1,147 research outputs found
Direct simulation of liquid-gas-solid flow with a free surface lattice Boltzmann method
Direct numerical simulation of liquid-gas-solid flows is uncommon due to the
considerable computational cost. As the grid spacing is determined by the
smallest involved length scale, large grid sizes become necessary -- in
particular if the bubble-particle aspect ratio is on the order of 10 or larger.
Hence, it arises the question of both feasibility and reasonability. In this
paper, we present a fully parallel, scalable method for direct numerical
simulation of bubble-particle interaction at a size ratio of 1-2 orders of
magnitude that makes simulations feasible on currently available
super-computing resources. With the presented approach, simulations of bubbles
in suspension columns consisting of more than fully resolved
particles become possible. Furthermore, we demonstrate the significance of
particle-resolved simulations by comparison to previous unresolved solutions.
The results indicate that fully-resolved direct numerical simulation is indeed
necessary to predict the flow structure of bubble-particle interaction problems
correctly.Comment: submitted to International Journal of Computational Fluid Dynamic
Parallel load balancing strategy for Volume-of-Fluid methods on 3-D unstructured meshes
© 2016. This version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/l Volume-of-Fluid (VOF) is one of the methods of choice to reproduce the interface motion in the simulation of multi-fluid flows. One of its main strengths is its accuracy in capturing sharp interface geometries, although requiring for it a number of geometric calculations. Under these circumstances, achieving parallel performance on current supercomputers is a must. The main obstacle for the parallelization is that the computing costs are concentrated only in the discrete elements that lie on the interface between fluids. Consequently, if the interface is not homogeneously distributed throughout the domain, standard domain decomposition (DD) strategies lead to imbalanced workload distributions. In this paper, we present a new parallelization strategy for general unstructured VOF solvers, based on a dynamic load balancing process complementary to the underlying DD. Its parallel efficiency has been analyzed and compared to the DD one using up to 1024 CPU-cores on an Intel SandyBridge based supercomputer. The results obtained on the solution of several artificially generated test cases show a speedup of up to similar to 12x with respect to the standard DD, depending on the interface size, the initial distribution and the number of parallel processes engaged. Moreover, the new parallelization strategy presented is of general purpose, therefore, it could be used to parallelize any VOF solver without requiring changes on the coupled flow solver. Finally, note that although designed for the VOF method, our approach could be easily adapted to other interface-capturing methods, such as the Level-Set, which may present similar workload imbalances. (C) 2014 Elsevier Inc. Allrights reserved.Peer ReviewedPostprint (author's final draft
Numerical simulation of super-square patterns in Faraday waves
We report the first simulations of the Faraday instability using the full
three-dimensional Navier-Stokes equations in domains much larger than the
characteristic wavelength of the pattern. We use a massively parallel code
based on a hybrid Front-Tracking/Level-set algorithm for Lagrangian tracking of
arbitrarily deformable phase interfaces. Simulations performed in rectangular
and cylindrical domains yield complex patterns. In particular, a
superlattice-like pattern similar to those of [Douady & Fauve, Europhys. Lett.
6, 221-226 (1988); Douady, J. Fluid Mech. 221, 383-409 (1990)] is observed. The
pattern consists of the superposition of two square superlattices. We
conjecture that such patterns are widespread if the square container is large
compared to the critical wavelength. In the cylinder, pentagonal cells near the
outer wall allow a square-wave pattern to be accommodated in the center
Diffusion-Based Coarse Graining in Hybrid Continuum-Discrete Solvers: Theoretical Formulation and A Priori Tests
Coarse graining is an important ingredient in many multi-scale
continuum-discrete solvers such as CFD--DEM (computational fluid
dynamics--discrete element method) solvers for dense particle-laden flows.
Although CFD--DEM solvers have become a mature technique that is widely used in
multiphase flow research and industrial flow simulations, a flexible and
easy-to-implement coarse graining algorithm that can work with CFD solvers of
arbitrary meshes is still lacking. In this work, we proposed a new coarse
graining algorithm for continuum--discrete solvers for dense particle-laden
flows based on solving a transient diffusion equation. Via theoretical analysis
we demonstrated that the proposed method is equivalent to the statistical
kernel method with a Gaussian kernel, but the current method is much more
straightforward to implement in CFD--DEM solvers. \textit{A priori} numerical
tests were performed to obtain the solid volume fraction fields based on given
particle distributions, the results obtained by using the proposed algorithm
were compared with those from other coarse graining methods in the literature
(e.g., the particle centroid method, the divided particle volume method, and
the two-grid formulation). The numerical tests demonstrated that the proposed
coarse graining procedure based on solving diffusion equations is theoretically
sound, easy to implement and parallelize in general CFD solvers, and has
improved mesh-convergence characteristics compared with existing coarse
graining methods. The diffusion-based coarse graining method has been
implemented into a CFD--DEM solver, the results of which are presented in a
separate work (R. Sun and H. Xiao, Diffusion-based coarse graining in hybrid
continuum-discrete solvers: Application in CFD-DEM solvers for particle laden
flows)
ASHEE: a compressible, equilibrium-Eulerian model for volcanic ash plumes
A new fluid-dynamic model is developed to numerically simulate the
non-equilibrium dynamics of polydisperse gas-particle mixtures forming volcanic
plumes. Starting from the three-dimensional N-phase Eulerian transport
equations for a mixture of gases and solid particles, we adopt an asymptotic
expansion strategy to derive a compressible version of the first-order
non-equilibrium model, valid for low concentration regimes and small particles
Stokes . When the model reduces to the dusty-gas one. The
new model is significantly faster than the Eulerian model while retaining the
capability to describe gas-particle non-equilibrium. Direct numerical
simulation accurately reproduce the dynamics of isotropic turbulence in
subsonic regime. For gas-particle mixtures, it describes the main features of
density fluctuations and the preferential concentration of particles by
turbulence, verifying the model reliability and suitability for the simulation
of high-Reynolds number and high-temperature regimes. On the other hand,
Large-Eddy Numerical Simulations of forced plumes are able to reproduce their
observed averaged and instantaneous properties. The self-similar radial profile
and the development of large-scale structures are reproduced, including the
rate of entrainment of atmospheric air. Application to the Large-Eddy
Simulation of the injection of the eruptive mixture in a stratified atmosphere
describes some of important features of turbulent volcanic plumes, including
air entrainment, buoyancy reversal, and maximum plume height. Coarse particles
partially decouple from the gas within eddies, modifying the turbulent
structure, and preferentially concentrate at the eddy periphery, eventually
being lost from the plume margins due to the gravity. By these mechanisms,
gas-particle non-equilibrium is able to influence the large-scale behavior of
volcanic plumes.Comment: 29 pages, 22 figure
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