Articles de revista
http://hdl.handle.net/2117/3191
20180426T02:02:34Z

Fluid flow simulation and permeability computation in deformed porous carbonate grainstones
http://hdl.handle.net/2117/116280
Fluid flow simulation and permeability computation in deformed porous carbonate grainstones
Zambrano, Miller; Tondi, Emanuele; Mancini, Lucia; Lanzafame, Gabriele; Trias Miquel, Francesc Xavier; Arzilli, Fabio; Materazzi, Fabio; Torrieri, Stefano
In deformed porous carbonates, the architecture of the pore network may be modified by deformation or diagenetic processes altering the permeability with respect to the pristine rock. The effects of the pore texture and morphology on permeability in porous rocks have been widely investigated due to the importance during the evaluation of geofluid reservoirs. In this study, these effects are assessed by combining synchrotron Xray computed microtomography (SR microCT) and computational fluid dynamics. The studied samples pertain to deformed porous carbonate grainstones highly affected by deformation bands (DBs) exposed in Northwestern Sicily and Abruzzo regions, Italy.
The highresolution SR microCT images of the samples, acquired at the SYRMEP beamline of the Elettra  Sincrotrone Trieste laboratory (Italy), were used for simulating a pressuredriven flow by using the latticeBoltzmann method (LBM). For the experiments, a multiple relaxation time (MRT) model with the D3Q19 scheme was used to avoid viscositydependent results of permeability. The permeability was calculated using Darcy's law once steady conditions were reached. After the simulations, the porenetwork properties (effective porosity, specific surface area, and geometrical tortuosity) were calculated using 3D images of the velocity fields. These images were segmented considering a velocity threshold value higher than zero.
The study showed that DBs may generate significant heterogeneity and anisotropy of the permeability of the evaluated rock samples. Cataclasis and cementation process taking place within the DBs reduce the effective porosity and therefore the permeability. Contrary to this, pressure dissolution and faulting may generate connected channels which contribute to the permeability only parallel to the DB.
20180413T16:38:56Z
Zambrano, Miller
Tondi, Emanuele
Mancini, Lucia
Lanzafame, Gabriele
Trias Miquel, Francesc Xavier
Arzilli, Fabio
Materazzi, Fabio
Torrieri, Stefano
In deformed porous carbonates, the architecture of the pore network may be modified by deformation or diagenetic processes altering the permeability with respect to the pristine rock. The effects of the pore texture and morphology on permeability in porous rocks have been widely investigated due to the importance during the evaluation of geofluid reservoirs. In this study, these effects are assessed by combining synchrotron Xray computed microtomography (SR microCT) and computational fluid dynamics. The studied samples pertain to deformed porous carbonate grainstones highly affected by deformation bands (DBs) exposed in Northwestern Sicily and Abruzzo regions, Italy.
The highresolution SR microCT images of the samples, acquired at the SYRMEP beamline of the Elettra  Sincrotrone Trieste laboratory (Italy), were used for simulating a pressuredriven flow by using the latticeBoltzmann method (LBM). For the experiments, a multiple relaxation time (MRT) model with the D3Q19 scheme was used to avoid viscositydependent results of permeability. The permeability was calculated using Darcy's law once steady conditions were reached. After the simulations, the porenetwork properties (effective porosity, specific surface area, and geometrical tortuosity) were calculated using 3D images of the velocity fields. These images were segmented considering a velocity threshold value higher than zero.
The study showed that DBs may generate significant heterogeneity and anisotropy of the permeability of the evaluated rock samples. Cataclasis and cementation process taking place within the DBs reduce the effective porosity and therefore the permeability. Contrary to this, pressure dissolution and faulting may generate connected channels which contribute to the permeability only parallel to the DB.

