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  • Symmetry-preserving discretization of Navier-Stokes equations on collocated unstructured meshes

     Trias Miquel, Francesc Xavier; Lehmkuhl Barba, Oriol; Oliva Llena, Asensio; Perez Segarra, Carlos David; Verstappen, R.W.C.P.
    Journal of computational physics
    Date of publication: 2014-02-01
    Journal article

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    A fully-conservative discretization is presented in this paper. The same principles followed by Verstappen and Veldman (2003) are generalized for unstructured meshes. Here, a collocated-mesh scheme is preferred over a staggered one due to its simpler form for such meshes. The basic idea behind this approach remains the same: mimicking the crucial symmetry properties of the underlying differential operators, i.e., the convective operator is approximated by a skew-symmetric matrix and the diffusive operator by a symmetric, positive-definite matrix. A novel approach to eliminate the checkerboard spurious modes without introducing any non-physical dissipation is proposed. To do so, a fully-conservative regularization of the convective term is used. The supraconvergence of the method is numerically showed and the treatment of boundary conditions is discussed. Finally, the new discretization method is successfully tested for a buoyancy-driven turbulent flow in a differentially heated cavity.

  • Numerical investigation on the role of discrete element method in combined LBM-IBM-DEM modeling

     Zhang, Hao; Tan, Yuanqiang; Shu, Shi; niu, Xiaodong; Trias Miquel, Francesc Xavier; Yang, Dongmin; Li, Hao; Sheng, Yong
    Computers and fluids
    Date of publication: 2014-05-01
    Journal article

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    Particle collisions play a very important role in determining the fluid-particle multiphase flow, and thus it is crucial to treat the particle-particle interaction using a felicitous method in numerical simulations. A novel combined lattice Boltzmann method (LBM)-immersed boundary method (IBM)-discrete element method (DEM) scheme is presented in this study with its application to model the sedimentation of 2D circular particles in incompressible Newtonian flows. The hydrodynamic model of the incompressible Newtonian flow is based on the Bhatnagar-Gross-Krook LBM, and a momentum exchange-based IBM is adopted to calculate the fluid-solid interaction force. The kinematics and trajectory of the discrete particles are evaluated by DEM, in which the particle-particle interaction rules are governed by theoretical contact mechanics to enable the direct use of real particle properties. This eliminates the need of artificial parameters and also improves the reliability of the numerical results. By using a more accurate and physical description of particle interaction, a 'safe zone' or threshold is also no longer required. Case studies of single particle settling in a cavity, and two particles settling in a channel were carried out, the velocity characteristics of the particle during settling and near the bottom were examined. A numerical example of sedimentation involving 504 particles was finally presented to demonstrate the capability of the combined scheme.

    Particle collisions play a very important role in determining the fluid-particle multiphase flow, and thus it is crucial to treat the particle-particle interaction using a felicitous method in numerical simulations. A novel combined lattice Boltzmann method (LBM)-immersed boundary method (IBM)-discrete element method (DEM) scheme is presented in this study with its application to model the sedimentation of 2D circular particles in incompressible Newtonian flows. The hydrodynamic model of the incompressible Newtonian flow is based on the Bhatnagar-Gross-Krook LBM, and a momentum exchange-based IBM is adopted to calculate the fluid-solid interaction force. The kinematics and trajectory of the discrete particles are evaluated by DEM, in which the particle-particle interaction rules are governed by theoretical contact mechanics to enable the direct use of real particle properties. This eliminates the need of artificial parameters and also improves the reliability of the numerical results. By using a more accurate and physical description of particle interaction, a 'safe zone' or threshold is also no longer required. Case studies of single particle settling in a cavity, and two particles settling in a channel were carried out, the velocity characteristics of the particle during settling and near the bottom were examined. A numerical example of sedimentation involving 504 particles was finally presented to demonstrate the capability of the combined scheme.

  • A simple approach to discretize the viscous term with spatially varying (eddy-)viscosity

     Trias Miquel, Francesc Xavier; Gorobets, Andrei; Oliva Llena, Asensio
    Journal of computational physics
    Date of publication: 2013-11-15
    Journal article

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    A simple approach to discretize the viscous dissipation term in the incompressible Navier¿Stokes equations with spatially varying viscosity is presented. Unlike the case where the viscosity remains constant, its discretization may be quite cumbersome especially for high-order staggered formulations. To circumvent this problem, we propose an alternative form of the viscous term whose discretization is straightforward. Notice that this approach is also suitable for eddy-viscosity models for Large-Eddy Simulation. Moreover, since it is based on already available operators, it can be easily implemented on any structured or unstructured code. The (supra)convergence of the method is numerically shown on both a fourth-order Cartesian staggered and an unstructured collocated formulation.

