Letteratura scientifica selezionata sul tema "CFD-DEM model"

The coupling of the Computational Fluid Dynamics (CFD) to the Discrete Element Method (DEM) to simulate fluidization is computationally demanding. Although the Linear Spring-Dahspot (LSD) model can help to reduce the CFD-DEM simulation runtime due to its simplicity, its applicability is not reasonable for all sorts of problems. The objective of the present work is to show the application of the LSD model to the CFD-DEM simulation of alumina fluidization. The simulations were carried out with the software ANSYS FLUENT 14.5 and divided into two parts: (1) the reproduction with ANSYS FLUENT of simulations from the literature in which the LSD model and a representative particle approach were used. (2) the simulation of alumina fluidization and validation with experimental data. The results of three main sets of parameters were analysed to include different DEM and CFD time steps, drag models, the representation of particles with both uniform size and particle size distribution, etc. The main conclusion of this work is that the LSD model and the CFD-DEM approach can be used to model the actual behaviour of alumina fluidized beds, but the high simulation runtime and the correct setting of the strategies used to control it are still limiting factors which deserve special attention.

Dry powder inhalations (DPIs) have gathered attention as a treatment for respiratory diseases due to the large effective absorption area in a human lung. A cascade impactor is generally used to investigate the inhalation performance of DPIs. For the improvement of the efficiency of DPIs, understanding the particle motion and deposition behavior in the human lung and the cascade impactor is required. In the present study, computer simulations were conducted to calculate the particle motion and deposition behavior in the human lung and the cascade impactor. As simulation methods, a coupling model of a computational fluid dynamics and a discrete phase method (CFD−DPM) and a coupling model of a CFD and a discrete element method (CFD−DEM) were used. The CFD−DEM simulation could reproduce the experimental particle deposition behavior in the cascade impactor, although it was difficult by the CFD−DPM simulation. Furthermore, the calculation results using the CFD−DEM simulation quantitatively demonstrated the higher particle reachability into the simple lung model when smaller particles were used. It was found that the CFD−DEM simulation is a powerful tool to calculate the particle motion and deposition behavior in the cascade impactor and human lung.

Dry powder inhalations (DPIs) have gathered attention as a treatment for respiratory diseases due to the large effective absorption area in a human lung. A cascade impactor is generally used to investigate the inhalation performance of DPIs. For the improvement of the efficiency of DPIs, understanding the particle motion and deposition behavior in the human lung and the cascade impactor is required. In the present study, computer simulations were conducted to calculate the particle motion and deposition behavior in the human lung and the cascade impactor. As simulation methods, a coupling model of a computational fluid dynamics and a discrete phase method (CFD−DPM) and a coupling model of a CFD and a discrete element method (CFD−DEM) were used. The CFD−DEM simulation could reproduce the experimental particle deposition behavior in the cascade impactor, although it was difficult by the CFD−DPM simulation. Furthermore, the calculation results using the CFD−DEM simulation quantitatively demonstrated the higher particle reachability into the simple lung model when smaller particles were used. It was found that the CFD−DEM simulation is a powerful tool to calculate the particle motion and deposition behavior in the cascade impactor and human lung.

Purpose – The macroscopic properties of dried sand can be correctly modelled when the accurate determination of the microscopic properties is available. The microscopic properties between the particles such as the coefficients of rolling (µ r) and sliding (µ s), are numerically determined in two different ways: with and without considering the fluid effect. In an earlier study, the microscopic properties were determined by discrete element method (DEM) and without considering the air effect on the macroscopic properties such as the Angle of Repose. The purpose of this paper is to recalibrate the microscopic properties through a coupling between the DEM and computational fluid dynamics (CFD). Design/methodology/approach – The first step is dedicated to the calibration of the CFD-DEM model through modelling a single particle sedimentation within air, water, and silicon oil. The voidage and drag models, the grid size ratio (D/dx), the domain size ratio (W/D), and the optimum coupling interval between the CFD and DEM were investigated through comparing the CFD-DEM results with the analytical solution and experimental data. The next step is about modelling an Hourglass with the calibrated CFD-DEM model to recalibrate the µ r and µ s of dried sand particles. Findings – It was concluded that the air has a minor effect on the macroscopic properties of the dried sand and the µ r and µ s that were obtained with the DEM can be utilized in the CFD-DEM simulation. Originality/value – Utilizing the granulometry of dried quartz sand in the calibration process of the CFD-DEM method has raised the possibility of using the µ r and µ s for other applications in future studies.

The primary motivation of this paper is to investigate the sand-retention mechanisms that occur at the opening of sand filters. Various retention mechanisms under various conditions are explored that have a particulate flow with a low concentration of sand particles (called slurry flow) such as particle shape, size, and concentration. The computational fluid dynamic (CFD)–discrete element method (DEM) model is applied to predict the retention mechanisms under steady flow conditions of the well-bore. By using coupled CFD–DEM (CFD to model the fluid flow, and DEM to model the particle flow), the physics involved in the retention mechanisms is studied. The coarse grid unresolved and the smoothed unresolved (refined grid unresolved) coupling approaches implemented in STAR-CCM+ (SIEMENS PLM) are used to transfer data between the fluid and solid phases and calculate the forces. The filter slots under investigation have different geometries: straight, keystone, wire-wrapped screen (WWS) and seamed slot and the particles are considered with different shapes and different aspect ratios and size distributions. The flow regime is laminar in all simulations conducted. The CFD–DEM model is validated from the perspectives of particle–fluid, particle–particle, and particle–wall interactions. Verification of the CFD–DEM model is conducted by mesh sensitivity analysis to investigate the coupling resolution between the CFD and DEM. By simulation of numerous slurry flow scenarios, three retention mechanisms including surface deposition, size exclusion, and sequential arching of particles are observed. However, the concentration of particles is too diluted to result in multiparticle arch formation. In the simulations, various conditions are tested to give us an insight into the parameters and conditions that could affect the occurrence of the retention mechanisms. As an example, the importance of the gravity force and interaction forces on retention mechanisms are confirmed at the microscale in comparison with others forces involved in retention mechanisms such as the drag force, lift force, cohesive force, buoyancy force, and virtual mass force.

AbstractCO2 capture using solid sorbents in fluidized bed reactors is a promising technology. The multiphase CFD model is increasingly developed to study the reactors, but it is difficult to model all the realistic details and it requires significant computational time. In this study, both the multiphase CFD model (i.e., CFD-DEM model coupled with reaction) and the simplified reactor models (i.e., plug flow model and bubbling two-phase model) are developed for modeling a fluidized bed CO2 capture reactor. The comparisons are made at different gas velocities from fixed bed to fluidized bed. The DEM based model reveals a detailed view of CO2 adsorption process with particle flow dynamics, based on which the assumptions in the simplified models can be evaluated. The plug flow model predictions generally show similar trends to the DEM model but there are quantitative differences; thus, it can be used to determine the reactor performance limit. The bubbling two-phase model gives better predictions than the plug flow model because the effect of bubbles on the inter-phase mass transfer and reaction is included. In the future, a closer combination of the multiphase CFD simulation and the simplified reactor models will likely be an efficient design method of CO2 capture fluidized bed reactors.