DNS of falling droplets in a vertical channel
http://hdl.handle.net/2117/116083
DNS of falling droplets in a vertical channel
Balcázar, Nestor; Castro González, Jesús; Chiva Segura, Jorge; Oliva Llena, Asensio
This paper presents Direct Numerical Simulation (DNS) of the falling motion of single and multiple deformable drops in a vertical channel. A systematic study of the wall effect on the motion of single drop is performed for Eötvös number (0.5=Eo=5), Morton number (103=M=108), and confinement ratio CR = 2. Second, the gravitydriven motion of multiple drops and their interactions are studied in a periodic vertical channel for CR = 4. These simulations are performed using a multiple marker levelset methodology, integrated in a finitevolume framework on a collocated unstructured grid. Each droplet is described by a levelset function, which allows capturing multiple interfaces in the same control volume, avoiding the numerical merging of the droplets. Numerical algorithms for fluid motion and interface capturing have been developed in the context of the finitevolume and levelset methodology, surface tension is modeled by means of the continuous surface force approach, and the pressurevelocity coupling is solved using a fractionalstep projection method. DNS of single drop shows that they migrate to the symmetry axis of the channel when the Reynolds number is low, following a monotonic approach or damped oscillations according to the dimensionless parameters. If Eötvös number increases, stronger oscillations around the symmetry axis are observed. Simulations of multiple drops show that the collision of two drops follows the draftingkissing tumbling (DKT) phenomenon. Deformable drops do not collide with the wall, whereas DKT phenomenon in the droplet swarm leads to the formation of groups which move through the center of the channel.
© 2018 WIT Press
20180409T13:03:18Z
Balcázar, Nestor
Castro González, Jesús
Chiva Segura, Jorge
Oliva Llena, Asensio
This paper presents Direct Numerical Simulation (DNS) of the falling motion of single and multiple deformable drops in a vertical channel. A systematic study of the wall effect on the motion of single drop is performed for Eötvös number (0.5=Eo=5), Morton number (103=M=108), and confinement ratio CR = 2. Second, the gravitydriven motion of multiple drops and their interactions are studied in a periodic vertical channel for CR = 4. These simulations are performed using a multiple marker levelset methodology, integrated in a finitevolume framework on a collocated unstructured grid. Each droplet is described by a levelset function, which allows capturing multiple interfaces in the same control volume, avoiding the numerical merging of the droplets. Numerical algorithms for fluid motion and interface capturing have been developed in the context of the finitevolume and levelset methodology, surface tension is modeled by means of the continuous surface force approach, and the pressurevelocity coupling is solved using a fractionalstep projection method. DNS of single drop shows that they migrate to the symmetry axis of the channel when the Reynolds number is low, following a monotonic approach or damped oscillations according to the dimensionless parameters. If Eötvös number increases, stronger oscillations around the symmetry axis are observed. Simulations of multiple drops show that the collision of two drops follows the draftingkissing tumbling (DKT) phenomenon. Deformable drops do not collide with the wall, whereas DKT phenomenon in the droplet swarm leads to the formation of groups which move through the center of the channel.

A new subgrid characteristic length for turbulence simulations on anisotropic grids
http://hdl.handle.net/2117/114556
A new subgrid characteristic length for turbulence simulations on anisotropic grids
Trias Miquel, Francesc Xavier; Gorobets, Andrei; Silvis, Maurits; Verstappen, Roel; Oliva Llena, Asensio
Direct numerical simulations of the incompressible NavierStokes equations are not feasible yet for most practical turbulent flows. Therefore, dynamically less complex mathematical formulations are necessary for coarsegrained simulations. In this regard, eddyviscosity models for LargeEddy Simulation (LES) are probably the most popular example thereof. This type of models requires the calculation of a subgrid characteristic length which is usually associated with the local grid size. For isotropic grids, this is equal to the mesh step. However, for anisotropic or unstructured grids, such as the pancakelike meshes that are often used to resolve nearwall turbulence or shear layers, a consensus on defining the subgrid characteristic length has not been reached yet despite the fact that it can strongly affect the performance of LES models. In this context, a new definition of the subgrid characteristic length is presented in this work. This flowdependent length scale is based on the turbulent, or subgrid stress, tensor and its representations on different grids. The simplicity and mathematical properties suggest that it can be a robust definition that minimizes the effects of mesh anisotropies on simulation results. The performance of the proposed subgrid characteristic length is successfully tested for decaying isotropic turbulence and a turbulent channel flow using artificially refined grids. Finally, a simple extension of the method for unstructured meshes is proposed and tested for a turbulent flow around a square cylinder. Comparisons with existing subgrid characteristic length scales show that the proposed definition is much more robust with respect to mesh anisotropies and has a great potential to be used in complex geometries where highly skewed (unstructured) meshes are present.