    A simple approach to discretize the viscous dissipation term in the incompressible Navier–Stokes equations with spatially varying viscosity is presented. Unlike the case where the viscosity remains constant, its discretization may be quite cumbersome especially for high-order staggered formulations. To circumvent this problem, we propose an alternative form of the viscous term whose discretization is straightforward. Notice that this approach is also suitable for eddy-viscosity models for Large-Eddy Simulation. Moreover, since it is based on already available operators, it can be easily implemented on any structured or unstructured code. The (supra)convergence of the method is numerically shown on both a fourth-order Cartesian staggered and an unstructured collocated formulation.

  • Parallel algorithms for Sn transport sweeps on unstructured meshes

     Colomer Rey, Guillem; Borrell Pol, Ricard; Trias Miquel, Francesc Xavier; Rodriguez Pérez, Ivette Maria
    Journal of computational physics
    Date of publication: 2013-01-01
    Journal article

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    The Boltzmann Transport Equation is solved on unstructured meshes using the Discrete Ordinates Method. The flux for each ordinate is swept across the computational grid, within a source iteration loop that accounts for the coupling between the different ordinates. In this paper, a spatial domain decomposition strategy is used to divide the work among the available CPUs. The sequential nature of the sweep process makes the parallelization of the overall algorithm the most challenging aspect. Several parallel sweep algorithms, which represent different options of interleaving communications and calculations in the solution process, are analysed. The option of grouping messages by means of buffering is also considered. One of the heuristics proposed consistently stands out as the best option in all the situations analyzed, which include different geometries and different sizes of the ordinate set. With this algorithm, good scalability results have been achieved regarding both weak and strong speedup tests with up to 2560 CPUs.

  • Conservation properties of unstructured finite-volume mesh schemes for the Navier-Stokes equations

     Jofre Cruanyes, Lluís; Lehmkuhl Barba, Oriol; Ventosa Molina, Jordi; Trias Miquel, Francesc Xavier; Oliva Llena, Asensio
    Numerical heat transfer. Part B, fundamentals
    Date of publication: 2013-11-09
    Journal article

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    The Navier-Stokes equations describe fluid flow by conserving mass and momentum. There are two main mesh discretizations for the computation of these equations, the collocated and staggered schemes. Collocated schemes locate the velocity field at the same grid points as the pressure one, while staggered discretizations locate variables at different points within the mesh. One of the most important characteristic of the discretization schemes, aside from accuracy, is their capacity to discretely conserve kinetic energy, specially when solving turbulent flow. Hence, this work analyzes the accuracy and conservation properties of two particular collocated and staggered schemes by solving various problems.

  • Direct and large-eddy simulation of non-oberbeck-boussinesq effects in a turbulent differentially heated cavity

     Kizildag, Deniz; Trias Miquel, Francesc Xavier; Rodriguez Pérez, Ivette Maria; Oliva Llena, Asensio
    ERCOFTAC Workshop on Direct and Large-Eddy Simulation
    Presentation's date: 2013-04-04
    Presentation of work at congresses

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  • DNS and regularization modeling of a turbulent differentially heated cavity of aspect ratio 5

     Trias Miquel, Francesc Xavier; Gorobets, Andrei; Perez Segarra, Carlos David; Oliva Llena, Asensio
    International journal of heat and mass transfer
    Date of publication: 2013-01-15
    Journal article

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  • A parallel MPI+OpenMP+OpenCL algorithm for hybrid supercomputations of incompressible flows

     Gorobets, Andrei; Trias Miquel, Francesc Xavier; Oliva Llena, Asensio
    Computers and fluids
    Date of publication: 2013-12
    Journal article

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    The work is devoted to the development of efficient parallel algorithms for large-scale simulations of incompressible flows on hybrid supercomputers based on massively-parallel accelerators. The governing equations are discretized using a high-order finite-volume scheme for Cartesian staggered meshes with the only restriction that, at least, one direction is periodic. Its ¿classical¿ MPI + OpenMP parallel implementation for CPUs was designed to scale till 100,000 CPU cores. The new hybrid algorithm is developed on a base of a multi-level parallel model that exploits several layers of parallelism of a modern hybrid supercomputer. In this model, MPI and OpenMP are used on the first two levels to couple nodes of a supercomputer and to engage its CPU cores. Then, computing accelerators are further used by means of the hardware independent OpenCL computing standard. In this way, the implementation is adapted to a general computing model with central processors and math co-processors. In this paper the work is focused on adapting the basic operations of the algorithm to architectures of Graphics Processing Units (GPU) without considering the multi-GPU communication scheme. Technology of porting the code to OpenCL is described, certain optimization approaches are presented and relevant performance results obtaining up to 80¿90 GFLOPS on a GPU accelerator are demonstrated. Moreover, the experience with different GPU architectures is summarized and a comparison based on the particular application is given for AMD and NVIDIA GPUs as well as for CUDA and OpenCL frameworks.

