Articles de revista
http://hdl.handle.net/2117/3191
20170620T05:35:40Z

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.

Three dimensionality in the wake of the flow around a circular cylinder at Reynolds number 5000
http://hdl.handle.net/2117/103891
Three dimensionality in the wake of the flow around a circular cylinder at Reynolds number 5000
Aljure Osorio, David E.; Lehmkuhl, Oriol; Rodríguez Pérez, Ivette María; Oliva Llena, Asensio
The turbulent flow around a circular cylinder has been investigated at Re=5000Re=5000 using direct numerical simulations. Low frequency behavior, vortex undulation, vortex splitting, vortex dislocations and three dimensional flow within the wake were found to happen at this flow regime. In order to successfully capture the wake three dimensionality, different spanwise lengths were considered. It was found that a length LZ=2pDLZ=2pD was enough to capture this behavior, correctly predicting different aspects of the flow such as drag coefficient, Strouhal number and pressure and velocity distributions when compared to experimental values. Two instability mechanisms were found to coexist in the present case study: a global type instability originating in the shear layer, which shows a characteristic frequency, and a convective type instability that seems to be constantly present in the near wake. Characteristics of both types of instabilities are identified and discussed in detail. As suggested by Norberg, a resonancetype effect takes place in the vortex formation region, as the coexistence of both instability mechanisms result in distorted vortex tubes. However, vortex coherence is never lost within the wake.
20170502T10:41:23Z
Aljure Osorio, David E.
Lehmkuhl, Oriol
Rodríguez Pérez, Ivette María
Oliva Llena, Asensio
The turbulent flow around a circular cylinder has been investigated at Re=5000Re=5000 using direct numerical simulations. Low frequency behavior, vortex undulation, vortex splitting, vortex dislocations and three dimensional flow within the wake were found to happen at this flow regime. In order to successfully capture the wake three dimensionality, different spanwise lengths were considered. It was found that a length LZ=2pDLZ=2pD was enough to capture this behavior, correctly predicting different aspects of the flow such as drag coefficient, Strouhal number and pressure and velocity distributions when compared to experimental values. Two instability mechanisms were found to coexist in the present case study: a global type instability originating in the shear layer, which shows a characteristic frequency, and a convective type instability that seems to be constantly present in the near wake. Characteristics of both types of instabilities are identified and discussed in detail. As suggested by Norberg, a resonancetype effect takes place in the vortex formation region, as the coexistence of both instability mechanisms result in distorted vortex tubes. However, vortex coherence is never lost within the wake.

Heat and moisture insulation by means of air curtains: application to refrigerated chambers
http://hdl.handle.net/2117/103512
Heat and moisture insulation by means of air curtains: application to refrigerated chambers
Giráldez García, Héctor; Pérez Segarra, Carlos David; Oliet Casasayas, Carles; Oliva Llena, Asensio
The present study is devoted to the determination of the efficiency of air curtain units (ACUs) applied to heat and moisture insulation of refrigerated chambers. A detailed study of the fluid dynamics and heat and mass transfer of the ACU in the refrigerated space and the external ambient is carried out by means of large eddy simulations (LES). The heat and moisture entrainment through the doorway and their transport inside the inner space are fully described. Three different configurations are studied: nonrecirculating, recirculating and twinjet air curtains. The condensation produced in the cool walls of the refrigerated space is evaluated considering the warm humid air from the ambient which penetrates inside the chamber through the doorway. The influence of both the discharge velocities and the discharge angles on the sealing capabilities of the three different tested ACU configurations is determined.
20170418T11:30:21Z
Giráldez García, Héctor
Pérez Segarra, Carlos David
Oliet Casasayas, Carles
Oliva Llena, Asensio
The present study is devoted to the determination of the efficiency of air curtain units (ACUs) applied to heat and moisture insulation of refrigerated chambers. A detailed study of the fluid dynamics and heat and mass transfer of the ACU in the refrigerated space and the external ambient is carried out by means of large eddy simulations (LES). The heat and moisture entrainment through the doorway and their transport inside the inner space are fully described. Three different configurations are studied: nonrecirculating, recirculating and twinjet air curtains. The condensation produced in the cool walls of the refrigerated space is evaluated considering the warm humid air from the ambient which penetrates inside the chamber through the doorway. The influence of both the discharge velocities and the discharge angles on the sealing capabilities of the three different tested ACU configurations is determined.