20180227T14:50:13Z
Trias Miquel, Francesc Xavier
Gorobets, Andrei
Silvis, Maurits
Verstappen, Roel
Oliva Llena, Asensio
Direct numerical simulations of the incompressible NavierStokes equations are not feasible yet for most practical turbulent flows. Therefore, dynamically less complex mathematical formulations are necessary for coarsegrained simulations. In this regard, eddyviscosity models for LargeEddy Simulation (LES) are probably the most popular example thereof. This type of models requires the calculation of a subgrid characteristic length which is usually associated with the local grid size. For isotropic grids, this is equal to the mesh step. However, for anisotropic or unstructured grids, such as the pancakelike meshes that are often used to resolve nearwall turbulence or shear layers, a consensus on defining the subgrid characteristic length has not been reached yet despite the fact that it can strongly affect the performance of LES models. In this context, a new definition of the subgrid characteristic length is presented in this work. This flowdependent length scale is based on the turbulent, or subgrid stress, tensor and its representations on different grids. The simplicity and mathematical properties suggest that it can be a robust definition that minimizes the effects of mesh anisotropies on simulation results. The performance of the proposed subgrid characteristic length is successfully tested for decaying isotropic turbulence and a turbulent channel flow using artificially refined grids. Finally, a simple extension of the method for unstructured meshes is proposed and tested for a turbulent flow around a square cylinder. Comparisons with existing subgrid characteristic length scales show that the proposed definition is much more robust with respect to mesh anisotropies and has a great potential to be used in complex geometries where highly skewed (unstructured) meshes are present.

Regional aerosol deposition in the human airways: the SimInhale benchmark case and a critical assessment of in silico methods
http://hdl.handle.net/2117/111950
Regional aerosol deposition in the human airways: the SimInhale benchmark case and a critical assessment of in silico methods
Koullapis, P.; Kassinos, S.C.; Muela Castro, Jordi; Pérez Segarra, Carlos David; Rigola Serrano, Joaquim; Lehmkuhl, Oriol; Cui, Y.; Sommerfeld, M.; Elcner, J.; Jicha, M.; Saveljic, I.; Filipovic, N.; Lizal, F.; Nicolaou, L.
Regional deposition effects are important in the pulmonary delivery of drugs intended for the topical treatment of respiratory ailments. They also play a critical role in the systemic delivery of drugs with limited lung bioavailability. In recent years, significant improvements in the quality of pulmonary imaging have taken place, however the resolution of current imaging modalities remains inadequate for quantifying regional deposition. Computational FluidParticle Dynamics (CFPD) can fill this gap by providing detailed information about regional deposition in the extrathoracic and conducting airways. It is therefore not surprising that the last 15. years have seen an exponential growth in the application of CFPD methods in this area. Survey of the recent literature however, reveals a wide variability in the range of modelling approaches used and in the assumptions made about important physical processes taking place during aerosol inhalation. The purpose of this work is to provide a concise critical review of the computational approaches used to date, and to present a benchmark case for validation of future studies in the upper airways. In the spirit of providing the wider community with a reference for quality assurance of CFPD studies, in vitro deposition measurements have been conducted in a humanbased model of the upper airways, and several groups within MP1404 SimInhale have computed the same case using a variety of simulation and discretization approaches. Here, we report the results of this collaborative effort and provide a critical discussion of the performance of the various simulation methods. The benchmark case, in vitro deposition data and in silico results will be published online and made available to the wider community. Particle image velocimetry measurements of the flow, as well as additional numerical results from the community, will be appended to the online database as they become available in the future.
© 2017. This version is made available under the CCBYNCND 4.0 license http://creativecommons.org/licenses/byncnd/4.0/
20171213T16:50:06Z
Koullapis, P.
Kassinos, S.C.