  • An openCL-based parallel CFD code for simulations on hybrid systems with massively-parallel accelerators

     Gorobets, Andrei; Trias Miquel, Francesc Xavier; Oliva Llena, Asensio
    Procedia engineering
    Date of publication: 2013
    Journal article

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    A parallel finite-volume CFD algorithm for modeling of incompressible flows on hybrid supercomputers is presented. It is based on a symmetry-preserving high-order numerical scheme for structured meshes. A multilevel approach that combines different parallel models is used for large-scale simulations on computing systems with massively-parallel accelerators. MPI is used on the first level within the distributed memory model to couple computing nodes of a supercomputer. On the second level OpenMP is used to engage multiple CPU cores of a computing node. The third level exploits the computing potential of massively-parallel accelerators such as GPU (Graphics Processing Units) of AMD and NVIDIA, or Intel Xeon Phi accelerators of the MIC (Many Integrated Core) architecture. The hardware independent OpenCL standard is used to compute on accelerators of different archi- tectures within a general model for a combination of a central processor and a math co-processor.

  • Blending regularization and large-eddy simulation. From homogeneous isotropic turbulence to wind farm boundary layers

     Folch Flórez, David; Trias Miquel, Francesc Xavier; Gorobets, Andrei; Oliva Llena, Asensio
    European Turbulence Conference
    Presentation's date: 2013-08
    Presentation of work at congresses

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    The incompressible Navier-Stokes equations form an excellent mathematical model for turbulent flows. However, direct simulations at high Reynolds numbers are not feasible because the convective term produces far too many relevant scales of motion. Therefore, in the foreseeable future numerical simulations of turbulent flows will have to resort to models of the small scales. Large-eddy simulation (LES) and regularization models are examples thereof. In the present work, we propose to combine both approaches. Restoring the Galilean invariance of the regularization method results into an additional hyperviscosity term. This approach provides a natural blending between regularization and LES. The performance of these recent improvements will be assessed through application to homogeneous isotropic turbulence, a turbulent channel flow and a wind-farm turbulent boundary layer.

    The incompressible Navier-Stokes equations form an excellent mathematical model for turbulent flows. However, direct simulations at high Reynolds numbers are not feasible because the convective term produces far too many relevant scales of motion. Therefore, in the foreseeable future numerical simulations of turbulent flows will have to resort to models of the small scales. Large-eddy simulation (LES) and regularization models are examples thereof. In the present work, we propose to combine both approaches. Restoring the Galilean invariance of the regularization method results into an additional hyperviscosity term. This approach provides a natural blending between regularization and LES. The performance of these recent improvements will be assessed through application to homogeneous isotropic turbulence, a turbulent channel flow and a wind-farm turbulent boundary layer.

  • New differential operators and discretization methods for eddy-viscosity models for LES

     Trias Miquel, Francesc Xavier; Gorobets, Andrei; Zhang, Hao; Oliva Llena, Asensio
    International Conference on Parallel Computational Fluid Dynamics
    Presentation's date: 2013-05-22
    Presentation of work at congresses

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    The incompressible Navier-Stokes equations constitute an excellent mathematical modelization of turbulence. Unfortunately, at- tempts at performing direct numerical simulations (DNS) are limited to relatively low-Reynolds numbers. Therefore, dynamically less complex mathematical formulations are necessary for coarse-grain simulations. Eddy-viscosity models for Large-Eddy Sim- ulation (LES) is an example thereof: they rely on differential operators that should be able to capture well different flow config- urations (laminar and 2D flows, near-wall behavior, transitional regime...). In the present work, several differential operators are derived from the criterion that vortex-stretching mechanism must stop at the smallest grid scale. Moreover, since the discretization errors may play an important role a novel approach to discretize the viscous term with spatially varying eddy-viscosity is used. It is based on basic operators; therefore, the implementation is straightforward even for staggered formulations. The performance of the proposed models will be assessed by means of direct comparison to DNS reference results.