Simulation of the twofluid model on incompressible flow with Fractional Step method for both resolved and unresolved scale interfaces
http://hdl.handle.net/2117/103410
Simulation of the twofluid model on incompressible flow with Fractional Step method for both resolved and unresolved scale interfaces
Hou, Xiaofei; Rigola Serrano, Joaquim; Lehmkuhl Barba, Oriol; Oliva Llena, Asensio
In the present paper, the Fractional Step method usually used in single fluid flow is here extended and applied for the twofluid model resolution using the finite volume discretization. The use of a projection method resolution instead of the usual pressurecorrection method for multifluid flow, successfully avoids iteration processes. On the other hand, the main weakness of the two fluid model used for simulations of free surface flows, which is the numerical diffusion of the interface, is also solved by means of the conservative Level Set method (interface sharpening) (Strubelj et al., 2009). Moreover, the use of the algorithm proposed has allowed presenting different freesurface cases with or without Level Set implementation even under coarse meshes under a wide range of density ratios. Thus, the numerical results presented, numerically verified, experimentally validated and converged under high density ratios, shows the capability and reliability of this resolution method for both mixed and unmixed flows.
20170406T09:19:18Z
Hou, Xiaofei
Rigola Serrano, Joaquim
Lehmkuhl Barba, Oriol
Oliva Llena, Asensio
In the present paper, the Fractional Step method usually used in single fluid flow is here extended and applied for the twofluid model resolution using the finite volume discretization. The use of a projection method resolution instead of the usual pressurecorrection method for multifluid flow, successfully avoids iteration processes. On the other hand, the main weakness of the two fluid model used for simulations of free surface flows, which is the numerical diffusion of the interface, is also solved by means of the conservative Level Set method (interface sharpening) (Strubelj et al., 2009). Moreover, the use of the algorithm proposed has allowed presenting different freesurface cases with or without Level Set implementation even under coarse meshes under a wide range of density ratios. Thus, the numerical results presented, numerically verified, experimentally validated and converged under high density ratios, shows the capability and reliability of this resolution method for both mixed and unmixed flows.

On the evolution of flow topology in turbulent RayleighBénard convection
http://hdl.handle.net/2117/103099
On the evolution of flow topology in turbulent RayleighBénard convection
Dabbagh, Firas; Trias Miquel, Francesc Xavier; Gorobets, Andrei; Oliva Llena, Asensio
Smallscale dynamics is the spirit of turbulence physics. It implicates many attributes of flow topology evolution, coherent structures, hairpin vorticity dynamics, and mechanism of the kinetic energy cascade. In this work, several dynamical aspects of the smallscale motions have been numerically studied in a framework of RayleighBenard convection (RBC). To do so, direct numerical simulations have been carried out at two Rayleigh numbers Ra = 10(8) and 10(10), inside an airfilled rectangular cell of aspect ratio unity and pi spanwise openended distance. As a main feature, the average rate of the invariants of the velocity gradient tensor (Q(G), RG) has displayed the socalled
Copyright 2016 AIP Publishing. This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing.
20170330T10:44:50Z
Dabbagh, Firas
Trias Miquel, Francesc Xavier
Gorobets, Andrei
Oliva Llena, Asensio
Smallscale dynamics is the spirit of turbulence physics. It implicates many attributes of flow topology evolution, coherent structures, hairpin vorticity dynamics, and mechanism of the kinetic energy cascade. In this work, several dynamical aspects of the smallscale motions have been numerically studied in a framework of RayleighBenard convection (RBC). To do so, direct numerical simulations have been carried out at two Rayleigh numbers Ra = 10(8) and 10(10), inside an airfilled rectangular cell of aspect ratio unity and pi spanwise openended distance. As a main feature, the average rate of the invariants of the velocity gradient tensor (Q(G), RG) has displayed the socalled