Muela Castro, Jordi
Pérez Segarra, Carlos David
Rigola Serrano, Joaquim
Lehmkuhl, Oriol
Cui, Y.
Sommerfeld, M.
Elcner, J.
Jicha, M.
Saveljic, I.
Filipovic, N.
Lizal, F.
Nicolaou, L.
Regional deposition effects are important in the pulmonary delivery of drugs intended for the topical treatment of respiratory ailments. They also play a critical role in the systemic delivery of drugs with limited lung bioavailability. In recent years, significant improvements in the quality of pulmonary imaging have taken place, however the resolution of current imaging modalities remains inadequate for quantifying regional deposition. Computational FluidParticle Dynamics (CFPD) can fill this gap by providing detailed information about regional deposition in the extrathoracic and conducting airways. It is therefore not surprising that the last 15. years have seen an exponential growth in the application of CFPD methods in this area. Survey of the recent literature however, reveals a wide variability in the range of modelling approaches used and in the assumptions made about important physical processes taking place during aerosol inhalation. The purpose of this work is to provide a concise critical review of the computational approaches used to date, and to present a benchmark case for validation of future studies in the upper airways. In the spirit of providing the wider community with a reference for quality assurance of CFPD studies, in vitro deposition measurements have been conducted in a humanbased model of the upper airways, and several groups within MP1404 SimInhale have computed the same case using a variety of simulation and discretization approaches. Here, we report the results of this collaborative effort and provide a critical discussion of the performance of the various simulation methods. The benchmark case, in vitro deposition data and in silico results will be published online and made available to the wider community. Particle image velocimetry measurements of the flow, as well as additional numerical results from the community, will be appended to the online database as they become available in the future.

Numerical study of an impulse wave generated by a sliding mass
http://hdl.handle.net/2117/111388
Numerical study of an impulse wave generated by a sliding mass
Schillaci, Eugenio; Favre Samarra, Federico; Antepara Zambrano, Óscar; Balcazar, Nestor; Oliva Llena, Asensio
In this work, a numerical framework for the direct numerical simulation of tsunami waves generated by landslide events is proposed. The method, implemented on the TermoFluids numerical platform, adopts a free surface model for the simulation of momentum equations; thus, considering the effect of air on the flow physics negligible. The effect of the solid motion on the flow is taken into account by means of a direct forcing immersed boundary method (IBM).
The method is available for 3D unstructured meshes; however, it can be integrated with an adaptive mesh refinement (AMR) tool to dynamically increase the local definition of the mesh in the vicinity of the interfaces, which separate the phases or in the presence of vortical structures.
The method is firstly validated by simulating the entrance of objects into still water surfaces for 2D and 3D configurations. Next, the case of tsunami generation from a subaerial landslide is studied and the results are validated by comparison to experimental and numerical measurements. Overall, the model demonstrates its efficiency in the simulation of this type of physics, and a wide versatility in the choice of the domain discretization.
© 2018 WIT Press
20171130T15:01:16Z
Schillaci, Eugenio
Favre Samarra, Federico
Antepara Zambrano, Óscar
Balcazar, Nestor
Oliva Llena, Asensio
In this work, a numerical framework for the direct numerical simulation of tsunami waves generated by landslide events is proposed. The method, implemented on the TermoFluids numerical platform, adopts a free surface model for the simulation of momentum equations; thus, considering the effect of air on the flow physics negligible. The effect of the solid motion on the flow is taken into account by means of a direct forcing immersed boundary method (IBM).
The method is available for 3D unstructured meshes; however, it can be integrated with an adaptive mesh refinement (AMR) tool to dynamically increase the local definition of the mesh in the vicinity of the interfaces, which separate the phases or in the presence of vortical structures.
The method is firstly validated by simulating the entrance of objects into still water surfaces for 2D and 3D configurations. Next, the case of tsunami generation from a subaerial landslide is studied and the results are validated by comparison to experimental and numerical measurements. Overall, the model demonstrates its efficiency in the simulation of this type of physics, and a wide versatility in the choice of the domain discretization.

