CTTC  Centre Tecnològic de la Transferència de Calor
http://hdl.handle.net/2117/3190
Sat, 03 Dec 2016 15:46:02 GMT
20161203T15:46:02Z

Parallel load balancing strategy for VolumeofFluid methods on 3D unstructured meshes
http://hdl.handle.net/2117/97703
Parallel load balancing strategy for VolumeofFluid methods on 3D unstructured meshes
Jofre Cruanyes, Lluís; Borrell Pol, Ricard; Lehmkuhl Barba, Oriol; Oliva Llena, Asensio
l VolumeofFluid (VOF) is one of the methods of choice to reproduce the interface motion in the simulation of multifluid 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 CPUcores 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 interfacecapturing methods, such as the LevelSet, which may present similar workload imbalances. (C) 2014 Elsevier Inc. Allrights reserved.
© 2016. This version is made available under the CCBYNCND 4.0 license http://creativecommons.org/licenses/byncnd/4.0/
Fri, 02 Dec 2016 13:23:56 GMT
http://hdl.handle.net/2117/97703
20161202T13:23:56Z
Jofre Cruanyes, Lluís
Borrell Pol, Ricard
Lehmkuhl Barba, Oriol
Oliva Llena, Asensio
l VolumeofFluid (VOF) is one of the methods of choice to reproduce the interface motion in the simulation of multifluid 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 CPUcores 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 interfacecapturing methods, such as the LevelSet, which may present similar workload imbalances. (C) 2014 Elsevier Inc. Allrights reserved.

Numerical simulation of roughness effects on the flow past a circular cylinder
http://hdl.handle.net/2117/97516
Numerical simulation of roughness effects on the flow past a circular cylinder
Rodríguez Pérez, Ivette María; Lehmkuhl, Oriol; Piomelli, Ugo; Chiva Segura, Jorge; Borrell, Ricard; Oliva Llena, Asensio
In the present work large eddy simulations of the flow past a rough cylinder are performed at a Reynolds number of Re = 4.2 × 105 and an equivalent sandgrain surface roughness height ks = 0.02D. In order to determine the effects of the surface roughness on the boundary layer transition and as a consequence on the wake topology, results
are compared to those of the smooth cylinder. It is shown that surface roughness triggers the transition to turbulence in the boundary layer, thus leading to an early separation caused by the increased drag and momentum deficit. Thus, the drag coefficient increases up to CD 1.122 (if compared to the smooth cylinder it should be about CD 0.3  0.5).
The wake topology also changes and resembles more the subcritical wake observed for the smooth cylinder at lower Reynolds numbers than the expected critical wake at this Reynolds number.
Wed, 30 Nov 2016 13:51:35 GMT
http://hdl.handle.net/2117/97516
20161130T13:51:35Z
Rodríguez Pérez, Ivette María
Lehmkuhl, Oriol
Piomelli, Ugo
Chiva Segura, Jorge
Borrell, Ricard
Oliva Llena, Asensio
In the present work large eddy simulations of the flow past a rough cylinder are performed at a Reynolds number of Re = 4.2 × 105 and an equivalent sandgrain surface roughness height ks = 0.02D. In order to determine the effects of the surface roughness on the boundary layer transition and as a consequence on the wake topology, results
are compared to those of the smooth cylinder. It is shown that surface roughness triggers the transition to turbulence in the boundary layer, thus leading to an early separation caused by the increased drag and momentum deficit. Thus, the drag coefficient increases up to CD 1.122 (if compared to the smooth cylinder it should be about CD 0.3  0.5).
The wake topology also changes and resembles more the subcritical wake observed for the smooth cylinder at lower Reynolds numbers than the expected critical wake at this Reynolds number.