The approach of combining computational fluid dynamics (CFD) for continuum fluid and the discrete element method (DEM) for discrete particles has been increasingly used to study the fundamentals of coupled particle–fluid flows. Different CFD–DEM models have been used. However, the origin and the applicability of these models are not clearly understood. In this paper, the origin of different model formulations is discussed first. It shows that, in connection with the continuum approach, three sets of formulations exist in the CFD–DEM approach: an original format set I, and subsequent derivations of set II and set III, respectively, corresponding to the so-called model A and model B in the literature. A comparison and the applicability of the three models are assessed theoretically and then verified from the study of three representative particle–fluid flow systems: fluidization, pneumatic conveying and hydrocyclones. It is demonstrated that sets I and II are essentially the same, with small differences resulting from different mathematical or numerical treatments of a few terms in the original equation. Set III is however a simplified version of set I. The testing cases show that all the three models are applicable to gas fluidization and, to a large extent, pneumatic conveying. However, the application of set III is conditional, as demonstrated in the case of hydrocyclones. Strictly speaking, set III is only valid when fluid flow is steady and uniform. Set II and, in particular, set I, which is somehow forgotten in the literature, are recommended for the future CFD–DEM modelling of complex particle–fluid flow.

The paper highlights the features of constructing a model of a wet semi-autogenous grinding mill based on the discrete element method and computational fluid dynamics. The model was built using Rocky DEM (v. 4.4.2, ESSS, Brazil) and Ansys Fluent (v. 2020 R2, Ansys, Inc., United States) software. A list of assumptions and boundary conditions necessary for modeling the process of wet semi-autogenous grinding by the finite element method is presented. The created model makes it possible to determine the energy-coarseness ratios of the semi-autogenous grinding (SAG) process under given conditions. To create the model in Rocky DEM the following models were used: The Linear Spring Rolling Limit rolling model, the Hysteretic Linear Spring model of the normal interaction forces and the Linear Spring Coulomb Limit for tangential forces. When constructing multiphase in Ansys Fluent, the Euler model was used with the primary phase in the form of a pulp with a given viscosity and density, and secondary phases in the form of air, crushing bodies and ore particles. The resistance of the solid phase to air and water was described by the Schiller–Naumann model, and viscosity by the realizable k-epsilon model with a dispersed multiphase turbulence model. The results of the work methods for material interaction coefficients determination were developed. A method for calculating the efficiency of the semi-autogenous grinding process based on the results of numerical simulation by the discrete element method is proposed.

CFD-DEM modelling of particle-laden turbulent flow is challenging in terms of the required and obtained CFD resolution, heavy DEM computations, and the limitations of the method. Here, we assess the efficiency of a particle-tracking solver in OpenFOAM with RANS-DEM and LES-DEM approaches under the unresolved CFD-DEM framework. Furthermore, we investigate aspects of the unresolved CFD-DEM method with regard to the coupling regime, particle boundary condition and turbulence modelling. Applying one-way and two-way coupling to our RANS-DEM simulations demonstrates that it is sufficient to include one-way coupling when the particle concentration is small (O ~ 10−5). Moreover, our study suggests an approach to estimate the particle boundary condition for cases when data is unavailable. In contrast to what has been previously reported for the adopted case, our RANS-DEM results demonstrate that simple dispersion models considerably underpredict particle dispersion and previously observed reasonable particle dispersion were due to an error in the numerical setup rather than the used dispersion model claiming to include turbulence effects on particle trajectories. LES-DEM may restrict extreme mesh refinement, and, under such scenarios, dynamic LES turbulence models seem to overcome the poor performance of static LES turbulence models. Sub-grade scale effects cannot be neglected when using coarse mesh resolution in LES-DEM and must be recovered with efficient modelling approaches to predict accurate particle dispersion.

Fluidization of solid particles using gas flow is an important process in chemical and pharmaceutical industries. The dynamics of fluidisation are intricately related to particle scale physics. Fluid-particle interactions dominate gas-solid fluidization behaviour for particles with average size and density greater than 10-4 m and 103 kg/m3, respectively, classified as Geldart B and D particles. Inter-particle forces, such as cohesion, play an increasingly important role in the fluidization dynamics of smaller particles, which are classified as Geldart A and C. In particular, interesting fluidization regimes have been noticed for weakly cohesive Geldart A particles, exhibiting a window of uniform fluidization before the onset of bubbling behaviour. Despite widespread industrial interests, the fundamental understanding of the mechanisms that underlie these fluidization regimes is poor. The present study aims to improve the understanding of fluidization dynamics of Geldart A regimes using numerical simulations. A DEM-CFD model was employed to capture the widely separated spatial and temporal scales associated with fluidization behaviour. The model couples the locally averaged Navier-Stokes equation for fluid with a discrete description of the particles. The methodology and its computer implementation are verified and validated to assess the extent of fluidization physics that it is able to capture. Verification cases check the implementation of the inter-phase momentum transfer term, drag model implementation and pressure-velocity coupling. The test cases are employed in order to cover a wide range of flow conditions. Robust validation tests for complex fluidization phenomena such as bubbling, spouting and bidisperse beds have been conducted to assess the predictive capabilities of the DEM-CFD solver. The simulation results for time and spatially averaged fluidziation behaviour are compared to experimental measurements obtained from the literature, and are shown to have capture fluidization physics qualitatively. Robust features of bubbling fluidization, such as minimum fluidization velocity, frequency of pressure drop fluctuations, segregation rates and solid circulation patterns were captured. Furthermore, the DEM-CFD model is critically assessed in terms of model conceptualization and parameter estimation, including those for drag closures, particle-wall boundary conditions, bed height and particle shape effects. The validation studies establish modelling best-practice guidelines and the level of discrepancy against the analytical solutions or experimental measurements. Having developed the model and established its predictive capability, it is used to probe the hydrodynamics of weakly cohesive particles. Cohesive interactions are captured by employing a pair-wise van derWaals force model. The cohesive strength of the granular bed is quantified by the ratio of the maximum van der Waals force to the particle gravitational force, defined as the granular Bond number. The Bond number of the bed is increased systematically from 0-10 to examine the role of cohesion in the fluidization behaviour of fine powders while keeping the particle size and density constant across all the simulations. The idea was to segregate the hydrodynamics associated with size and density of the particles from the inter-particle interactions. The size and density of the particles are carefully chosen at a scale where inter-particle forces are present but minimal [Seville et al., 2000]. The Geldart A fluidization behaviour is captured for granular beds with Bond numbers ranging from 1 to 3. Many robust features of Geldart A fluidization, such as pressure drop overshoot, delay in the onset of bubbling, macroscopic Umf predictions and uniform bed expansion are captured in the DEM-CFD framework. The expanded bed was characterized according to criteria that the particles are highly immobile in this regime and the expanded porosity is related to inlet velocity by Richardson–Zaki correlations. Sudden jumps in the magnitudes of global granular temperature were found near the regime transitions. This observation was used an indicator of the onset of bubbling and quantification of minimum bubbling velocity (Umb). The window of the expanded bed regime (quantified as Umb - Umf) was shown to be an increasing function of cohesive strength of the bed. Furthermore, the stability of the expanded bed was probed by studying the response of the expanded bed to sudden inertial and voidage shocks. A kinematic wave, generated as a response to the voidage shock, was shown to slow down with increasing cohesion and decreasing hydrodynamic forces. Furthermore, predictions of Umb by DEM-CFD simulations for weakly cohesive beds were compared against empirical correlations by Valverde [2013] with an excellent match. Stress analysis of the expanded bed revealed the presence of tensile stresses. As the inlet velocity is increased beyond the minimum fluidization velocity, a longitudinal shift of these negative stresses is observed until they reach the top of the bed. Negative stresses were seen at the bed surface at the onset of bubbling. The role of cohesion stresses in the formation of expanded bed and suppression of bubbling was highlighted. Finally, the microstructure of the expanded bed was probed at different local micro and mescoscopic length scales. Evidence of clustering, agglomeration and cavities were presented in the expanded bed. Expanded bed expansion was shown to have mesostructural inhomogeneities present, which is contrary to the belief of homogeneous expansion.