  • An OpenCL-based parallel CFD code for simulations on hybrid systems with massively-parallel accelerators

     Gorobets, Andrei; Trias Miquel, Francesc Xavier; Oliva Llena, Asensio
    International Conference on Parallel Computational Fluid Dynamics
    Presentation's date: 2013-05-23
    Presentation of work at congresses

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  • Progress on eddy-viscosity models for LES: new differential operators and discretization methods

     Trias Miquel, Francesc Xavier; Verstappen, Roel; Gorobets, Andrei; Oliva Llena, Asensio
    European Turbulence Conference
    Presentation's date: 2013-09-04
    Presentation of work at congresses

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    The incompressible Navier-Stokes equations constitute an excellent mathematical modelization of turbulence. Unfortunately, attempts at performing direct simulations are limited to relatively low-Reynolds numbers. Therefore, dynamically less complex mathematical formulations are necessary for coarse-grain simulations. Eddy-viscosity models for Large-Eddy Simulation (LES) is an example thereof: they rely on differential operators that should be able to capture well different flow configurations (laminar and 2D flows, near-wall behavior, transitional regime...). In the present work, several differential operators are derived from the criterion that vortex-stretching mechanism must stop at the smallest grid scale. Moreover, since the discretization errors may play a n important role a novel approach to discretize the viscous term with spatially varying eddy-viscosity is used. It is based on basic operators; therefore, the implementation is straightforward even for staggered formulations.

  • Acumulador de energía térmica en base a materiales de cambio de fase sólido-líquido y método de fabricación de la unidad

     Oliva Llena, Asensio; Perez Segarra, Carlos David; Rigola Serrano, Joaquim; Castro Gonzalez, Jesus; Oliet Casasayas, Carles; Rodriguez Pérez, Ivette Maria; Lehmkuhl Barba, Oriol; Trias Miquel, Francesc Xavier; Capdevila Paramio, Roser; Alba Queipo, Ramiro; Ordoño Martinez, Manuel Miguel; Morales Ruiz, Sergio
    Date of request: 2013-03-01
    Invention patent

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  • Máquina de absorción refrigerada por aire

     Oliva Llena, Asensio; Perez Segarra, Carlos David; Rigola Serrano, Joaquim; Castro Gonzalez, Jesus; Oliet Casasayas, Carles; Rodriguez Pérez, Ivette Maria; Lehmkuhl Barba, Oriol; Trias Miquel, Francesc Xavier; Capdevila Paramio, Roser; Alba Queipo, Ramiro; Ordoño Martinez, Manuel Miguel; Farnos Baulenas, Joan
    Date of request: 2013-06-14
    Invention patent

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  • Numerical simulation of turbulence at lower cost: regularization modeling

     Trias Miquel, Francesc Xavier; Gorobets, Andrey; Perez Segarra, Carlos David; Oliva Llena, Asensio
    Computers and fluids
    Date of publication: 2012-03-13
    Journal article

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  • Spectrally-consistent regularization modeling of turbulence and its connections with LES

     Trias Miquel, Francesc Xavier
    Connections Between Regularized and Large-Eddy Simulation Methods for Turbulence
    Presentation's date: 2012-05-13
    Presentation of work at congresses

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  • An eulerian-lagrangian modeling of fluidized bed

     Zhang, Hao; Yuan-Qiang, Tan; Dong-Min, Yang; Trias Miquel, Francesc Xavier; Sheng, Yong; Oliva Llena, Asensio
    National Conference on Computational Mechanics of Granular Materials
    Presentation's date: 2012-09-17
    Presentation of work at congresses

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  • Eulerian-Lagrangian modeling of fluidized bed

     Tan, Y; Zhang, Hao; Yang, D; Trias Miquel, Francesc Xavier; Sheng, Y.; Oliva Llena, Asensio
    National Conference on Computational Mechanics of Granular Materials
    Presentation's date: 2012-06-14
    Presentation of work at congresses

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  • Sistema de almacenamiento de energía térmica combinando material sólido de calor sensible y material de cambio de fase

     Oliva Llena, Asensio; Perez Segarra, Carlos David; Rigola Serrano, Joaquim; Castro Gonzalez, Jesus; Oliet Casasayas, Carles; Rodriguez Pérez, Ivette Maria; Lehmkuhl Barba, Oriol; Trias Miquel, Francesc Xavier; Capdevila Paramio, Roser; Galione Klot, Pedro Andres
    Date of request: 2012-12-27
    Invention patent

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  • Spectrally-consistent regularization modeling at very high Rayleigh numbers

     Trias Miquel, Francesc Xavier; Verstappen, R.W.C.P.; Gorobets, Andrei; Oliva Llena, Asensio
    International Symposium on Turbulence, Heat and Mass Transfer
    Presentation's date: 2012-09-19
    Presentation of work at congresses

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    The incompressible Navier-Stokes equations constitute an excellent mathematical modelization of turbulence. Unfortunately, attempts at performing direct simulations are limited to relatively low-Rayleigh numbers because of the almost numberless small scales produced by the non-linear convective term. Alternatively, a dynamically less complex formulation is proposed here. Namely, regularizations of the Navier-Stokes equations that preserve the symmetry and conservation properties exactly. To do so, both convective and diffusive term are altered in the same vein. In this way, the convective production of small scales is effectively restrained whereas the modified diffusive term introduces a hyperviscosity effect and consequently enhances the destruction of small scales. In practice, the only additional ingredient is a selfadjoint linear filter whose local filter length is determined from the requirement that vortex-stretching must stop at the smallest grid scale. The performance of the novel regularization modeling approach is assessed through application to turbulent natural convection flows at very high Rayleigh numbers