A new statistical model for subgrid dispersion in large eddy simulations of particleladen flows
http://hdl.handle.net/2117/97201
A new statistical model for subgrid dispersion in large eddy simulations of particleladen flows
Muela Castro, Jordi; Lehmkuhl Barba, Oriol; Pérez Segarra, Carlos David; Oliva Llena, Asensio
Dispersed multiphase turbulent flows are present in many industrial and commercial applications like
internal combustion engines, turbofans, dispersion of contaminants, steam turbines, etc. Therefore, there is a clear
interest in the development of models and numerical tools capable of performing detailed and reliable simulations
about these kind of flows. Large Eddy Simulations offer good accuracy and reliable results together with reasonable
computational requirements, making it a really interesting method to develop numerical tools for particleladen turbulent
flows. Nonetheless, in multiphase dispersed flows additional difficulties arises in LES, since the effect of the unresolved
scales of the continuous phase over the dispersed phase is lost due to the filtering procedure. In order to solve this
issue a model able to reconstruct the subgrid velocity seen by the particles is required. In this work a new model for
the reconstruction of the subgrid scale effects over the dispersed phase is presented and assessed. This innovative
methodology is based in the reconstruction of statistics via Probability Density Functions (PDFs).
This article is published under a CC BY licence. The Version of Record is available online at: http.//dx.doi.org/10.1088/17426596/745/3/032115
Thu, 24 Nov 2016 16:18:01 GMT
http://hdl.handle.net/2117/97201
20161124T16:18:01Z
Muela Castro, Jordi
Lehmkuhl Barba, Oriol
Pérez Segarra, Carlos David
Oliva Llena, Asensio
Dispersed multiphase turbulent flows are present in many industrial and commercial applications like
internal combustion engines, turbofans, dispersion of contaminants, steam turbines, etc. Therefore, there is a clear
interest in the development of models and numerical tools capable of performing detailed and reliable simulations
about these kind of flows. Large Eddy Simulations offer good accuracy and reliable results together with reasonable
computational requirements, making it a really interesting method to develop numerical tools for particleladen turbulent
flows. Nonetheless, in multiphase dispersed flows additional difficulties arises in LES, since the effect of the unresolved
scales of the continuous phase over the dispersed phase is lost due to the filtering procedure. In order to solve this
issue a model able to reconstruct the subgrid velocity seen by the particles is required. In this work a new model for
the reconstruction of the subgrid scale effects over the dispersed phase is presented and assessed. This innovative
methodology is based in the reconstruction of statistics via Probability Density Functions (PDFs).

Partitioned semiimplicit methods for simulation of biomechanical fluidstructure interaction problems
http://hdl.handle.net/2117/97189
Partitioned semiimplicit methods for simulation of biomechanical fluidstructure interaction problems
Naseri, Alireza; Lehmkuhl Barba, Oriol; González Acedo, Ignacio; Oliva Llena, Asensio
This paper represents numerical simulation of fluidstructure interaction (FSI) system involving an
incompressible viscous fluid and a lightweight elastic structure. We follow a semiimplicit approach in which we
implicitly couple the addedmass term (pressure stress) of the fluid to the structure, while other terms are coupled
explicitly. This significantly reduces the computational cost of the simulations while showing adequate stability.
Several coupling schemes are tested including fixedpoint method with different static and dynamic relaxation,
as well as NewtonKrylov method with approximated Jacobian. Numerical tests are conducted in the context of a
biomechanical problem. Results indicate that the NewtonKrylov solver outperforms fixed point ones while introducing
more complexity to the problem due to the evaluation of the Jacobian. Fixedpoint solver with Aitken's relaxation
method also proved to be a simple, yet efficient method for FSI simulations.
This article is published under a CC BY licence. The Version of Record is available online at: http://dx.doi.org/10.1088/17426596/745/3/032020.
Thu, 24 Nov 2016 14:59:40 GMT
http://hdl.handle.net/2117/97189
20161124T14:59:40Z
Naseri, Alireza
Lehmkuhl Barba, Oriol
González Acedo, Ignacio
Oliva Llena, Asensio
This paper represents numerical simulation of fluidstructure interaction (FSI) system involving an
incompressible viscous fluid and a lightweight elastic structure. We follow a semiimplicit approach in which we
implicitly couple the addedmass term (pressure stress) of the fluid to the structure, while other terms are coupled
explicitly. This significantly reduces the computational cost of the simulations while showing adequate stability.
Several coupling schemes are tested including fixedpoint method with different static and dynamic relaxation,
as well as NewtonKrylov method with approximated Jacobian. Numerical tests are conducted in the context of a
biomechanical problem. Results indicate that the NewtonKrylov solver outperforms fixed point ones while introducing
more complexity to the problem due to the evaluation of the Jacobian. Fixedpoint solver with Aitken's relaxation
method also proved to be a simple, yet efficient method for FSI simulations.