Numerical study of Taylor bubbles rising in a stagnant liquid using a levelset/movingmesh method
http://hdl.handle.net/2117/105970
Numerical study of Taylor bubbles rising in a stagnant liquid using a levelset/movingmesh method
Gutiérrez González, Ernesto; Balcázar Arciniega, Néstor; Bartrons Casademont, Eduard; Rigola Serrano, Joaquim
An Arbitrary LagrangianEulerian formulation has been posed to solve the challenging problem of the threedimensional Taylor bubble, within a Conservative Level Set (CLS) framework. By employing a domain optimization method (i.e. the moving mesh method), smaller domains can be used to simulate rising bubbles, thus saving computational resources. As the method requires the use of open boundaries, a careful treatment of both inflow and outflow boundary conditions has been carried out. The coupled CLS  moving mesh method has been verified by means of extensive numerical tests. The challenging problem of the full threedimensional Taylor bubble has then been thoroughly addressed, providing a detailed description of its features. The study also includes a sensitivity analyses with respect to the initial shape of the bubble, the initial volume of the bubble, the flow regime and the inclination of the channel.
20170629T08:29:59Z
Gutiérrez González, Ernesto
Balcázar Arciniega, Néstor
Bartrons Casademont, Eduard
Rigola Serrano, Joaquim
An Arbitrary LagrangianEulerian formulation has been posed to solve the challenging problem of the threedimensional Taylor bubble, within a Conservative Level Set (CLS) framework. By employing a domain optimization method (i.e. the moving mesh method), smaller domains can be used to simulate rising bubbles, thus saving computational resources. As the method requires the use of open boundaries, a careful treatment of both inflow and outflow boundary conditions has been carried out. The coupled CLS  moving mesh method has been verified by means of extensive numerical tests. The challenging problem of the full threedimensional Taylor bubble has then been thoroughly addressed, providing a detailed description of its features. The study also includes a sensitivity analyses with respect to the initial shape of the bubble, the initial volume of the bubble, the flow regime and the inclination of the channel.

A 3D volumeoffluid advection method based on cellvertex velocities for unstructured meshes
http://hdl.handle.net/2117/105736
A 3D volumeoffluid advection method based on cellvertex velocities for unstructured meshes
Jofre Cruanyes, Lluís; Lehmkuhl Barba, Oriol; Castro González, Jesús; Oliva Llena, Asensio
A new geometrical VolumeofFluid (VOF) method for capturing interfaces on threedimensional (3D) Cartesian and unstructured meshes is introduced. The method reconstructs interfaces as first and secondorder piecewise planar approximations (PLIC), and advects volumes in a single unsplit Lagrangian–Eulerian (LE) geometrical algorithm based on constructing flux polyhedrons by tracing back the Lagrangian trajectories of the cellvertex velocities. In this way, the situations of overlapping between flux polyhedrons are minimized, consequently, the accuracy in the solution of the advection equation is improved by minimizing the creation of overshoots (volume fractions over one), undershoots (volume fractions below zero) and wisps (fluid in void regions or vice versa). However, if not treated carefully, the use of cellvertex velocities may result in the construction of flux polyhedrons that contain nonplanar faces and that do not conserve volume. Therefore, this work explains in detail a set of geometric algorithms necessary to overcome these two drawbacks. In addition, the new VOF method is analyzed numerically on 3D Cartesian and unstructured meshes, first, by reconstructing the interface of spherical geometries and, second, by evaluating the final advection result of a sphere placed in a rotation, shear and deformation field.
20170622T14:39:33Z
Jofre Cruanyes, Lluís
Lehmkuhl Barba, Oriol
Castro González, Jesús
Oliva Llena, Asensio
A new geometrical VolumeofFluid (VOF) method for capturing interfaces on threedimensional (3D) Cartesian and unstructured meshes is introduced. The method reconstructs interfaces as first and secondorder piecewise planar approximations (PLIC), and advects volumes in a single unsplit Lagrangian–Eulerian (LE) geometrical algorithm based on constructing flux polyhedrons by tracing back the Lagrangian trajectories of the cellvertex velocities. In this way, the situations of overlapping between flux polyhedrons are minimized, consequently, the accuracy in the solution of the advection equation is improved by minimizing the creation of overshoots (volume fractions over one), undershoots (volume fractions below zero) and wisps (fluid in void regions or vice versa). However, if not treated carefully, the use of cellvertex velocities may result in the construction of flux polyhedrons that contain nonplanar faces and that do not conserve volume. Therefore, this work explains in detail a set of geometric algorithms necessary to overcome these two drawbacks. In addition, the new VOF method is analyzed numerically on 3D Cartesian and unstructured meshes, first, by reconstructing the interface of spherical geometries and, second, by evaluating the final advection result of a sphere placed in a rotation, shear and deformation field.