  • Towards the regularization modeling of wind farm boundary layers

     Trias Miquel, Francesc Xavier; Folch, David; Gorobets, Andrey; Oliva Llena, Asensio
    EUROMECH Colloquium
    Presentation's date: 2012-02-15
    Presentation of work at congresses

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    Spectrally-consistent regularization modeling of turbulent natural convection flows  Open access

     Trias Miquel, Francesc Xavier; Verstappen, R.W.C.P.; Oliva Llena, Asensio; Gorobets, Andrei
    European Thermal Sciences Conference
    Presentation's date: 2012-09-05
    Presentation of work at congresses

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    The incompressible Navier-Stokes equations constitute an excellent mathematical modelization of turbulence. Unfortunately, attempts at performing direct simulations are limited to relatively low-Reynolds numbers because of the almost numberless small scales produced by the non-linear convective term. Alternatively, a dynamically less complex formulation is proposed here. Namely, regularizations of the Navier-Stokes equations that preserve the symmetry and conservation properties exactly. To do so, both convective and diffusive terms are altered in the same vein. In this way, the convective production of small scales is effectively restrained whereas the modified diffusive term introduces a hyperviscosity effect and consequently enhances the destruction of small scales. In practice, the only additional ingredient is a self-adjoint linear filter whose local filter length is determined from the requirement that vortex-stretching must stop at the smallest grid scale. In the present work, the performance of the above-mentioned recent improvements is assessed through application to turbulent natural convection flows by means of comparison with DNS reference data.

    The incompressible Navier-Stokes equations constitute an excellent mathematical modelization of turbulence. Unfortunately, attempts at performing direct simulations are limited to relatively low-Reynolds numbers because of the almost numberless small scales produced by the non-linear convective term. Alternatively, a dynamically less complex formulation is proposed here. Namely, regularizations of the Navier-Stokes equations that preserve the symmetry and conservation properties exactly. To do so, both convective and diffusive terms are altered in the same vein. In this way, the convective production of small scales is effectively restrained whereas the modified diffusive term introduces a hyperviscosity effect and consequently enhances the destruction of small scales. In practice, the only additional ingredient is a self-adjoint linear filter whose local filter length is determined from the requirement that vortex-stretching must stop at the smallest grid scale. In the present work, the performance of the above-mentioned recent improvements is assessed through application to turbulent natural convection flows by means of comparison with DNS reference data.

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    Spectrally-consistent regularization modeling of wind farm boundary layers  Open access

     Trias Miquel, Francesc Xavier; Gorobets, Andrey; Folch, David; Oliva Llena, Asensio
    Conference on Modelling Fluid Flow
    Presentation's date: 2012-09-06
    Presentation of work at congresses

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    The incompressible Navier-Stokes equations constitute an excellent mathematical modelization of turbulence. Unfortunately, attempts at performing direct simulations are limited to relatively low-Reynolds numbers because of the almost numberless small scales produced by the non-linear convective term. Alternatively, a dynamically less complex formulation is proposed here. Namely, regularizations of the Navier-Stokes equations that preserve the symmetry and conservation properties exactly. To do so, both convective and diffusive term are altered in the same vein. In this way, the convective production of small scales is effectively restrained whereas the modified diffusive term introduces a hyperviscosity effect and consequently enhances the destruction of small scales. In practise, the only additional ingredient is a self-adjoint linear filter whose local filter length is determined from the requirement that vortex-stretching must stop at the smallest grid scale. In the present work, the performance of the above-mentioned recent improvements is assessed through application to homogeneous isotropic turbulence, a turbulent channel flow and a turbulent boundary layer. As a final application, regularization modelling will be applied for large-scale numerical simulation of the atmospheric boundary layer through wind farms.