During the oil recovery a hole is drilled, the sandstone is left unsupported next to the cavity and disloged sand grains can enter the oil recovery system. This process is called sand production and several problems may arise due to that process, as clogging up of the well or damage to the well equipment. The study of sand production process is of paramout importance for safe and economical hydrocarbon production. The majority of numerical models to predict sand production that have been used to date are continuum-based. However, a continuum approach cannot easily capture important features of the sanding problem, such as erosion, and it requires the formulation recognized as a difficult task because of the large number of interactions and non-lineaities intrinsic to the problem. On the other hand, discrete-element based approaches allow a simpler formulation of the problem and a better understanding of some of its features. Discrete Element Methods (DEM) describe more naturally the disaggregation and erosion of sand particles and the fluid-solid interaction. In this research rock behavior has been epresented in DEM using the parallel-bond model (PBM) because it mimics the effect of cement between particles. The study has involved the calibration of the DEM rock model agains real data. Moreover, limitations of the DEM model have been explored and sensitivity analyses examining the effects of the local damping have been performed. The main aim of this research is to improve the understanding of sand production based on a Computational Fluid Dynamics (CFD) -DEM coupling model. CFD-DEM is frequently used for process and chemical engineering problems (Zhu et al., 2007). To simulate the interaction of the particles with the fluid, the solid DEM model is coupled with a fluid model (CFD). A validation of the CFD-DEM model has been carried out in this thesis by performing single particle simulations and analyses of ermeability tests. Simulations of sand production using a omogeneous sandstone analogue and, finally, the simulation of sand production under realistic conditions are presented.
En este estudio se ha representado el comportamiento de la roca en DEM utilizando el modelo de enlace paralelo (parallel-bond model - PBM) porque simulat el efecto del cemento entre partículas. El estudio ha implicado la calibración del model en DEM de la roca en comparación don datos reales. Además, las limitaciones del modelo en DEM se ha exploradoy se han llevado a cabo análisis de sensibilidad para examinar los efectos del parámetro de amortiguación (local damping). El principal objetivo de este estudio es mejorar la comprensión de la producción de arena basado en un modelo de acoplamiento con Dinámica de Fluidos Computacional (Computational Fluid Dynamics - CFD) CFD-DEM. El CFD-DEM se usa frecuentemente para problemas de procesos y de ingeniería química. Para simular la interacción de las partículas con el fluido, el modelo sólido DEM es acoplado con el modelo de fluido (CFD). En esta tesis se ha llevado a cabo una validación del modelo CFD-DEM realizando simulaciones con una sola partícula y con análisis de tests de permeabilidad. Se han presentado simulaciones de producción de arena utilizando un modelo análogo a una arenisca homogénea, y finalmente se han presentado simulaciones de producción de arena bajo condiciones realistas. Durante el proceso de extracción de petróleo se perfora un agujero y la arenisca queda sin apoyo junto a la vacidad y los granos de arena sueltos pueden entrar en el sistema de extracción del petróleo. Este proceso se llama producción de arena y pueden producirse varios problemas debido a este proceso, como obstruir el pozo o dañar el equipo instalado en el pozo. El estudio del proceso de producción de arena es de primordial importancia para una producción de hidrocarburo segura y rentable. La mayoría de los modelos numéricos para predecir la producción de arena que se han estado usando hasta el momento están basados en medio continuo. No obstante, un método continuo no puede captar fácilmente algunas características importantes del problema de la producción de arena, como es la erosión, y requiere una formulación que se reconoce como una tarea difícil debido al gran número de interacciones y no-linearidades intrínsecas al problema. Por otro lado, los métodos basados en elementos discretos permiten una formulación más simple del problema y un entendimiento mejor de algunas de sus características. Los Métodos de Elementos Discretos (Discrete Element Methods - DEM) describen de una forma más natural la desagregación y la erosión de partículas de arena y de la interacción sólido-fluido.

In recent years, the increasing impacts of landslide hazards on human lives and lifeline facilities worldwide has advanced the necessity to find out both economically acceptable and useful techniques to predict the occurrence and destructive power of landslides. Though many projects exist to attain this goal, the current investigation set out to establish an understanding of the initiation and propagation mechanisms of landslides via numerical simulations, so that mitigation strategies to reduce the long-term losses from landslide hazards can be made. In this research, the Discrete Element Method (DEM) and Computational Fluid Dynamics (CFD) have been used to investigate the mechanical and hydraulic behaviour of granular materials involved in landslides. The main challenge is to provide rational analyses of large scale landslides via small scale numerical simulations. To solve this problem, dimensional analyses have been performed on a simple granular column collapse model. The influence of governing dimensionless groups on the debris runout distance and deposit height has been studied for the terrestrial and submerged granular flows. 3D DEM investigations of granular flows in plane strain conditions have been performed in this research. The input parameters of the DEM model have been calibrated by the numerical triaxial tests, based on which, the relationships between the microscopic variables and the macroscopic soil strength properties are analysed. Using the simple granular column collapse model, the influences of column aspect ratio, characteristic strain, model size ratio and material internal friction angle on the runout distance and deposit height of granular materials have been examined. Additionally, the deformation and energy evolution of dry granular materials are also discussed. The DEM-CFD coupling model has been employed to study the mechanical and hydraulic behaviour of highly mobilized terrestrial / submarine landslides. This model has been validated via numerical simulations of fluid flow through a porous soil sample and grain batch sedimentations. The simulations of granular flows in the submerged environment have led to some meaningful insights into the flow mechanisms, such as the mobilization of sediments, the generation and dissipation of excess pore water pressures and the evolution of effective stresses. Overall, this study shows that the proposed numerical tools are capable of modelling the mechanical and hydraulic behaviour of terrestrial and submarine landslides.

