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•Solid–liquid mixing of viscous suspensions is investigated using CFD-DEM.•Fraction of suspended particles is measured numerically and compared to experiments.•Only the rolling and ...sliding friction affect the fraction of suspended particles.•For clearances of C=T/4 and C=T/3 a dead zone remains below the impeller.•Decreasing the clearance to C=T/5 removes the dead zone.
In chemical engineering, numerous processes require the suspension of particles in a laminar or transitional regime. For such operations, predicting the fraction of suspended particles as well as their distribution and homogeneity is a major concern. In this work, the unresolved CFD-DEM model introduced by our group for solid–liquid mixing is used to investigate the mixing dynamics of viscous suspensions. The techniques chosen to characterize the degree of suspension, the homogeneity and the distribution of the particles are presented. They are used to assess the efficiency of a pitched blade turbine with a clearance of C=T/4. The impact of solid properties on mixing dynamics is investigated by varying the Young's modulus, the coefficient of restitution and the sliding friction coefficient in the DEM model. Lastly, five alternative configurations of the mixing rig are investigated by varying the clearance of the impeller and introducing baffles.
•A semi-implicit finite volume immersed boundary method based is developed.•It is parallel and compatible with unstructured meshes and dynamic mesh refinement.•Its accuracy is similar to that of ...sliding mesh and single reference frame techniques.•It is flexible enough to simulate flows in stirred tanks with overlapping impellers.•It can be part of solid–liquid mixing CFD-DEM models.
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.
In this research, we investigate the influence of a load-balancing strategy and parametrization on the speed-up of discrete element method simulations using Lethe-DEM. Lethe-DEM is an open-source DEM ...code which uses a cell-based load-balancing strategy. We compare the computational performance of different cell-weighing strategies based on the number of particles per cell (linear and quadratic). We observe two minimums for particle to cell weights (at 3, 40 for quadratic, and 15, 50 for linear) in both linear and quadratic strategies. The first and second minimums are attributed to the suitable distribution of cell-based and particle-based functions, respectively. We use four benchmark simulations (packing, rotating drum, silo, and V blender) to investigate the computational performances of different load-balancing schemes (namely, single-step, frequent and dynamic). These benchmarks are chosen to demonstrate different scenarios that may occur in a DEM simulation. In a large-scale rotating drum simulation, which shows the systems in which particles occupy a constant region after reaching steady-state, single-step load-balancing shows the best performance. In a silo and V blender, where particles move in one direction or have a reciprocating motion, frequent and dynamic schemes are preferred. We propose an automatic load-balancing scheme (dynamic) that finds the best load-balancing steps according to the imbalance of computational load between the processes. Furthermore, we show the high computational performance of Lethe-DEM in the simulation of the packing of 108 particles on 4800 processes. We show that simulations with optimum load-balancing need ≈40% less time compared to the simulations with no load-balancing.
Sub-aerial (dry) and submerged dense granular collapses are studied by means of a three-phase unresolved computational fluid dynamics-discrete element method (CFD-DEM) numerical model. Physical ...experiments are also performed to provide data for validation and further analysis. Validations show good compatibility between the numerical and experimental results. Collapse mechanism as well as post-collapse morphological parameters, such as granular surface profile and runout distance, are analyzed. The spatiotemporal variation of solid volume fraction is also investigated. The effect granular column aspect ratio is studied and found to be a key factor in granular morphology for both submerged and dry conditions. The volume fraction analysis evolution shows an expansion and re-compaction trend, correlated with the granular movement.
Databases catalogue the corpus of research literature into scientific categories and report classes of bibliometric data such as the number of citations to articles, the number of authors, journals, ...funding agencies, institutes, references, etc. The number of articles and citations in a category are gauges of productivity and scientific impact but a quantitative basis to compare researchers between categories is limited. Here, we compile a list of bibliometric indicators for 236 science categories and citation rates of the 500 most cited articles of each category. The number of citations per paper vary by several orders of magnitude and are highest in multidisciplinary sciences, general internal medicine, and biochemistry and lowest in literature, poetry, and dance. A regression model demonstrates that citation rates to the top articles in each category increase with the square root of the number of articles in a category and decrease proportionately with the age of the references: articles in categories that cite recent research are also cited more frequently. The citation rate correlates positively with the number of funding agencies that finance the research. The category h-index correlates with the average number of cites to the top 500 ranked articles of each category (R2=0.997). Furthermore, only a few journals publish the top 500 cited articles in each category: four journals publish 60% (σ=±20%) of these and ten publish 81% (σ=±15%).