  • DNS and regularization modeling of a turbulent flow through a square duct up to Re _tau = 1200

     Trias Miquel, Francesc Xavier; Gorobets, Andrei; Zhang, Hao; Oliva Llena, Asensio
    International Conference on Parallel Computational Fluid Dynamics
    Presentation's date: 2012-05-20
    Presentation of work at congresses

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  • A parallel OpenCL-based solver for large-scale DNS of incompressible flows on heterogenous systems

     Trias Miquel, Francesc Xavier; Gorobets, Andrei; Oliva Llena, Asensio
    International Conference on Parallel Computational Fluid Dynamics
    Presentation's date: 2012-05-20
    Presentation of work at congresses

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    Numerical investigation of the location of maximum erosive wear damage in elbow: Effect of slurry velocity, bend orientation and angle of elbow  Open access

     Zhang, Hao; Tan, Yuanqiang; Yang, Dongmin; Trias Miquel, Francesc Xavier; Jiang, Shengqiang; Sheng, Yong; Oliva Llena, Asensio
    Powder technology
    Date of publication: 2011-02-01
    Journal article

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  • Hybrid MPI+OpenMP parallelization of an FFT-based 3D Poisson solver with one periodic direction

     Gorobets, Andrey; Trias Miquel, Francesc Xavier; Borrell Pol, Ricard; Lehmkuhl Barba, Oriol; Oliva Llena, Asensio
    Computers and fluids
    Date of publication: 2011-10
    Journal article

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    This work is devoted to the development of efficient parallel algorithms for the direct numerical simulation (DNS) of incompressible flows on modern supercomputers. In doing so, a Poisson equation needs to be solved at each time-step to project the velocity field onto a divergence-free space. Due to the non-local nature of its solution, this elliptic system is the part of the algorithm that is most difficult to parallelize. The Poisson solver presented here is restricted to problems with one uniform periodic direction. It is a combination of a block preconditioned Conjugate Gradient (PCG) and an FFT diagonalization. The latter decomposes the original system into a set of mutually independent 2D systems that are solved by means of the PCG algorithm. For the most ill-conditioned systems, that correspond to the lowest Fourier frequencies, the PCG is replaced by a direct Schur-complement based solver. The previous version of the Poisson solver was conceived for single-core (also dual-core) processors and therefore, the distributed memory model with message-passing interface (MPI) was used. The irruption of multi-core architectures motivated the use of a two-level hybrid MPI + OpenMP parallelization with the shared memory model on the second level. Advantages and implementation details for the additional OpenMP parallelization are presented and discussed in this paper. Numerical experiments show that, within its range of efficient scalability, the previous MPI-only parallelization is slightly outperformed by the MPI + OpenMP approach. But more importantly, the hybrid parallelization has allowed to significantly extend the range of efficient scalability. Here, the solver has been successfully tested up to 12800 CPU cores for meshes with up to 109 grid points. However, estimations based on the presented results show that this range can be potentially stretched up until 200,000 cores approximately. Finally, several examples of DNS simulations are briefly presented to illustrate some potential applications of the solver.

  • On the construction of discrete filters for symmetry-preserving regularization models

     Trias Miquel, Francesc Xavier; Verstappen, R.W.C.P.
    Computers and fluids
    Date of publication: 2011-01
    Journal article

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  • DNS and RANS modelling of a turbulent plane impinging jet

     Jaramillo Ibarra, Julian Ernesto; Trias Miquel, Francesc Xavier; Gorobets, Andrey; Perez Segarra, Carlos David; Oliva Llena, Asensio
    International journal of heat and mass transfer
    Date of publication: 2011-01-31
    Journal article

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  • Parallel direct Poisson solver for discretisations with one Fourier diagonalisable direction

     Borrell Pol, Ricard; Lehmkuhl Barba, Oriol; Trias Miquel, Francesc Xavier; Oliva Llena, Asensio
    Journal of computational physics
    Date of publication: 2011-06-01
    Journal article

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  • A self-adaptive strategy for the time integration of Navier-Stokes equations

     Trias Miquel, Francesc Xavier; Lehmkuhl Barba, Oriol
    Numerical heat transfer. Part B, fundamentals
    Date of publication: 2011-08
    Journal article

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    An efficient self-adaptive strategy for the explicit time integration of Navier-Stokes equations is presented. Unlike the conventional explicit integration schemes, it is not based on a standard CFL condition. Instead, the eigenvalues of the dynamical system are analytically bounded and the linear stability domain of the time-integration scheme is adapted in order to maximize the time step. The method works independently of the underlying spatial mesh; therefore, it can be easily integrated into structured or unstructured codes. The additional computational cost is minimal, and a significant increase of the time step is achieved without losing accuracy. The effectiveness and robustness of the method are demonstrated on both a Cartesian staggered and an unstructured collocated formulation. In practice, CPU cost reductions up to more than 4 with respect to the conventional approach have been measured.