Le dépassement de la chute de pression du lit à la vitesse minimale de fluidisation, qui se produit pendant la transition d'un état de lit fixe à un état de lit fluidisé, est un phénomène courant pour les particules fines classées dans le groupe A selon la classification de Geldart. Ces particules présentent une hystérésis entre les courbes de chute de pression pour les trajectoires de vitesse de gaz décroissante et croissante. Cette étude utilise deux modèles de pression de particules adhésives dans des simulations de modèles à deux fluides pour incorporer l'influence de la force de Van der Waals interparticulaire, dans le but de prédire le dépassement de la pression. Le premier modèle de pression adhésive, développé dans le cadre de la théorie cinétique des écoulements granulaires rapides, n'a pas réussi à capturer le dépassement en raison de la prévalence de contacts multiples et prolongés dans les lits fixes. Nous avons proposé une fermeture alternative basée sur le nombre de coordination, générant une contribution adhésive significativement plus élevée que le modèle de la théorie cinétique et reproduisant avec succès le dépassement de la chute de pression.En outre, nous avons construit une base de données numériques CFD-DEM (Computational Fluid Dynamics-Discrete Element Method) pour prédire l'hystérésis dans la chute de pression. Cette base de données peut guider la formulation d'une équation de transport eulérienne pour le nombre de coordination, permettant l'incorporation des effets de l'historique des déformations. Nous avons étudié l'impact de la force de Van der Waals et de la friction statique sur la fluidisation des solides fins à l'échelle moyenne en utilisant des simulations CFD-DEM et leur rôle dans l'apparition du phénomène de dépassement de pression. Notre analyse examine des paramètres tels que la chute de pression du gaz, le vide du lit, le nombre de coordination, les pressions répulsives et adhésives des solides, le gradient vertical de vitesse des solides, le tenseur de tissu et la contrainte de cisaillement particule-paroi tout au long des processus de défluidisation et de fluidisation. Nous avons démontré qu'il est nécessaire de prendre en compte l'adhésion de Van der Waals pour prédire l'expansion homogène du lit sur toute la gamme des vitesses, du minimum requis pour la fluidisation au minimum pour le bullage. L'ensemble de données CFD-DEM généré peut guider le développement de fermetures de contraintes solides pour les modèles à deux fluides afin d'incorporer les effets de l'adhésion de Van der Waals et de la friction statique sur l'hydrodynamique de la fluidisation, ce qui permet de prédire l'hystérésis dans la chute de pression du lit à l'échelle macroscopique. Dans ce travail, nous avons incorporé un modèle de frottement statique-dynamique dans le code CFD-DEM massivement parallèle YALES2 à l'aide d'un algorithme en deux étapes, afin de remédier aux lacunes du modèle de frottement dynamique de Coulomb, qui est pratique pour les écoulements granulaires rapides mais ne s'applique pas aux lits stationnaires. Nous avons validé notre mise en œuvre par une série de tests à macro- et micro-échelle. En outre, nous avons introduit dans YALES2 les forces de Van der Waals entre particules et entre particules et parois, et validé cet ajout à l'échelle microscopique. En outre, nous avons postulé une expression de relaxation pour le terme source dans l'équation de transport des nombres de coordination et déterminé le temps de relaxation des nombres de coordination à l'aide de données de simulation CFD-DEM. En outre, nous avons utilisé une technique de pénalisation pour coupler de manière semi-implicite les phases gazeuse et solide, en particulier par le traitement implicite des forces de traînée et d'Archimède. Cette approche vise à résoudre les problèmes de stabilité rencontrés lorsque le couplage interphase est explicite
The overshoot in bed pressure drop at the minimum fluidization velocity, occurring during the transition from a fixed to a fluidized bed state, is a common phenomenon for fine particles categorized under Group A according to Geldart's classification. These particles exhibit hysteresis between the pressure drop curves for the decreasing and increasing gas velocity paths. This study employs two adhesive particle pressure models within two-fluid model simulations to incorporate the influence of interparticle Van der Waals force, aiming to predict the pressure overshoot. The first adhesive pressure model, developed within the kinetic theory of rapid granular flows framework, failed to capture the overshoot due to the prevalence of multiple and prolonged contacts in fixed beds. We proposed an alternative closure based on coordination number, generating a significantly higher adhesive contribution than the kinetic theory model and successfully reproducing the pressure drop overshoot.In addition, we constructed a Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) numerical database to predict hysteresis in pressure drop. This database can guide the formulation of an Eulerian transport equation for the coordination number, enabling the incorporation of deformation history effects. We explored the impact of Van der Waals force and static friction on the fluidization of fine solids at the mesoscale using CFD-DEM simulations and their role in causing the pressure overshoot phenomenon. Our analysis examines parameters such as gas pressure drop, bed voidage, coordination number, repulsive and adhesive solid pressures, vertical solid velocity gradient, fabric tensor, and particle-wall shear stress throughout the defluidization and fluidization processes. We demonstrated that it is necessary to consider the Van der Waals adhesion to predict the homogeneous expansion of the bed across the range of velocities from the minimum required for fluidization to the minimum for bubbling. The generated CFD-DEM dataset can guide the development of solid stress closures for two-fluid models to incorporate the effects of Van der Waals adhesion and static friction on fluidization hydrodynamics, allowing for the prediction of hysteresis in bed pressure drop at the macroscale.In this work, we incorporated a static-dynamic friction model into the massively parallel CFD-DEM code YALES2 using a two-step algorithm, aiming to address the shortcomings of the Coulomb dynamic friction model, which is practical for fast granular flows but not applicable to stationary beds. We validated our implementation through a series of macro- and microscale tests. Furthermore, we introduced interparticle and particle-wall Van der Waals forces into YALES2 and validated this addition at the microscale. Additionally, we postulated a relaxation expression for the source term in the coordination number transport equation and determined the coordination number relaxation time using CFD-DEM simulation data. Moreover, we employed a penalization technique to semi-implicitly couple gas and solid phases, specifically through the implicit handling of drag and Archimedes forces. This approach aimed to resolve the stability issues encountered when the interphase coupling is explicit

Computational Fluid Dynamics Coupled to Discrete Element Method (CFD-DEM) is widely used in simulating a large variety of particulate flow system. This Eulerian-Lagrangian technique tracks all the particles included in the system by the application of point mass models in their equation of motion. CFD-DEM is a more accurate (and more expensive) technique compared to an Eulerian-Eulerian representation. Compared to Particle Resolved Simulations (PRS), CFD-DEM is less expensive since it does not require resolving the flow around each particles and thus can be applied to larger scale systems. Nevertheless, simulating industrial and natural scale systems is a challenge for this numerical technique. This is because the cost of CFD-DEM is proportional to the number of particles in the system under consideration. Thus, massively parallel codes are used to tackle these problems with the help of supercomputers. In this thesis, the CFD-DEM capability in the in-house code Generalized Incompressible Direct and Large Eddy Simulation of Turbulence (GenIDLEST) is used to investigate large scale dense particulate flow systems. Central to the contributions made by this work are developments to reduce the computational cost of CFD-DEM. This includes the development and validation of reduced order history force model for use in large scale systems and validation of the representative particle model, which lumps multiple particles into one, thus reducing the number of particles that need to be tracked in the system. Numerical difficulties in the form of long integration times and instabilities encountered in fully coupling the fluid and particle phases in highly energetic systems are alleviated by proposing a partial coupling scheme which maintains the accuracy of full-coupling to a large extent but at a reduced computational cost. The proposed partial-coupling is found to have a better convergence behavior compared to the full coupling in large systems and can be used in cases where full coupling is not feasible or impractical to use. Alternative modeling approaches for the tangential treatment of the soft-sphere impact model to avoid storing individual impact deformation are proposed and tested. A time advancement technique is developed and proposed for use in dense particulate systems with a hard-sphere impact model. The new advancement technique allows for the use of larger time steps which can speed-up the time to solution by as much as an order of magnitude.
PHD