High-order Computational Fluid Dynamics (CFD) methods have the potential to deliver higher accuracy at lower computational cost than conventional second-order methods. In this work, we introduce ...Lethe, an object-oriented, open-source, high-order (space and time) CFD software which leverages the well-established deal.II library. Lethe solves incompressible flow problems on 2D and 3D unstructured quad and hex meshes and allows dynamic mesh adaptation. It is parallel and it is adapted to the solution of large problems (>108 degrees of freedom) on distributed computer architectures. We illustrate some of its fundamental capacities through two benchmarks: a manufactured solution and the DNS of Taylor–Green vortices at Re=1600.
We propose a novel method to calculate the local void fraction in solid–fluid Euler–Lagrange models in an attempt to better predict the behavior of multiphase flows in complex computational fluid ...dynamics–discrete element method (CFD-DEM) simulations. This method is efficient, continuous in both time and space, and valid in structured and unstructured meshes. Cheap and common methods such as the particle centroid method (PCM) are commonly used to determine the void fraction. When particles are partially located in a cell, their volume contribution is not evaluated accurately, leading to inaccurate prediction of the void fraction and, consequently, inaccurate simulations. This also affects the stability of the simulation and introduces constraints on the elements’ sizes as well as the time steps for the CFD and DEM. There are several analytical approaches that accurately determine the void fraction, but these methods might not be always feasible due to their high computational cost as well as their inability to properly function on unstructured meshes. The proposed method aims at resolving these problems. We demonstrate the capacity of this method using its implementation in Lethe, an open-source CFD-DEM software.
Experiments for the validation of unresolved CFD-DEM software for solid–liquid flows are often expensive, time consuming and generally provide little insight into the local particles dynamics. ...Additionally, several DEM parameters such as the particle surface properties are often obtained from experiments in air and used for all CFD-DEM simulations even when the fluid is a liquid. We design and perform a simple and time efficient gas–solid and liquid–solid rotary kiln benchmark for the purpose of creating a validation case for unresolved CFD-DEM software which we use to validate the unresolved CFD-DEM model of the open-source software Lethe. This case, which contains dense solid–solid contacts and strong solid–fluid forces gives insight on the importance of proper calibration of DEM surface properties in solid–liquid flows as well as on the importance of the lift force. Furthermore, it is highly sensitive to the accuracy of the CFD discretization.
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•CFD-DEM fidelity depends heavily on accurate calibration of DEM contact properties.•High-order CFD discretization results in mesh- and order-independent calibration and results.•Sliding friction coefficient increases with an increase in fluid’s viscosity.•Particle’s coefficient of restitution changes drastically between gas and liquids.•Magnus lift force plays a vital role in numerical predictions of correct regimes.
CFD-DEM is used to simulate solid–fluid systems. DEM models the motion of discrete particles while CFD models the dynamics of the fluid phase. Coupling both necessitates the calculation of the void ...fraction and the solid–fluid forces resulting in a computationally expensive method. Additionally, evaluating volume-averaged quantities locally restricts particle to cell size ratios limiting the accuracy of the CFD. To mitigate these limitations, we develop a unified finite element CFD-DEM solver which integrates the CFD and DEM solvers into a single software resulting in faster and cheaper coupling between the solvers. It supports dynamically load-balanced parallelization. This allows for more efficient simulations as load balancing ensures the even distribution of workloads among processors; thus, exploiting available resources efficiently. Our solver supports high order schemes; thus, allowing the use of larger elements enhancing the validity and stability of the void fraction schemes while achieving better accuracy. We verify and validate our CFD-DEM solver with a large array of test cases: particle sedimentation, a fluidized bed, the Rayleigh Taylor instability, and a spouted bed.
•A novel stabilized immersed boundary method for melting and freezing is proposed.•The method is robust and handles phase transition in small temperature intervals.•It preserves the second order ...accuracy of the stabilized finite element scheme.•It is validated by studying the melting of gallium in rectangular or prismatic cavity.•There are differences in the melted profile and Nusselt number between 2D and 3D.
Numerous processes in the automotive, additive manufacturing or energy storage industries require an accurate prediction of the solidification (freezing) and melting (thawing) dynamics of substances. The numerical modeling of these phase changes is highly complex because it includes sharp moving interfaces and strong discontinuities in the material properties. This complexity is often exacerbated by the occurrence of natural convection, which induces a strong coupling between the motion of the liquid and the position of the solid–liquid interface. This leads to strongly coupled non-linear thermo-fluid problems which have to be solved in complex geometries.
In this work, we introduce two novel stabilized finite element models to predict the phase change with natural convection. The first model uses a more classical viscosity approach to impose stasis in the solid region whereas the second one is based on an immersed boundary formulation to accurately describe the solid–fluid interface.
The efficiency of the stabilization is first demonstrated by studying the Stefan problem. The two approaches to impose stasis are then compared using 2D test cases before they are both used to study melting in a rectangular (2D) and prismatic (3D) cavity. Significant differences are observed in the flow profiles and the solid–liquid interface position between the 2D and the 3D simulations.
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