A levelset method for thermal motion of bubbles and droplets
http://hdl.handle.net/2117/96631
A levelset method for thermal motion of bubbles and droplets
Balcázar Arciniega, Néstor; Oliva Llena, Asensio; Rigola Serrano, Joaquim
A conservative levelset model for direct simulation of twophase flows with thermocapillary effects at dynamically deformable interface is presented. The NavierStokes equations coupled with the energy conservation equation are solved by means of a finitevolume/levelset method. Some numerical examples including thermocapillary motion of single and multiple fluid particles are computed by means of the present method. The results are compared with analytical solutions and numerical results from the literature as validations of the proposed model.
Published under licence in Journal of Physics: Conference Series by IOP Publishing Ltd.
Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Mon, 14 Nov 2016 15:25:45 GMT
http://hdl.handle.net/2117/96631
20161114T15:25:45Z
Balcázar Arciniega, Néstor
Oliva Llena, Asensio
Rigola Serrano, Joaquim
A conservative levelset model for direct simulation of twophase flows with thermocapillary effects at dynamically deformable interface is presented. The NavierStokes equations coupled with the energy conservation equation are solved by means of a finitevolume/levelset method. Some numerical examples including thermocapillary motion of single and multiple fluid particles are computed by means of the present method. The results are compared with analytical solutions and numerical results from the literature as validations of the proposed model.

Frost formation: optimizing solutions under a finite volume approach
http://hdl.handle.net/2117/96630
Frost formation: optimizing solutions under a finite volume approach
Bartrons Casademont, Eduard; Pérez Segarra, Carlos David; Oliet Casasayas, Carles
A threedimensional transient formulation of the frost formation process is developed by means of a finite volume approach. Emphasis is put on the frost surface boundary condition as well as the wide range of empirical correlations related to the thermophysical and transport properties of frost. A study of the numerical solution is
made, establishing the parameters that ensure grid independence. Attention is given to the algorithm, the discretised equations and the code optimization through dynamic relaxation techniques. A critical analysis of four cases is carried out by comparing solutions of several empirical models against tested experiments. As a result, a discussion on the performance of such parameters is started and a proposal of the most suitable models is presented.
Published under licence in Journal of Physics: Conference Series by IOP Publishing Ltd.
Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Mon, 14 Nov 2016 15:20:51 GMT
http://hdl.handle.net/2117/96630
20161114T15:20:51Z
Bartrons Casademont, Eduard
Pérez Segarra, Carlos David
Oliet Casasayas, Carles
A threedimensional transient formulation of the frost formation process is developed by means of a finite volume approach. Emphasis is put on the frost surface boundary condition as well as the wide range of empirical correlations related to the thermophysical and transport properties of frost. A study of the numerical solution is
made, establishing the parameters that ensure grid independence. Attention is given to the algorithm, the discretised equations and the code optimization through dynamic relaxation techniques. A critical analysis of four cases is carried out by comparing solutions of several empirical models against tested experiments. As a result, a discussion on the performance of such parameters is started and a proposal of the most suitable models is presented.