On the properties of discrete spatial filters for CFD
http://hdl.handle.net/2117/102792
On the properties of discrete spatial filters for CFD
Baez Vidal, Aleix; Lehmkuhl Barba, Oriol; Trias Miquel, Francesc Xavier; Pérez Segarra, Carlos David
The spatial filtering of variables in the context of Computational Fluid Dynamics (CFD) is a common practice. Most of the discrete filters used in CFD simulations are locally accurate models of continuous operators. However, when filters are adaptative, i.e. the filter width is not constant, or meshes are irregular, discrete filters sometimes break relevant global properties of the continuous models they are based on. For example, the principle of maxima and minima reduction or conservation are eventually infringed. In this paper, we analyze the properties of analytic continuous convolution filters and extract those we consider to define filtering. Then, we impose the accomplishment of these properties on explicit discrete filters by means of constraints. Three filters satisfying the derived conditions are deduced and compared to common differential discrete CFD filters on synthetic fields. Tests on the developed discrete filters show the fulfillment of the imposed properties. In particular, the problem of maxima and minima generation is resolved for physically relevant cases. The tests are conducted on the basis of the eigenvectors of graph Laplacian matrices of meshes. Thus, insight into the relations between filtering and oscillation growth on general meshes is provided. Further tests on singularity fields and on isentropic vortices have also been conducted to evaluate the performance of filters on basic CFD fields. Results confirm that imposing the proposed conditions makes discrete filters properties consistent with those of the continuous ones.
© 2016. This version is made available under the CCBYNCND 4.0 license http://creativecommons.org/licenses/byncnd/4.0/
20170322T12:10:06Z
Baez Vidal, Aleix
Lehmkuhl Barba, Oriol
Trias Miquel, Francesc Xavier
Pérez Segarra, Carlos David
The spatial filtering of variables in the context of Computational Fluid Dynamics (CFD) is a common practice. Most of the discrete filters used in CFD simulations are locally accurate models of continuous operators. However, when filters are adaptative, i.e. the filter width is not constant, or meshes are irregular, discrete filters sometimes break relevant global properties of the continuous models they are based on. For example, the principle of maxima and minima reduction or conservation are eventually infringed. In this paper, we analyze the properties of analytic continuous convolution filters and extract those we consider to define filtering. Then, we impose the accomplishment of these properties on explicit discrete filters by means of constraints. Three filters satisfying the derived conditions are deduced and compared to common differential discrete CFD filters on synthetic fields. Tests on the developed discrete filters show the fulfillment of the imposed properties. In particular, the problem of maxima and minima generation is resolved for physically relevant cases. The tests are conducted on the basis of the eigenvectors of graph Laplacian matrices of meshes. Thus, insight into the relations between filtering and oscillation growth on general meshes is provided. Further tests on singularity fields and on isentropic vortices have also been conducted to evaluate the performance of filters on basic CFD fields. Results confirm that imposing the proposed conditions makes discrete filters properties consistent with those of the continuous ones.