The study and modelling of two-phase flow, even the simplest ones such as the bubbly flow, remains a challenge that requires exploring the physical phenomena from different spatial and temporal resolution levels. CFD (Computational Fluid Dynamics) is a widespread and promising tool for modelling, but nowadays, there is no single approach or method to predict the dynamics of these systems at the different resolution levels providing enough precision of the results. The inherent difficulties of the events occurring in this flow, mainly those related with the interface between phases, makes that low or intermediate resolution level approaches as system codes (RELAP, TRACE, ...) or 3D TFM (Two-Fluid Model) have significant issues to reproduce acceptable results, unless well-known scenarios and global values are considered. Instead, methods based on high resolution level such as Interfacial Tracking Method (ITM) or Volume Of Fluid (VOF) require a high computational effort that makes unfeasible its use in complex systems. In this thesis, an open-source simulation framework has been designed and developed using the OpenFOAM library to analyze the cases from microescale to macroscale levels. The different approaches and the information that is required in each one of them have been studied for bubbly flow. In the first part, the dynamics of single bubbles at a high resolution level have been examined through VOF. This technique has allowed to obtain accurate results related to the bubble formation, terminal velocity, path, wake and instabilities produced by the wake. However, this approach has been impractical for real scenarios with more than dozens of bubbles. Alternatively, this thesis proposes a CFD Discrete Element Method (CFD-DEM) technique, where each bubble is represented discretely. A novel solver for bubbly flow has been developed in this thesis. This includes a large number of improvements necessary to reproduce the bubble-bubble and bubble-wall interactions, turbulence, velocity seen by the bubbles, momentum and mass exchange term over the cells or bubble expansion, among others. But also new implementations as an algorithm to seed the bubbles in the system have been incorporated. As a result, this new solver gives more accurate results as the provided up to date. Following the decrease on resolution level, and therefore the required computational resources, a 3D TFM have been developed with a population balance equation solved with an implementation of the Quadrature Method Of Moments (QMOM). The solver is implemented with the same closure models as the CFD-DEM to analyze the effects involved with the lost of information due to the averaging of the instantaneous Navier-Stokes equation. The analysis of the results with CFD-DEM reveals the discrepancies found by considering averaged values and homogeneous flow in the models of the classical TFM formulation. Finally, for the lowest resolution level approach, the system code RELAP5/MOD3 is used for modelling the bubbly flow regime. The code has been modified to reproduce properly the two-phase flow characteristics in vertical pipes, comparing the performance of the calculation of the drag term based on drift-velocity and drag coefficient approaches.
El estudio y modelado de flujos bifásicos, incluso los más simples como el bubbly flow, sigue siendo un reto que conlleva aproximarse a los fenómenos físicos que lo rigen desde diferentes niveles de resolución espacial y temporal. El uso de códigos CFD (Computational Fluid Dynamics) como herramienta de modelado está muy extendida y resulta prometedora, pero hoy por hoy, no existe una única aproximación o técnica de resolución que permita predecir la dinámica de estos sistemas en los diferentes niveles de resolución, y que ofrezca suficiente precisión en sus resultados. La dificultad intrínseca de los fenómenos que allí ocurren, sobre todo los ligados a la interfase entre ambas fases, hace que los códigos de bajo o medio nivel de resolución, como pueden ser los códigos de sistema (RELAP, TRACE, etc.) o los basados en aproximaciones 3D TFM (Two-Fluid Model) tengan serios problemas para ofrecer resultados aceptables, a no ser que se trate de escenarios muy conocidos y se busquen resultados globales. En cambio, códigos basados en alto nivel de resolución, como los que utilizan VOF (Volume Of Fluid), requirieren de un esfuerzo computacional tan elevado que no pueden ser aplicados a sistemas complejos. En esta tesis, mediante el uso de la librería OpenFOAM se ha creado un marco de simulación de código abierto para analizar los escenarios desde niveles de resolución de microescala a macroescala, analizando las diferentes aproximaciones, así como la información que es necesaria aportar en cada una de ellas, para el estudio del régimen de bubbly flow. En la primera parte se estudia la dinámica de burbujas individuales a un alto nivel de resolución mediante el uso del método VOF (Volume Of Fluid). Esta técnica ha permitido obtener resultados precisos como la formación de la burbuja, velocidad terminal, camino recorrido, estela producida por la burbuja e inestabilidades que produce en su camino. Pero esta aproximación resulta inviable para entornos reales con la participación de más de unas pocas decenas de burbujas. Como alternativa, se propone el uso de técnicas CFD-DEM (Discrete Element Methods) en la que se representa a las burbujas como partículas discretas. En esta tesis se ha desarrollado un nuevo solver para bubbly flow en el que se han añadido un gran número de nuevos modelos, como los necesarios para contemplar los choques entre burbujas o con las paredes, la turbulencia, la velocidad vista por las burbujas, la distribución del intercambio de momento y masas con el fluido en las diferentes celdas por cada una de las burbujas o la expansión de la fase gaseosa entre otros. Pero también se han tenido que incluir nuevos algoritmos como el necesario para inyectar de forma adecuada la fase gaseosa en el sistema. Este nuevo solver ofrece resultados con un nivel de resolución superior a los desarrollados hasta la fecha. Siguiendo con la reducción del nivel de resolución, y por tanto los recursos computacionales necesarios, se efectúa el desarrollo de un solver tridimensional de TFM en el que se ha implementado el método QMOM (Quadrature Method Of Moments) para resolver la ecuación de balance poblacional. El solver se desarrolla con los mismos modelos de cierre que el CFD-DEM para analizar los efectos relacionados con la pérdida de información debido al promediado de las ecuaciones instantáneas de Navier-Stokes. El análisis de resultados de CFD-DEM permite determinar las discrepancias encontradas por considerar los valores promediados y el flujo homogéneo de los modelos clásicos de TFM. Por último, como aproximación de nivel de resolución más bajo, se investiga el uso uso de códigos de sistema, utilizando el código RELAP5/MOD3 para analizar el modelado del flujo en condiciones de bubbly flow. El código es modificado para reproducir correctamente el flujo bifásico en tuberías verticales, comparando el comportamiento de aproximaciones para el cálculo del término d
L'estudi i modelatge de fluxos bifàsics, fins i tot els més simples com bubbly flow, segueix sent un repte que comporta aproximar-se als fenòmens físics que ho regeixen des de diferents nivells de resolució espacial i temporal. L'ús de codis CFD (Computational Fluid Dynamics) com a eina de modelatge està molt estesa i resulta prometedora, però ara per ara, no existeix una única aproximació o tècnica de resolució que permeta predir la dinàmica d'aquests sistemes en els diferents nivells de resolució, i que oferisca suficient precisió en els seus resultats. Les dificultat intrínseques dels fenòmens que allí ocorren, sobre tots els lligats a la interfase entre les dues fases, fa que els codis de baix o mig nivell de resolució, com poden ser els codis de sistema (RELAP,TRACE, etc.) o els basats en aproximacions 3D TFM (Two-Fluid Model) tinguen seriosos problemes per a oferir resultats acceptables , llevat que es tracte d'escenaris molt coneguts i se persegueixen resultats globals. En canvi, codis basats en alt nivell de resolució, com els que utilitzen VOF (Volume Of Fluid), requereixen d'un esforç computacional tan elevat que no poden ser aplicats a sistemes complexos. En aquesta tesi, mitjançant l'ús de la llibreria OpenFOAM s'ha creat un marc de simulació de codi obert per a analitzar els escenaris des de nivells de resolució de microescala a macroescala, analitzant les diferents aproximacions, així com la informació que és necessària aportar en cadascuna d'elles, per a l'estudi del règim de bubbly flow. En la primera part s'estudia la dinàmica de bambolles individuals a un alt nivell de resolució mitjançant l'ús del mètode VOF. Aquesta tècnica ha permès obtenir resultats precisos com la formació de la bambolla, velocitat terminal, camí recorregut, estela produida per la bambolla i inestabilitats que produeix en el seu camí. Però aquesta aproximació resulta inviable per a entorns reals amb la participació de més d'unes poques desenes de bambolles. Com a alternativa en aqueix cas es proposa l'ús de tècniques CFD-DEM (Discrete Element Methods) en la qual es representa a les bambolles com a partícules discretes. En aquesta tesi s'ha desenvolupat un nou solver per a bubbly flow en el qual s'han afegit un gran nombre de nous models, com els necessaris per a contemplar els xocs entre bambolles o amb les parets, la turbulència, la velocitat vista per les bambolles, la distribució de l'intercanvi de moment i masses amb el fluid en les diferents cel·les per cadascuna de les bambolles o els models d'expansió de la fase gasosa entre uns altres. Però també s'ha hagut d'incloure nous algoritmes com el necessari per a injectar de forma adequada la fase gasosa en el sistema. Aquest nou solver ofereix resultats amb un nivell de resolució superior als desenvolupat fins la data. Seguint amb la reducció del nivell de resolució, i per tant els recursos computacionals necessaris, s'efectua el desenvolupament d'un solver tridimensional de TFM en el qual s'ha implementat el mètode QMOM (Quadrature Method Of Moments) per a resoldre l'equació de balanç poblacional. El solver es desenvolupa amb els mateixos models de tancament que el CFD-DEM per a analitzar els efectes relacionats amb la pèrdua d'informació a causa del promitjat de les equacions instantànies de Navier-Stokes. L'anàlisi de resultats de CFD-DEM permet determinar les discrepàncies ocasionades per considerar els valors promitjats i el flux homogeni dels models clàssics de TFM. Finalment, com a aproximació de nivell de resolució més baix, s'analitza l'ús de codis de sistema, utilitzant el codi RELAP5/MOD3 per a analitzar el modelatge del fluxos en règim de bubbly flow. El codi és modificat per a reproduir correctament les característiques del flux bifàsic en canonades verticals, comparant el comportament d'aproximacions per al càlcul del terme de drag basades en velocitat de drift flux model i de les basades en coe
Peña Monferrer, C. (2017). Computational fluid dynamics multiscale modelling of bubbly flow. A critical study and new developments on volume of fluid, discrete element and two-fluid methods [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/90493
TESIS

