Strategies for simulation software quality assurance applied to open source DEM

We present a strategy to improve the software quality for scientific simulation software, applied to the open source DEM code LIGGGHTS [1] [2]. We aim to improve the quality of the LIGGGHTS DEM code by two measures: Firstly, making the simulation code open source gives the whole user community the possibility to detect bugs in the source code and make suggestions to improve the code quality. Secondly, we apply a test harness, which is an important part of the work-flow for quality assurance in software engineering [5]. In the case of scientific simulation software, it consists of a set of simulation examples that should span the range of applicability of the software as good as possible. Technically, in our case it consists of a set of 10-50 LIGGGHTS simulations and is being run automatically on our cluster, where the number of processors, the code features and the numerical models are varied. Qualitative results are automatically extracted and are plotted for comparison, so thus a huge parameter space of flow regimes, numerical models, code features and parallelization situations can be governed. A test harness can aid in (a) finding bugs in the software, (b) checking parallel efficiency and consistency, (c) comparing different numerical models, and, most importantly, (d) experimental validation. Parallel consistency means that within a parallel framework, we need to have the possibility to compare the answers that a run with a different number of processors gives and the time that it takes to compute them. Experimental validation is especially important for scientific simulations. If experimental data is available for a test case, the experimental data is automatically compared to the numerical results, by means of global quantities such number of particles in the simulation, translational and rotational kinetic energy, thermal energy etc. The LIGGGHTS test harness aims to be a transparent and open community effort that everybody can contribute to in order to improve the quality of the LIGGGHTS code. We illustrate the usefulness of the test harness with several examples, where we especially focus on experimental validation

A 3D-1D model for the simulation of plant-scale chemical reactors

A 3D-1D model has been developed to simulate the methane dehydroaromatization (MDA) process in plant-scale catalytic reactors. The 3D part of the model consists of CFD-DEM coupled simulations of some relevant volume elements (RVEs), while the 1D part is a low-order model bridging the solution between the RVEs. The CFD-DEM model, implemented in the CFDEM®coupling software, uses an immerse boundary method to resolve: 1) the flow around the catalytic structures, 2) the heat exchange between solid and fluid, 3) the MDA reaction at the fluid-catalyst interface. The CFD-DEM solution is scaled-up by the 1D model to allow the simulation of industrial-scale processes at acceptable computational cost. The effect of design parameters (e.g., catalyst geometry) and operating conditions (e.g., reactor operating temperature) on the methane conversion rate and pressure drop can be investigated using the proposed model and the main results will be presented

A 3D-1D model for the simulation of plant-scale chemical reactors

A 3D-1D model has been developed to simulate the methane dehydroaromatization (MDA) process in plant-scale catalytic reactors. The 3D part of the model consists of CFD-DEM coupled simulations of some relevant volume elements (RVEs), while the 1D part is a low-order model bridging the solution between the RVEs. The CFD-DEM model, implemented in the CFDEM®coupling software, uses an immerse boundary method to resolve: 1) the flow around the catalytic structures, 2) the heat exchange between solid and fluid, 3) the MDA reaction at the fluid-catalyst interface. The CFD-DEM solution is scaled-up by the 1D model to allow the simulation of industrial-scale processes at acceptable computational cost. The effect of design parameters (e.g., catalyst geometry) and operating conditions (e.g., reactor operating temperature) on the methane conversion rate and pressure drop can be investigated using the proposed model and the main results will be presented

Development of an unresolved CFD–DEM model for the flow of viscous suspensions and its application to solid–liquid mixing

Although viscous solid–liquid mixing plays a key role in the industry, the vast majority of the literature on the mixing of suspensions is centered around the turbulent regime of operation. However, the laminar and transitional regimes face considerable challenges. In particular, it is important to know the minimum impeller speed () that guarantees the suspension of all particles. In addition, local information on the flow patterns is necessary to evaluate the quality of mixing and identify the presence of dead zones. Multiphase computational fluid dynamics (CFD) is a powerful tool that can be used to gain insight into local and macroscopic properties of mixing processes. Among the variety of numerical models available in the literature, which are reviewed in this work, unresolved CFD–DEM, which combines CFD for the fluid phase with the discrete element method (DEM) for the solid particles, is an interesting approach due to its accurate prediction of the granular dynamics and its capability to simulate large amounts of particles. In this work, the unresolved CFD–DEM method is extended to viscous solid–liquid flows. Different solid–liquid momentum coupling strategies, along with their stability criteria, are investigated and their accuracies are compared. Furthermore, it is shown that an additional sub-grid viscosity model is necessary to ensure the correct rheology of the suspensions. The proposed model is used to study solid–liquid mixing in a stirred tank equipped with a pitched blade turbine. It is validated qualitatively by comparing the particle distribution against experimental observations, and quantitatively by compairing the fraction of suspended solids with results obtained via the pressure gauge technique