Numerical and experimental investigation of a vertical LiBr falling film absorber considering wave regimes and in presence of mist flow
http://hdl.handle.net/2117/102637
Numerical and experimental investigation of a vertical LiBr falling film absorber considering wave regimes and in presence of mist flow
Garcia Rivera, Eduardo; Castro González, Jesús; Farnós Baulenas, Joan; Oliva Llena, Asensio
The absorber represents the most critical component in absorption systems and one of the key issues. In this component complex heat and mass transfer phenomena during the absorption process takes place simultaneously. For this reason the development of mathematical models validated against experimental data always constitutes useful tools for the design and improvement of falling film absorbers. A testing device has been designed and built to reproduce absorption phenomena in vertical LiBrH2O falling film absorbers with the primary objective to obtain experimental data. On the other hand, a mathematical model of falling film absorption of H2O vapour in LiBr aqueous solutions has been implemented. Wave regime is considered by including and solving the Free Surface Deflection Equation. The numerical results are validated using the experimental data.
During the development of this work, the authors have paid careful attention to the verification of experimental data. Such verification consists of performing energy and mass balances in the fluid film side. Important discrepancies were found in our experimental data. Therefore, an extensive study was carried out in order to find the source of such errors. The conclusion is that there is a drag of LiBr solution in the water vapour which increases with the Re number. This mist flow cannot be measured experimentally, but can be evaluated in an indirect way. The mathematical models have been adapted in order to consider the influence of mist flow. On the other hand, in the literature there are not many experimental works related to falling film absorbers which expose enough information to verify the reliability of their experimental data.
20170320T08:44:26Z
Garcia Rivera, Eduardo
Castro González, Jesús
Farnós Baulenas, Joan
Oliva Llena, Asensio
The absorber represents the most critical component in absorption systems and one of the key issues. In this component complex heat and mass transfer phenomena during the absorption process takes place simultaneously. For this reason the development of mathematical models validated against experimental data always constitutes useful tools for the design and improvement of falling film absorbers. A testing device has been designed and built to reproduce absorption phenomena in vertical LiBrH2O falling film absorbers with the primary objective to obtain experimental data. On the other hand, a mathematical model of falling film absorption of H2O vapour in LiBr aqueous solutions has been implemented. Wave regime is considered by including and solving the Free Surface Deflection Equation. The numerical results are validated using the experimental data.
During the development of this work, the authors have paid careful attention to the verification of experimental data. Such verification consists of performing energy and mass balances in the fluid film side. Important discrepancies were found in our experimental data. Therefore, an extensive study was carried out in order to find the source of such errors. The conclusion is that there is a drag of LiBr solution in the water vapour which increases with the Re number. This mist flow cannot be measured experimentally, but can be evaluated in an indirect way. The mathematical models have been adapted in order to consider the influence of mist flow. On the other hand, in the literature there are not many experimental works related to falling film absorbers which expose enough information to verify the reliability of their experimental data.

Drain water heat recovery storagetype unit for residential housing
http://hdl.handle.net/2117/102396
Drain water heat recovery storagetype unit for residential housing
Torras Ortiz, Santiago; Oliet Casasayas, Carles; Rigola Serrano, Joaquim; Oliva Llena, Asensio
The drain water heat recovery (DWHR) system is an interesting household technology to reduce energy costs and environmental impact. The objective of the utilisation of these devices is the recovery of the waste heat from domestic warm drain water, and transferring it to cold water entering the house. A drain water heat recovery unit has been built in this work. The authors are using both numerical and experimental tools to design and study the performance of this device, focusing on the analysis of a specific drain water heat recovery storagetype based on a cylindrical tank with an internal coiled pipe. The numerical simulation has been performed using an inhouse platform, where the different elements of the DWHR storage are linked to solve the system. On the other hand, an experimental infrastructure has been developed to analyse of the system, which has been instrumented to provide detailed information of its heat recovery and storage capacities and temperature map. Different internal flow rates and operational temperatures have been studied. From the results obtained it can be said that the device shows interesting heat recovery and storage capacities, while the numerical platform shows promising comparison results against the experiments.
© 2016. This version is made available under the CCBYNCND 4.0 license http://creativecommons.org/licenses/byncnd/4.0/
20170313T12:49:47Z
Torras Ortiz, Santiago
Oliet Casasayas, Carles
Rigola Serrano, Joaquim
Oliva Llena, Asensio
The drain water heat recovery (DWHR) system is an interesting household technology to reduce energy costs and environmental impact. The objective of the utilisation of these devices is the recovery of the waste heat from domestic warm drain water, and transferring it to cold water entering the house. A drain water heat recovery unit has been built in this work. The authors are using both numerical and experimental tools to design and study the performance of this device, focusing on the analysis of a specific drain water heat recovery storagetype based on a cylindrical tank with an internal coiled pipe. The numerical simulation has been performed using an inhouse platform, where the different elements of the DWHR storage are linked to solve the system. On the other hand, an experimental infrastructure has been developed to analyse of the system, which has been instrumented to provide detailed information of its heat recovery and storage capacities and temperature map. Different internal flow rates and operational temperatures have been studied. From the results obtained it can be said that the device shows interesting heat recovery and storage capacities, while the numerical platform shows promising comparison results against the experiments.