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    Symmetry-preserving regularization of wall-bounded turbulent flows  Open access

     Trias Miquel, Francesc Xavier; Gorobets, Andrey; Verstappen, Roel; Oliva Llena, Asensio
    Journal of physics: conference series
    Date of publication: 2011
    Journal article

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    The incompressible Navier-Stokes equations constitute an excellent mathematical modelization of turbulence. Unfortunately, attempts at performing direct simulations are limited to relatively low-Reynolds numbers because of the almost numberless small scales produced by the non-linear convective term. Alternatively, a dynamically less complex formulation is proposed here. Namely, regularizations of the Navier-Stokes equations that preserve the symmetry and conservation properties exactly. To do so, both convective and diffusive term are altered in the same vein. In this way, the convective production of small scales is effectively restrained whereas the modified diffusive term introduces an hyper-viscosity effect and consequently enhances the destruction of small scales. In practice, the only additional ingredient is a self-adjoint linear filter whose local filter length is determined from the requirement that vortex-stretching must stop at the smallest grid scale. To do so, a new criterion based on the invariants of the local strain tensor is proposed here. Altogether, the proposed method constitutes a parameter-free turbulence model.

    The incompressible Navier-Stokes equations constitute an excellent mathematical modelization of turbulence. Unfortunately, attempts at performing direct simulations are limited to relatively low-Reynolds numbers because of the almost numberless small scales produced by the non-linear convective term. Alternatively, a dynamically less complex formulation is proposed here. Namely, regularizations of the Navier-Stokes equations that preserve the symmetry and conservation properties exactly. To do so, both convective and diffusive term are altered in the same vein. In this way, the convective production of small scales is effectively restrained whereas the modified diffusive term introduces an hyper-viscosity effect and consequently enhances the destruction of small scales. In practice, the only additional ingredient is a self-adjoint linear filter whose local filter length is determined from the requirement that vortex-stretching must stop at the smallest grid scale. To do so, a new criterion based on the invariants of the local strain tensor is proposed here. Altogether, the proposed method constitutes a parameter-free turbulence model.

  • FI-2011-3-0019: Exploring new frontiers in turbulent natural convection flows

     Trias Miquel, Francesc Xavier
    Participation in a competitive project

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  • DESARROLLO CODIGOS Y ALGORITMOS PARALELOS ALTA PRESTACIONES FINES DISEÑO OPTIMIZADO SISTEMAS Y EQUIPOS TERMICOS

     Perez Segarra, Carlos David; Trias Miquel, Francesc Xavier; Capdevila Paramio, Roser; Chiva Segura, Jorge; Gorobets, Andrey; Giraldez Garcia, Hector; Ventosa Molina, Jordi; Oliva Llena, Asensio
    Participation in a competitive project

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  • FI-2011-2-0011: Exploring new frontiers in turbulent natural convection flows

     Trias Miquel, Francesc Xavier
    Participation in a competitive project

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  • FI-2011-1-0013: Regularization Modeling of Turbulent Natural Convection Flows

     Trias Miquel, Francesc Xavier
    Participation in a competitive project

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  • Access to the full text
    FFT-based Poisson Solver for large scale numerical simulations of incompressible flows  Open access

     Borrell Pol, Ricard; Lehmkuhl Barba, Oriol; Trias Miquel, Francesc Xavier; Oyarzun Altamirano, Guillermo; Oliva Llena, Asensio
    International Conference on Parallel Computational Fluid Dynamics
    Presentation's date: 2011-05-16
    Presentation of work at congresses

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    In the context of time-accurate numerical simulation of incompressible flows, a Poisson equation needs to be solved at least once per time-step to project the velocity field onto a divergence-free space. Due to the non-local nature of its solution, this elliptic system is one of the most time consuming and difficult to parallelise parts of the code. In this paper, a parallel direct Poisson solver restricted to problems with one uniform periodic direction is presented. It is a combination of a Direct Schur-complement based Decomposition (DSD) and a Fourier diagonalisation. The latter decomposes the original system into a set of mutually independent 2D systems which are solved by means of the DSD algorithm. Since no restrictions are imposed in the non-periodic directions, the overall algorithm is well-suited for solving problems discretised on extruded 2D unstructured meshes. A new overall parallelisation strategy with respect to our earlier works is presented. This has allowed us to solve discrete Poisson equations with up to 109 grid points in less than half a second, using up to 8192 CPU cores of the MareNostrum Supercomputer.

    Postprint (author’s final draft)

  • Symmetry-preserving regularization of wall-bounded turbulent flows

     Trias Miquel, Francesc Xavier; Gorobets, Andrey; Verstappen, R.W.C.P.; Oliva Llena, Asensio
    European Turbulence Conference
    Presentation's date: 2011-09-12
    Presentation of work at congresses

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    The incompressible Navier-Stokes equations constitute an excellent mathematical modelization of turbulence. Unfortunately, attempts at performing direct simulations are limited to relatively low-Reynolds numbers because of the almost numberless small scales produced by the non-linear convective term. Alternatively, a dynamically less complex formulation is proposed here. Namely, regularizations of the Navier-Stokes equations that preserve the symmetry and conservation properties exactly. To do so, both convective and diffusive term are altered in the same vein. In this way, the convective production of small scales is effectively restrained whereas the modified diffusive term introduces an hyper-viscosity effect and consequently enhances the destruction of small scales. In practice, the only additional ingredient is a self-adjoint linear filter whose local filter length is determined from the requirement that vortex-stretching must stop at the smallest grid scale. To do so, a new criterion based on the invariants of the local strain tensor is proposed here. Altogether, the proposed method constitutes a parameter-free turbulence model.