A semi-implicit immersed boundary method and its application to viscous mixing

Computational fluid dynamics (CFD) simulations in the context of single-phase mixing remain challenging notably due the presence of a complex rotating geometry within the domain. In this work, we develop a parallel semi-implicit immersed boundary method based on Open∇FOAM, which is applicable to unstructured meshes. This method is first verified on academic test cases before it is applied to single phase mixing. It is then applied to baffled and unbaffled stirred tanks equipped with a pitched blade impeller. The results obtained are compared to experimental data and those predicted with the single rotating frame and sliding mesh techniques. The proposed method is found to be of comparable accuracy in predicting the flow patterns and the torque values while being straightforwardly applicable to complex systems with multiples impellers for which the swept volumes overlap

Strategies for simulation software quality assurance applied to open source DEM

We present a strategy to improve the software quality for scientific simulation software, applied to the open source DEM code LIGGGHTS [1] [2]. We aim to improve the quality of the LIGGGHTS DEM code by two measures: Firstly, making the simulation code open source gives the whole user community the possibility to detect bugs in the source code and make suggestions to improve the code quality. Secondly, we apply a test harness, which is an important part of the work-flow for quality assurance in software engineering [5]. In the case of scientific simulation software, it consists of a set of simulation examples that should span the range of applicability of the software as good as possible. Technically, in our case it consists of a set of 10-50 LIGGGHTS simulations and is being run automatically on our cluster, where the number of processors, the code features and the numerical models are varied. Qualitative results are automatically extracted and are plotted for comparison, so thus a huge parameter space of flow regimes, numerical models, code features and parallelization situations can be governed. A test harness can aid in (a) finding bugs in the software, (b) checking parallel efficiency and consistency, (c) comparing different numerical models, and, most importantly, (d) experimental validation. Parallel consistency means that within a parallel framework, we need to have the possibility to compare the answers that a run with a different number of processors gives and the time that it takes to compute them. Experimental validation is especially important for scientific simulations. If experimental data is available for a test case, the experimental data is automatically compared to the numerical results, by means of global quantities such number of particles in the simulation, translational and rotational kinetic energy, thermal energy etc. The LIGGGHTS test harness aims to be a transparent and open community effort that everybody can contribute to in order to improve the quality of the LIGGGHTS code. We illustrate the usefulness of the test harness with several examples, where we especially focus on experimental validation

Progress in Applied CFD. Selected papers from 10th International Conference on Computational Fluid Dynamics in the Oil & Gas, Metallurgical and Process Industries

When installing gravity foundations for offshore structures such as wind power stations or oil platforms, the seabed needs to be excavated for providing enough stability. To minimize the impact on the surrounding fauna and the installation costs, steep but stable slopes are desired. The work presented is done in a research project on the numerical investigation of the stability of submarine slopes, particularly under the impact of influences like material removal or wave-induced disturbances. The method used in the current project is coupled CFD-DEM: while the dynamics of the fluid phase (water and in some cases water and air) are handled with computational fluid dynamics (CFD), the soil is modelled by spheres, whose motion is calculated with a discrete element method (DEM). Force models are used for considering the particles’ effect on the fluid and vice versa, a void fraction field accounts for the volume of the particles on the CFD side. Due to the high number of particles in the domain only unresolved CFD-DEM (cf., e.g. Zhou (2010)) is suitable: in this case the particles are smaller than the cells of the CFD mesh. In the presented work the investigations concentrated on the validation of the CFD-DEM models against small-scale experiments that were conducted by the authors. In a first step, the used materials were characterized and a lubrication force model was implemented. Furthermore, some basic investigations on the topic of dilatancy were carried out. Then an experimental setup and an according simulation were compared. In addition to that a three phase (air, water, particles) solver was used to depict the effect of surface waves onto the particle bed. For the calculations CFDEM®coupling was used. CFDEM®coupling is an Open Source software for coupled CFD-DEM simulations. It uses the CFD framework of the Open Source CFD code OpenFOAM® and the DEM framework of the Open Source code LIGGGHTS®. Both CFDEM®coupling and LIGGGHTS® have been presented before (cf., e.g. Goniva et al. (2012), Kloss et al. (2012)), the used model equations were validated against analytical solutions and literature.publishedVersio

A Novel Modeling Approach for Plastics Melting within a CFD-DEM Framework

Existing three-dimensional modeling approaches to single-screw extrusion can be classified according to the process sections. The discrete element method (DEM) allows describing solids transport in the feed section. The melt flow in the melt section can be calculated by means of computational fluid dynamics (CFD). However, the current state of the art only allows a separate consideration of the respective sections. A joint examination of the process sections still remains challenging. In this study, a novel modeling approach is presented, allowing a joint consideration of solids and melt transport and, beyond that, the formation of melt. For this purpose, the phase transition from the solid to liquid states is modeled for the first time within the framework CFDEMCoupling®, combining CFD and DEM by a novel melting model implemented in this study. In addition, a melting apparatus for the validation of the novel melting model is set up and put into operation. CFD-DEM simulations are carried out in order to calculate the melting rate and are compared to experimental results. A good agreement between the simulation and experimental results is found. From the findings, it can be assumed that the CFD-DEM simulation of single-screw extruder with a joint consideration of the feed and melt section is feasible