A lowdissipation convection scheme for the stable discretization of turbulent interfacial flow
http://hdl.handle.net/2117/105415
A lowdissipation convection scheme for the stable discretization of turbulent interfacial flow
Schillaci, Eugenio; Jofre Cruanyes, Lluís; Balcázar Arciniega, Néstor; Antepara Zambrano, Óscar; Oliva Llena, Asensio
This paper analyzes a lowdissipation discretization for the resolution of immiscible, incompressible multiphase flow by means of interfacecapturing schemes. The discretization is built on a threedimensional, unstructured finitevolume framework and aims at minimizing the differences in kinetic energy preservation with respect to the continuous governing equations. This property plays a fundamental role in the case of flows presenting significant levels of turbulence. At the same time, the hybrid form of the convective operator proposed in this work incorporates localized lowdispersion characteristics to limit the growth of spurious flow solutions. The lowdissipation discrete framework is presented in detail and, in order to expose the advantages with respect to commonly used methodologies, its conservation properties and accuracy are extensively studied, both theoretically and numerically. Numerical tests are performed by considering a threedimensional vortex, an exact sinusoidal function, and a spherical drop subjected to surface tension forces in equilibrium and immersed in a swirling velocity field. Finally, the turbulent atomization of a liquidgas jet is numerically analyzed to further assess the capabilities of the method.
© 2017. This version is made available under the CCBYNCND 4.0 license http://creativecommons.org/licenses/byncnd/4.0/
20170614T12:46:47Z
Schillaci, Eugenio
Jofre Cruanyes, Lluís
Balcázar Arciniega, Néstor
Antepara Zambrano, Óscar
Oliva Llena, Asensio
This paper analyzes a lowdissipation discretization for the resolution of immiscible, incompressible multiphase flow by means of interfacecapturing schemes. The discretization is built on a threedimensional, unstructured finitevolume framework and aims at minimizing the differences in kinetic energy preservation with respect to the continuous governing equations. This property plays a fundamental role in the case of flows presenting significant levels of turbulence. At the same time, the hybrid form of the convective operator proposed in this work incorporates localized lowdispersion characteristics to limit the growth of spurious flow solutions. The lowdissipation discrete framework is presented in detail and, in order to expose the advantages with respect to commonly used methodologies, its conservation properties and accuracy are extensively studied, both theoretically and numerically. Numerical tests are performed by considering a threedimensional vortex, an exact sinusoidal function, and a spherical drop subjected to surface tension forces in equilibrium and immersed in a swirling velocity field. Finally, the turbulent atomization of a liquidgas jet is numerically analyzed to further assess the capabilities of the method.

