described by Discrete Element multispheres. From the performed simulations, one can see that the coupled particle methods together with meshbased Computational Fluid Dynamics simulations are suitable for the description of the flow field and the chip evacuation. Furthermore, the simulations of the cooling lubricant for a single-lip drill show a velocity sink behind the cutting edge at the tip of the drilling tool. This result indicates great potential for the optimization of the tool geometry, presumably leading to an increased tool life as well as an improved cooling behaviour. Considering inlet pressures above 20 bar, and a drill diameter of 2 mm, the presented systems are challenging, with particle velocities above 100 m/s. However, the results of the performed simulations show a great potential for the application of coupled particle methods and meshbased Computational Fluid Dynamics for the investigation of the coolant flow. It can be seen that the Smoothed Particle Hydrodynamics method is an interesting alternative compared to meshbased methods, due to its property to describe the transient evolution of the cooling lubricant without any special treatment of free surfaces and interfaces that experience great topological changes. Especially for simulations which do not assume the evacuation channel to be entirely filled with cooling lubricant, the meshless nature of particle methods is beneficial.

On modification of pressure gradient operator in integrated ISPH for multifluid and porous media flow with free-surface

In Incompressible Smoothed Particle Hydrodynamics (ISPH) simulation, choice of pressure gradient operator plays an important role. Variations in effective porosity, fluid density, and free-surface conditions dictate the nature of the formulation. This study proposes an integrated ISPH framework with an implicit free-surface treatment. Pressure variation at the multi-fluid interface is maintained using a modified density-weighted pressure gradient with linear momentum conservation. Different pressure gradients with diffused interfaces for a porous domain as well as multifluid interface have been compared with the proposed operator. A unified form of Brinkman and Navier-Stokes equations are utilized to describe the flow-physics inside and outside the porous domain. Density variation in fluids is modelled by solving the scalar-transport equation. Effect of the porous domain is incorporated in terms of varying representative volume of the fluid particles. Porous media interface conditions are implicitly implemented using Darcy velocity and by introducing porosity into Pressure Poisson Equation (PPE). The present model is capable of minimizing error in velocity-divergence due to implicit free-surface treatment combined with linear momentum conservation. Proposed framework is validated by using existing experimental data of density-dependent flow with very low-density ratio and flow through porous blocks. A result of density-current passing through porous domain demonstrates the capability of the developed model for complex scenarios.