  • Effect of mesh partition on the scalability of the parallel solution of the radiative transfer equation

     Colomer Rey, Guillem; Borrell Pol, Ricard; Trias Miquel, Francesc Xavier; Oliva Llena, Asensio
    International Conference on Computational Heat and Mass Transfer
    Presentation's date: 2011-07-18
    Presentation of work at congresses

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    The radiative transfer equation is solved in parallel using spatial domain decomposition. The effect of several kinds of mesh partitioning strategies is analyzed in this work. Geometries discretized with both structured and unstructured meshes have been considered, and significant differences in performance have been observed in both cases.

  • Hybrid MPI+OpenMP parallelization of a Navier-Stokes solver for large-scale DNS

     Gorobets, Andrey; Trias Miquel, Francesc Xavier; Borrell Pol, Ricard; Soria Guerrero, Manel; Oliva Llena, Asensio
    International Conference on Computational Heat and Mass Transfer
    Presentation's date: 2011-07-07
    Presentation of work at congresses

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    This work presents a parallel Navier-Stokes solver for the large-scale direct numerical simulation (DNS) of incompressible flows with one periodic direction. It is based on a scalable Poisson solver that combines an FFT diagonalization with a preconditioned conjugate gradient (PCG) method and a direct Schurcomplement based method. Modern architectures of supercomputers with multi-core nodes motivate the use of a two-level hybrid MPI+OpenMP parallelization. The use of the more complex two-level approach has allowed to significantly extend the number of CPUs the solver can efficiently use. Here, the solver has been successfully tested on up to 12800 CPU cores for meshes with up to 109 grid points. However, estimations based on presented results show that more than 50000 CPU cores can potentially be exploited.

  • Turbulent differentially heated cavity of aspect ratio 5 : direct numerical simulation and regularization modeling

     Trias Miquel, Francesc Xavier; Gorobets, Andrey; Oliva Llena, Asensio
    International Conference on Computational Heat and Mass Transfer
    Presentation's date: 2011-07-07
    Presentation of work at congresses

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    This work is devoted to the study of turbulent natural convection flows in differentially heated cavities. The adopted configuration corresponds to an air-filled (Pr = 0.7) cavity of aspect ratio 5 and Rayleigh number Ra = 4.5 × 1010 (based on the cavity height). Firstly, a complete direct numerical simulation (DNS) has been performed. Then, the DNS results have been used as reference solution to assess the performance of symmetry-preserving regularization as a simulation shortcut: a novel class of regularization that restrain the convective production of small scales of motion in an unconditionally stable manner. In this way, the new set of equations are dynamically less complex than the original Navier-Stokes equations, and therefore more amenable to be numerically solved. Time-averaged DNS results have revealed that the transition of the vertical boundary layer occurs at more downstream positions than those observed in the experiments.

  • Access to the full text
    Hybrid MPI+OpenMP parallelization of an FFT-based 3D Poisson solver that can reach 100000 CPU cores  Open access

     Gorobets, Andrey; Trias Miquel, Francesc Xavier; Borrell Pol, Ricard; Soria Guerrero, Manel; Oliva Llena, Asensio
    International Conference on Parallel Computational Fluid Dynamics
    Presentation's date: 2011-05-07
    Presentation of work at congresses

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    This work is devoted to the development of efficient parallel algorithms for the direct numerical simulation (DNS) of incompressible flows on modern supercomputers. A Poisson solver for problems with one uniform periodic direction is presented here. It is extended with a two-level hybrid MPI+OpenMP parallelization. Advantages and implementation details for the additional OpenMP parallelization are presented and discussed. This upgrade has allowed to significantly extend the range of efficient scalability. Here, the solver has been tested up to 12800 CPU cores for meshes with up to 10 9 nodes. However, estimations based on the presented results show that this range can be potentially stretched beyond 10 5 cores.

    Postprint (author’s final draft)

  • Numerical simulation of turbulence at lower costs: regularization modeling

     Trias Miquel, Francesc Xavier; Gorobets, Andrey; Soria Guerrero, Manel; Oliva Llena, Asensio
    International Conference on Parallel Computational Fluid Dynamics
    Presentation's date: 2011-05-07
    Presentation of work at congresses

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