Numerical simulations of conjugate convection combined with surface thermal radiation using an ImmersedBoundary Method
http://hdl.handle.net/2117/96629
Numerical simulations of conjugate convection combined with surface thermal radiation using an ImmersedBoundary Method
Favre Samarra, Federico; Colomer Rey, Guillem; Lehmkuhl Barba, Oriol; Oliva Llena, Asensio
Dynamic and thermal interaction problems involving fluids and solids were studied through a finite volume based NavierStokes solver, combined with immersedboundary techniques and the net radiation method. Source terms were included in the momentum and energy equations to enforce the nonslip condition and the conjugate boundary condition including the radiative heat exchange. Code validation was performed through the simulation of two cases from the literature: conjugate natural convection in a square cavity with a conducting side wall; and a cubical cavity with conducting walls and a heat source. The accuracy of the methodology and the validation of the inclusion of moving bodies into the simulation was performed via a theoretical case.
Published under licence in Journal of Physics: Conference Series by IOP Publishing Ltd.
Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Mon, 14 Nov 2016 15:04:32 GMT
http://hdl.handle.net/2117/96629
20161114T15:04:32Z
Favre Samarra, Federico
Colomer Rey, Guillem
Lehmkuhl Barba, Oriol
Oliva Llena, Asensio
Dynamic and thermal interaction problems involving fluids and solids were studied through a finite volume based NavierStokes solver, combined with immersedboundary techniques and the net radiation method. Source terms were included in the momentum and energy equations to enforce the nonslip condition and the conjugate boundary condition including the radiative heat exchange. Code validation was performed through the simulation of two cases from the literature: conjugate natural convection in a square cavity with a conducting side wall; and a cubical cavity with conducting walls and a heat source. The accuracy of the methodology and the validation of the inclusion of moving bodies into the simulation was performed via a theoretical case.

On the solution of the full threedimensional Taylor bubble problem by using a coupled Conservative Level Set  Moving Mesh method
http://hdl.handle.net/2117/96626
On the solution of the full threedimensional Taylor bubble problem by using a coupled Conservative Level Set  Moving Mesh method
Gutiérrez Álvarez, Enrique; Balcázar Arciniega, Néstor; Lehmkuhl Barba, Oriol; Oliva Llena, Asensio
The challenging problem of the full threedimensional Taylor bubble has been addressed by using a Conservative Level Set method in order to deal with the multiphase flow. A moving mesh is used, aiming to optimize the simulation domain. The mesh is moved as the bubble rises, so the region of study can be limited to the surroundings of the bubble, notably reducing the domain's size. This saving in the computational resources facilitates to face the Taylor bubble problem without the axisymmetric assumption. By doing so, a detailed description of the fluid field is provided, comparing the results against numerical and experimental data.
Published under licence in Journal of Physics: Conference Series by IOP Publishing Ltd.
Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Mon, 14 Nov 2016 14:48:19 GMT
http://hdl.handle.net/2117/96626
20161114T14:48:19Z
Gutiérrez Álvarez, Enrique
Balcázar Arciniega, Néstor
Lehmkuhl Barba, Oriol
Oliva Llena, Asensio
The challenging problem of the full threedimensional Taylor bubble has been addressed by using a Conservative Level Set method in order to deal with the multiphase flow. A moving mesh is used, aiming to optimize the simulation domain. The mesh is moved as the bubble rises, so the region of study can be limited to the surroundings of the bubble, notably reducing the domain's size. This saving in the computational resources facilitates to face the Taylor bubble problem without the axisymmetric assumption. By doing so, a detailed description of the fluid field is provided, comparing the results against numerical and experimental data.