Es ergab sich die Notwendigkeit, die Vermischungseffekte mit einem 1:5 skalierten Modell nachzuvollziehen. In dieser Arbeit wurden Skalierungseffekte hervorgehoben und ein Vergleich der Strömungen im Originalreaktor und 1:5 Plexiglasmodell mit Hilfe eines numerischen Strömungsberechnungsprogrammes vollzogen. Dabei wurde das Modell und der Originalreaktor möglichtst originalgetreu abgebildet und mit den kalten Strängen zusammen modelliert.Die Vergleichsrechnungen belegen, daß es ausreichend ist, die Vermischungsvorgänge in einem mindestens 1:6.6 skalierten Modell eines DWR zu untersuchen. Die Parameter (Druck, Temperatur, Geschwindigkeit) erlauben den Aufbau als Plexiglasmodell, das eine optische Beobachtung der Vermischung ermöglicht. Das Forschungszentrum Rossendorf hat mit dem Aufbau eines 1:5 Modells 1997 begonnen.

Es ergab sich die Notwendigkeit, die Vermischungseffekte mit einem 1:5 skalierten Modell nachzuvollziehen. In dieser Arbeit wurden Skalierungseffekte hervorgehoben und ein Vergleich der Strömungen im Originalreaktor und 1:5 Plexiglasmodell mit Hilfe eines numerischen Strömungsberechnungsprogrammes vollzogen. Dabei wurde das Modell und der Originalreaktor möglichtst originalgetreu abgebildet und mit den kalten Strängen zusammen modelliert.Die Vergleichsrechnungen belegen, daß es ausreichend ist, die Vermischungsvorgänge in einem mindestens 1:6.6 skalierten Modell eines DWR zu untersuchen. Die Parameter (Druck, Temperatur, Geschwindigkeit) erlauben den Aufbau als Plexiglasmodell, das eine optische Beobachtung der Vermischung ermöglicht. Das Forschungszentrum Rossendorf hat mit dem Aufbau eines 1:5 Modells 1997 begonnen.

The charge motion in a tumbling mill has been mostly described by empirical, mechanistic and computational models. The computational model presented in this work is a three phase approach for the tumbling mill that combines a particle description for the solids modelled by the Discrete Element Method, and the continuum description for the uid by CFD approach. In the present work a phase coupled approach is developed using C++ subroutines to model the charge and slurry dynamics inside a tumbling mill by mapping the particles on the CFD mesh and resolving the particle volume and velocities on per cell basis. The coupled DEM-CFD approach is implemented and the eect of drag force on the slurry by the particle motion. The set of coupled simulation were run varying the slurry rheology and results were validated with equivalent PEPT experiment of lab scale mills and a very good agreement is found in some cases. The Beeststra drag correlation was used to calculate the drag force between the uid and the particles. The free surface prole of the charge as well as the slurry is calculated as well as the axial center of mass prole of the mill. The second part of the thesis deals with development of a two way coupled DEM-CFD approach that incorporates the eect of slurry on the charge as well as vice versa emulating the Virtual Commutation Machine as developed by Cleary but using CFD in place of SPH more extensively method for modeling uid ow.

AbstractThe coupled Computational Fluid Dynamics and Discrete Element Method (CFD-DEM) is a useful tool for modeling the dynamics of sand production that occurs in oil and gas reservoirs. To perform accurate, physically relevant and efficient calculations, the optimal size of the simulation time-step should be selected. In this study, we investigate the selection of an appropriate time-step interval between CFD and DEM models in sand production simulations. The CPU time, speedup and root mean squared relative error of the obtained results are examined to compare the sand production phenomenon at different coupling numbers. Most of the results including the final sand production rate, bond number and bond ratio indicate that the simulations with coupling numbers of N = 10 and N = 100 produce more accurate results. Moreover, these outcomes demonstrate significant improvements in terms of acceleration of the modeling process.