Large eddy simulation of a turbulent diffusion flame: some aspects of subgrid modelling consistency
http://hdl.handle.net/2117/104310
Large eddy simulation of a turbulent diffusion flame: some aspects of subgrid modelling consistency
Ventosa Molina, Jordi; Lehmkuhl, Oriol; Pérez Segarra, Carlos David; Oliva Llena, Asensio
In the context of Large Eddy Simulation (LES) solely for the momentum
transport equation there may be found several models for the turbulent subgrid fluxes. Furthermore, among those relying on the eddy diffusivity approach, each model may be based on different invariants of the strain rate. Besides, when heat and mass transfer are also considered, closures for the subgrid turbulent scalar fluxes are also required. Hence, different model combinations may be considered. Additionally, when other physical phenomena are included, such as combustion, further subgrid modelling is involved. Therefore, in the present study a LES simulation of a turbulent diffusion flame is performed and different combination of subgrid models are used in order to analyse the numerical effects in the simulations. Several models for the turbulent momentum subgrid fluxes are considered, both constant and dynamically evaluated Schmidt numbers. Regarding combustion, in the context of the Flamelet/ProgressVariable (FPV) model, with an assumed probability density function for the turbulentchemistry interactions and four different closures for the subgrid mixture fraction variance are considered. Hence, a large number of model combinations are possible. The present study highlights the need for a consistent closure of subgrid effects. It is shown that, in the context of an FPV modelling, incorrect capture of subgrid mixing results in a flame liftoff for the studied flame (DLR A diffusion flame), even though experimentally an attached flame was reported. It is found that a consistent formulation is required, that is, all subgrid closures should become active in the same regions of the domain to avoid modelling inconsistencies. In contrast, when the classical flamelet approach is used, no liftoff is observed. The reason is that the classical flamelet includes only a limited subset of the possible flame states, i.e. only includes burning flamelets and extinguished flamelets for scalar dissipation rates past the extinction one.
This is a copy of the author 's final draft version of an article published in the journal Flow turbulence and combustion. The final publication is available at Springer via http://dx.doi.org/10.1007/s1049401798132
20170511T10:22:17Z
Ventosa Molina, Jordi
Lehmkuhl, Oriol
Pérez Segarra, Carlos David
Oliva Llena, Asensio
In the context of Large Eddy Simulation (LES) solely for the momentum
transport equation there may be found several models for the turbulent subgrid fluxes. Furthermore, among those relying on the eddy diffusivity approach, each model may be based on different invariants of the strain rate. Besides, when heat and mass transfer are also considered, closures for the subgrid turbulent scalar fluxes are also required. Hence, different model combinations may be considered. Additionally, when other physical phenomena are included, such as combustion, further subgrid modelling is involved. Therefore, in the present study a LES simulation of a turbulent diffusion flame is performed and different combination of subgrid models are used in order to analyse the numerical effects in the simulations. Several models for the turbulent momentum subgrid fluxes are considered, both constant and dynamically evaluated Schmidt numbers. Regarding combustion, in the context of the Flamelet/ProgressVariable (FPV) model, with an assumed probability density function for the turbulentchemistry interactions and four different closures for the subgrid mixture fraction variance are considered. Hence, a large number of model combinations are possible. The present study highlights the need for a consistent closure of subgrid effects. It is shown that, in the context of an FPV modelling, incorrect capture of subgrid mixing results in a flame liftoff for the studied flame (DLR A diffusion flame), even though experimentally an attached flame was reported. It is found that a consistent formulation is required, that is, all subgrid closures should become active in the same regions of the domain to avoid modelling inconsistencies. In contrast, when the classical flamelet approach is used, no liftoff is observed. The reason is that the classical flamelet includes only a limited subset of the possible flame states, i.e. only includes burning flamelets and extinguished flamelets for scalar dissipation rates past the extinction one.

DNS and regularization modeling of a turbulent differentially heated cavity of aspect ratio 5
http://hdl.handle.net/2117/103914
DNS and regularization modeling of a turbulent differentially heated cavity of aspect ratio 5
Trias Miquel, Francesc Xavier; Gorobets, Andrei; Oliva Llena, Asensio; Pérez Segarra, Carlos David
This work is devoted to the study of turbulent natural convection flows in
differentially heated cavities. The adopted configuration corresponds to an airfilled
(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 symmetrypreserving 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 is dynamically less complex than the original
NavierStokes equations, and therefore more amenable to be numerically solved.
Direct comparison with the DNS results shows fairly good agreement even for very coarse grids.
20170502T14:37:13Z
Trias Miquel, Francesc Xavier
Gorobets, Andrei
Oliva Llena, Asensio
Pérez Segarra, Carlos David
This work is devoted to the study of turbulent natural convection flows in
differentially heated cavities. The adopted configuration corresponds to an airfilled
(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 symmetrypreserving 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 is dynamically less complex than the original
NavierStokes equations, and therefore more amenable to be numerically solved.
Direct comparison with the DNS results shows fairly good agreement even for very coarse grids.