New subgridscale models for largeeddy simulation of RayleighBénard convection
http://hdl.handle.net/2117/96616
New subgridscale models for largeeddy simulation of RayleighBénard convection
Dabbagh, Firas; Trias Miquel, Francesc Xavier; Gorobets, Andrei; Oliva Llena, Asensio
At the crossroad between flow topology analysis and the theory of turbulence, a new eddyviscosity model for Largeeddy simulation has been recently proposed by Trias et al.[PoF, 27, 065103 (2015)]. The S3PQRmodel has the proper cubic nearwall behaviour and no intrinsic limitations for statistically inhomogeneous flows. In this work, the new model has been tested for an air turbulent RayleighBenard convection in a rectangular cell of aspect ratio unity and n spanwise openended distance. To do so, direct numerical simulation has been carried out at two Rayleigh numbers Ra = 108 and 1010, to assess the model performance and investigate a priori the effect of the turbulent Prandtl number. Using an approximate formula based on the Taylor series expansion, the turbulent Prandtl number has been calculated and revealed a constant and Raindependent value across the bulk region equals to 0.55. It is found that the turbulent components of eddyviscosity and eddydiffusivity are positively prevalent to maintain a turbulent wind essentially driven by the mean buoyant force at the sidewalls. On the other hand, the new eddyviscosity model is preliminary tested for the case of Ra = 108 and showed overestimation of heat flux within the boundary layer but fairly good prediction of turbulent kinetics at this moderate turbulent flow.
Published under licence in Journal of Physics: Conference Series by IOP Publishing Ltd.
Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Mon, 14 Nov 2016 13:32:43 GMT
http://hdl.handle.net/2117/96616
20161114T13:32:43Z
Dabbagh, Firas
Trias Miquel, Francesc Xavier
Gorobets, Andrei
Oliva Llena, Asensio
At the crossroad between flow topology analysis and the theory of turbulence, a new eddyviscosity model for Largeeddy simulation has been recently proposed by Trias et al.[PoF, 27, 065103 (2015)]. The S3PQRmodel has the proper cubic nearwall behaviour and no intrinsic limitations for statistically inhomogeneous flows. In this work, the new model has been tested for an air turbulent RayleighBenard convection in a rectangular cell of aspect ratio unity and n spanwise openended distance. To do so, direct numerical simulation has been carried out at two Rayleigh numbers Ra = 108 and 1010, to assess the model performance and investigate a priori the effect of the turbulent Prandtl number. Using an approximate formula based on the Taylor series expansion, the turbulent Prandtl number has been calculated and revealed a constant and Raindependent value across the bulk region equals to 0.55. It is found that the turbulent components of eddyviscosity and eddydiffusivity are positively prevalent to maintain a turbulent wind essentially driven by the mean buoyant force at the sidewalls. On the other hand, the new eddyviscosity model is preliminary tested for the case of Ra = 108 and showed overestimation of heat flux within the boundary layer but fairly good prediction of turbulent kinetics at this moderate turbulent flow.

On the extension of LES methods from incompressible to compressible turbulent flows with application to turbulent channel flow
http://hdl.handle.net/2117/96615
On the extension of LES methods from incompressible to compressible turbulent flows with application to turbulent channel flow
Pedro Costa, Juan Bautista; Baez Vidal, Aleix; Lehmkuhl Barba, Oriol; Pérez Segarra, Carlos David; Oliva Llena, Asensio
The objective of the present work is to validate the compressible LargeEddy Simulation (LES) models implemented in the in house parallel unstructured CFD code TermoFluids. Our research team has implemented and tested several LES models over the past years for the incompressible regimen. In order to be able to solve complex
turbulent compressible flows, the models are revisited and modified if necessary. In addition, the performance of the implemented hybrid advection scheme is an issue of interest for the numerical simulation of turbulent compressible flows. The models are tested in the well known turbulent channel flow problem at different compressible regimens.
Published under licence in Journal of Physics: Conference Series by IOP Publishing Ltd.
Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Mon, 14 Nov 2016 13:22:50 GMT
http://hdl.handle.net/2117/96615
20161114T13:22:50Z
Pedro Costa, Juan Bautista
Baez Vidal, Aleix
Lehmkuhl Barba, Oriol
Pérez Segarra, Carlos David
Oliva Llena, Asensio
The objective of the present work is to validate the compressible LargeEddy Simulation (LES) models implemented in the in house parallel unstructured CFD code TermoFluids. Our research team has implemented and tested several LES models over the past years for the incompressible regimen. In order to be able to solve complex
turbulent compressible flows, the models are revisited and modified if necessary. In addition, the performance of the implemented hybrid advection scheme is an issue of interest for the numerical simulation of turbulent compressible flows. The models are tested in the well known turbulent channel flow problem at different compressible regimens.