This paper presents a numerical investigation of sedimentation and separation of particles in liquids using a modified discrete element method coupled with computational fluid dynamics (DEM/CFD). In the present DEM/CFD, a weakly compressible fluid model is incorporated to model continuum carrier (liquid) phases, while a soft-sphere based DEM is used to model discrete particles. The key advantage of DEM over continuum methods is that it can be used to analyze sedimentation behaviour at the particle level and to model explicitly particle–particle and particle–wall collisions. Using the modified DEM/CFD, computational simulations of settling of spherical particles in a container fully filled with water have been conducted to examine effects of particle size, particle density, particle concentration and fluid viscosity on gravitational sedimentation of particles. A three dimensional numerical simulation of stratification and particle segregation in a liquid is also performed. It has been demonstrated that the modified DEM/CFD is a robust numerical technique for analyzing complex sedimentation and separation process of particles in liquids in 3D.

The coupled Discrete Element Method with Computational Fluid Dynamics (DEM/CFD) is a numerical technique for modelling fluid-solid particle flows, in which the motion of particles is modeled using DEM and that of fluid is solved using CFD. The interaction between the fluid and particles is approximated by an empirical correlation for the drag force. In this study, an immersed boundary method (IBM) is incorporated into DEM/CFD in order to model complex fluid-solid particle flows involving large objects and moving arbitrary shaped boundaries. The modified DEM/CFD is verified by comparing the numerical simulations with the experimental observation of the separation of binary mixtures in a vibrated bed. Its capacity is demonstrated with two case studies: i) fluidisation with a large immersed inclusion and ii) roll compaction in the presence of air. It is shown that the capacity of DEM/CFD is significantly enhanced with IBM and it can be used to simulate a range of particulate flows consisting of fluids and particles with large objects, complex and/or moving boundaries.

The transmission of particle-bearing liquids in pipes has motivated continuing research into erosion mechanisms and the distribution of erosion rates over wetted surfaces. This chapter covers these initiatives with particular reference to erosion-corrosion modelling within bends and straight sections of cylindrical pipes manufactured in a variety of materials and transporting a variety of liquids. Erosion-corrosion modelling techniques such as submerged slurry jets and rotating cylinder electrodes have been used to study factors influencing material degradation. Improvements in computational fluid dynamics (CFD), such as the development of a moving deforming mesh (MDM) have improved the accuracy of CFD models in predicting pipe wall erosion rates. Combined discrete phase tracking approaches such as the CFD-DPM-DEM (discrete phase-discrete element model) have helped improve computational efficiency. Wall impact erosion models are calibrated using laboratory scale tests. Validation of CFD models using full-scale test data is rare, meaning their accuracy is still largely unreported. Material testing has helped to identify the resilience of prospective pipeline materials to erosion-corrosion, while modifications to internal geometry and pipe section have shown potential to improve erosion-corrosion resistance.

Two-phase computational fluid dynamics - discrete element method (CFD-DEM) framework has gained attention in cohesive sediment transport due to its capability of resolving particle-particle interactions and capturing the time evolution of individual flocs and hence the flocculation dynamics of cohesive sediment in turbulent flows. For cohesive sediments of size smaller than the Kolmogorov length scale, the point-particle approach is commonly used, in which the flow around particles is not fully resolved, and the hydrodynamic force on particles is parameterized by the drag law. The accuracy of floc dynamics, aggregation, breakup, and reshaping therefore strongly depends on force parameterization of individual point-particles that make up the floc. In this chapter, we review recent advances in the state-of-art two-phase CFD-DEM model approach on cohesive sediment transport and make recommendation for future research.

To enhance the separation efficiency in the separator during the process of separating tobacco leaves from the stems, it is necessary to study how the airflow affects the motion of the tobacco in the separation chamber. Therefore, a DEM-CFD coupling model is established to simulate the interaction between the airflow and the mixed tobacco leaves and stems. Different inlet wind speeds, between 7.5 m/s–15 m/s are set to study the effect of inlet velocity on the stems separation efficiency. The results show that the inlet wind speed of 12.5 m/s–15 m/s can lead to a better separation effect, giving a 76% destemming rate with only 4% tobacco leaves waste.

Discrete Element Method (DEM) coupled to Computational Fluid Mechanics (CFD) is a powerful tool for simulating complex multiphase flows. The cost of this Eulerian-Lagrangian description, however, increases with the increase of the number of particles ∼O(Np) which limit its use in natural and industrial scale systems. Efforts to reduce the cost of CFD-DEM capability include reducing the total number of simulated particles by lumping them in larger size representative particles (RP Model). The scaled Representative Particle simulations are less compute intensive compared to the more expensive high fidelity un-scaled/resolved simulations. The prediction accuracy of the RP model, however, decreases as larger scaling factors are used. In the current work, we study the possibility of getting improved results from RP model by using two different techniques. First attempts will be made to identify reasons for reduction in RP model prediction accuracy, then different techniques for improvement of RP model are suggested and tested. In the second part, the ability of the co-kriging surrogate model to improve CFD-DEM predictions by combining many reduced-order RP model simulations with a few high-fidelity unscaled calculations is tested. Appropriate systems are selected to evaluate each method.

Abstract Dry powder inhalers (DPIs), used as a means for pulmonary drug delivery, typically contain a combination of active pharmaceutical ingredient (API) and significantly larger carrier particles. The micro-sized drug particles — which have a strong propensity to aggregate and poor aerosolization performance — mixed with significantly large carrier particles that are unable to penetrate the mouth-throat region to deagglomerate and entrain the smaller API particles in the inhaled airflow. The performance of a DPI, therefore, depends on entrainment the carrier-API combination particles and the time and thoroughness of the deagglomeration of the individual API particles from the carrier particles. Since DPI particle transport is significantly affected by particle-particle interactions, very different particles sizes and shapes, various forces including electrostatic and van der Waals forces, they present significant challenges to Computational Fluid Dynamics (CFD) modelers to model regional lung deposition from a DPI. In the current work, we present a novel high fidelity CFD discrete element modeling (CFD-DEM) and sensitivity analysis framework for predicting the transport of DPI carrier and API particles. The work integrates exascale capable CFD-DEM and sensitivity analysis capabilities by leveraging the Department of Energy (DOE) laboratories libraries: Multiphase Flow Interface Flow Exchange (MFiX) for CFD-DEM, and Trilinos for leading-edge portable/scalable linear algebra. We carried out a sensitivity analysis of various formulation properties and their effects on particle size distribution with Dakota, an open source software designed to exploit High-Performance Computing (HPC) capabilities of a massively parallel supercomputer. We developed wrappers to exchange information among these state-of-the-art tools for DPI.

Abstract Results of a flume experiment and numerical model of plastic accumulation in front of a plate are presented. The single phase CFD-DEM model formed a successful benchmark case to model plastic accumulation inside an ocean cleanup system. A fixed wooden plate was placed in a steady cross flow and plastic was released upstream of it. We recorded the evolution of the plastic accumulation profiles under slowly increasing plastic load. Experimental parameters were the flow velocity, draft of the plate (varying the plate Froude number) as well as three different types of plastic particles. The accumulation of oil in front of barriers and parallels to the phenomena of plastic accumulation were reviewed. As a second part of the project we used the open source CFD-DEM code CFDEM® to reproduce the flume experiment. It couples the discrete element method (DEM) software LIGGGHTS® and the open source computational fluid dynamics (CFD) software OpenFOAM®. A linear relationship of the relative depth of the accumulation with the Froude number of the plate was found for a given type of particle and reproduced in the numerical model. We identified limitations of the experimental setup, calibration experiments and the single phase CFD-DEM approach and outlined the steps for further research.

Abstract The motions of ice floes in linear waves were simulated by coupling Discrete Element Method (DEM) and Computational Fluid Dynamics (CFD). The interactions between ice floes are investigated by DEM. The hydrodynamics of ice floes, mainly including the drag force and the buoyancy, are calculated by CFD. In the simulation, the ice floes are treated as discrete elements, and the contact forces between ice floes are determined by the Hertz-Mindlin (no-slip) contact model. The shape of ice floes is an approximate square composed of a number of spherical faces, which can reduce the computation cost. The waves are treated as First Order Airy wave, which is linear in nature and applied to small amplitude waves in shallow liquid depth ranges. The volume of fluid (VOF) method was adopted to capture the free surface. The simulation results are in agreement with the actual situation to a certain extent.

Abstract Arctic shipping activities have been steadily increased due to global warming and economic benefit. As a result, there are more demands to estimate the ship performance in various ice conditions at the early design stage. In this paper, a CFD-DEM coupled approach was applied to estimate the ice resistance and broken ice behavior around the hull including hydrodynamic interactions. For the simulation, the moving ship in the stationary ice field was implemented using the overset grid technique. The estimated ice resistance as well as ice behaviors around the hull from the simulations were compared with model test results and underwater videos of the same vessel in a similar ice condition. The results demonstrated good agreement with model test measurements and further improvement for actual application was discussed in the paper.

Successful verification and validation is crucial to build confidence in the application of coupled Computational Fluid Dynamics - Discrete Element Method (CFD-DEM). Model verification includes ensuring a mesh-independent solution, which poses a major difficulty in CFD-DEM due to the complicated solution relationship with computational cell size. In this paper, we investigate the theoretical relationship between the solution and computational cell size by tracing the effects of a change in cell size through the mathematical model. The porosity profile for simulations of fixed-particle beds is determined to be Gaussian, and the average and standard deviation of the representative distribution are reported against cell size. We find the standard deviation of bed porosity increases exponentially as the cell size is reduced, and the drag calculations are very sensitive to changes in the porosity standard deviation, resulting in an exponential change in expected drag when the cell size is small relative to the particle diameter. The divided volume fraction method of porosity calculation is shown to be superior to the centred volume fraction method, as it reduces the porosity standard deviation. The sensitivity of five popular drag laws to changes in the porosity profile is presented, and the Ergun and Beetstra drag laws are shown to be the least sensitive to changes in the cell size.

Abstract Predicting accurate erosion rate due to sand particles in oil and gas production is important for maintaining safe and reliable operations while maximizing output efficiency. Computational Fluid Dynamic (CFD) is a powerful tool for erosion prediction as it provides detailed erosion pattern in complex geometry. In an effort to improve accuracy of erosion prediction, this paper proposes an algorithm to accurately represent particle shape in CFD erosion simulation through coupling with Discrete Element Method (DEM) for non-spherical shape particles. The fluid motions are predicted by CFD and the particle movements (including particle-particle and particle-wall collisions) and fluid-particle interaction are calculated using DEM. It is widely known that sand particles are of finite volume with a non-spherical shape, accurate representation of sand particles is important in CFD modelling for accurate prediction of erosion rate. Traditional CFD approach usages lagrangian tracking of sand particles through Discrete Phase Model (DPM), where a particle is assumed as a point mass for the calculation of trajectory and particle-wall interaction. Particle impact velocity and impact angle are important parameter in determining erosion. Assumption of point mass in DPM approach, will not capture particle-wall interaction accurately especially when particles are of non-spherical in shape. In additional, DPM approach ignores particle-particle interactions. This can adversary affect the accuracy of erosion predictions. Integrating non-spherical DEM collision algorithm with CFD erosion simulation, will overcome these limitations and improve erosion predictions. Benefits of this CFD-DEM erosion modelling was demonstrated for gas-solid flow in a 2" pipework which consists of out-of-plane elbows in series and blind-tees. Experimental dataset [1] for erosion pattern on each elbow was used to validate CFD predictions. Three different erosion CFD simulations were performed, traditional DPM based CFD simulation, CFD-DEM simulation for spherical shape particles and CFD-DEM simulation for non-spherical shape particles. CFD-DEM coupled simulations clearly show an improvement on erosion predictions compared to DPM based CFD simulation. Effect of non-spherical shape on rebound angle during particle-wall collision is captured accurately in CFD-DEM simulation. CFD-DEM simulation using non-spherical particle, was able to predict erosion pattern closer to experimental observations. This paper will demonstrate an increase in accuracy of sand erosion prediction by integrating DEM collision algorithm in CFD modelling. The prediction results of elbow erosion subject to a condition of dilute gas-particle flow are validated against experimental data. Improved prediction of erosion risk will increase the safety and reliability of oil & gas operations, while maximizing output efficiency.

Recent concerns regarding global warming and energy security have accelerated research and developmental efforts to produce biofuels from agricultural and forestry residues, and energy crops. Anaerobic digestion is a promising process for producing biogas-biofuel from biomass feedstocks. However, there is a need for new reactor designs and operating considerations to process fibrous biomass feedstocks. In this research project, the multiphase flow behavior of biomass particles was investigated. The objective was accomplished through both simulation and experimentation. The simulations included both particle-level and bulk flow simulations. Successful computational fluid dynamics (CFD) simulation of multiphase flow in the digester is dependent on the accuracy of constitutive models which describe (1) the particle phase stress due to particle interactions, (2) the particle phase dissipation due to inelastic interactions between particles and (3) the drag force between the fibres and the digester fluid. Discrete Element Method (DEM) simulations of Homogeneous Cooling Systems (HCS) were used to develop a particle phase dissipation rate model for non-spherical particle systems that was incorporated in a two-fluid CFDmultiphase flow model framework. Two types of frictionless, elongated particle models were compared in the HCS simulations: glued-sphere and true cylinder. A new model for drag for elongated fibres was developed which depends on Reynolds number, solids fraction, and fibre aspect ratio. Schulze shear test results could be used to calibrate particle-particle friction for DEM simulations. Several experimental measurements were taken for biomass particles like olive pulp, orange peels, wheat straw, semolina, and wheat grains. Using a compression tester, the breakage force, breakage energy, yield force, elastic stiffness and Young’s modulus were measured. Measurements were made in a shear tester to determine unconfined yield stress, major principal stress, effective angle of internal friction and internal friction angle. A liquid fludized bed system was used to determine critical velocity of fluidization for these materials. Transport measurements for pneumatic conveying were also assessed. Anaerobic digestion experiments were conducted using orange peel waste, olive pulp and wheat straw. Orange peel waste and olive pulp could be anaerobically digested to produce high methane yields. Wheat straw was not digestible. In a packed bed reactor, anaerobic digestion was not initiated above bulk densities of 100 kg/m³ for peel waste and 75 kg/m³ for olive pulp. Interestingly, after the digestion has been initiated and balanced methanogenesis established, the decomposing biomass could be packed to higher densities and successfully digested. These observations provided useful insights for high throughput reactor designs. Another outcome from this project was the development of low cost devices to measure methane content of biogas for off-line (US$37), field (US$50), and online (US$107) applications.