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http://hdl.handle.net/2117/3190
Sun, 31 Aug 2014 10:37:51 GMT2014-08-31T10:37:51Zwebmaster.bupc@upc.eduUniversitat Politècnica de Catalunya. Servei de Biblioteques i DocumentaciónoLarge eddy simulation of a turbulent jet diffusion flame using the Flamelet-Progress Variable model
http://hdl.handle.net/2117/22039
Title: Large eddy simulation of a turbulent jet diffusion flame using the Flamelet-Progress Variable model
Authors: Ventosa, Jordi; Lehmkuhl Barba, Oriol; Pérez Segarra, Carlos David; Oliva Llena, Asensio
Abstract: In this work an hydrogen enriched methane flame in a non-premixed configuration is studied, which corresponds to the DLR flame A. Large Eddy Simulation (LES) will be used to numerically analyse the case. Unstructured meshes are used and coupled with conservative discretisations of the differential operators. Chemical kinetics are modelled using the Flamelet/Progress-Variable model, taking into account differential diffusion effects. Computed first and second moments of the transported variables are shown to be in agreement with the experimental data.Thu, 13 Mar 2014 15:40:47 GMThttp://hdl.handle.net/2117/220392014-03-13T15:40:47ZVentosa, Jordi; Lehmkuhl Barba, Oriol; Pérez Segarra, Carlos David; Oliva Llena, AsensionoIn this work an hydrogen enriched methane flame in a non-premixed configuration is studied, which corresponds to the DLR flame A. Large Eddy Simulation (LES) will be used to numerically analyse the case. Unstructured meshes are used and coupled with conservative discretisations of the differential operators. Chemical kinetics are modelled using the Flamelet/Progress-Variable model, taking into account differential diffusion effects. Computed first and second moments of the transported variables are shown to be in agreement with the experimental data.Study of the autoignition of a hydrogen jet in a turbulent co-flow of heated air using LES modelling
http://hdl.handle.net/2117/22038
Title: Study of the autoignition of a hydrogen jet in a turbulent co-flow of heated air using LES modelling
Authors: Muela Castro, Jordi; Ventosa, Jordi; Lehmkuhl Barba, Oriol; Oliva Llena, Asensio
Abstract: The autoignition process of a hydrogen jet into a preheated turbulent air stream is numerically studied. A Progress-variable model with the turbulence-chemistry interaction s modelled using a Presumed Conditional Moment (PCM) closure has been used. Furthermore, the same case is studied using a Finite Rates model without closure for the reaction rate. The PV-PCM model reproduces satisfactorily the physical behaviour found in the experiments, although the model tends to underpredict the autoignition length. The results of the Finite Rates also capture accurately the autoignition phenomenology observed experimentally and the autoignition lengths are closer to those obtained in the experiment.Thu, 13 Mar 2014 15:33:05 GMThttp://hdl.handle.net/2117/220382014-03-13T15:33:05ZMuela Castro, Jordi; Ventosa, Jordi; Lehmkuhl Barba, Oriol; Oliva Llena, AsensionoThe autoignition process of a hydrogen jet into a preheated turbulent air stream is numerically studied. A Progress-variable model with the turbulence-chemistry interaction s modelled using a Presumed Conditional Moment (PCM) closure has been used. Furthermore, the same case is studied using a Finite Rates model without closure for the reaction rate. The PV-PCM model reproduces satisfactorily the physical behaviour found in the experiments, although the model tends to underpredict the autoignition length. The results of the Finite Rates also capture accurately the autoignition phenomenology observed experimentally and the autoignition lengths are closer to those obtained in the experiment.Large-eddy simulations of turbulent flow around a wall-mounted cube using an adaptive mesh refinement approach
http://hdl.handle.net/2117/22037
Title: Large-eddy simulations of turbulent flow around a wall-mounted cube using an adaptive mesh refinement approach
Authors: Antepara Zambrano, Óscar; Lehmkuhl Barba, Oriol; Oliva Llena, Asensio; Favre, Federico
Abstract: In the present work two LES models for predicting turbulent flow and an Adaptive Mesh Refinement (AMR) technique are proposed and tested for a fully 3D geometry: turbulent flow around a wall-mounted cube at Reh=7235. The wall-adapting eddy viscosity model within a variational multiscale method (VMS-WALE) and the QR model are tested to predict the flow around the body. The numerical algorithm used to solve the governing equations preserves the symmetry and conservation properties. AMR algorithm is applied to get enough grid-resolution to solve the vortical structures near the body, adapting the mesh according to physics-based refinement criteria. High order conservative schemes are applied in the connection between coarse and fine regions. The numerical results obtained are assessed and compared to the results of the direct numerical simulations (DNS) on the basis of first and second order statistics.Thu, 13 Mar 2014 15:28:58 GMThttp://hdl.handle.net/2117/220372014-03-13T15:28:58ZAntepara Zambrano, Óscar; Lehmkuhl Barba, Oriol; Oliva Llena, Asensio; Favre, FedericonoIn the present work two LES models for predicting turbulent flow and an Adaptive Mesh Refinement (AMR) technique are proposed and tested for a fully 3D geometry: turbulent flow around a wall-mounted cube at Reh=7235. The wall-adapting eddy viscosity model within a variational multiscale method (VMS-WALE) and the QR model are tested to predict the flow around the body. The numerical algorithm used to solve the governing equations preserves the symmetry and conservation properties. AMR algorithm is applied to get enough grid-resolution to solve the vortical structures near the body, adapting the mesh according to physics-based refinement criteria. High order conservative schemes are applied in the connection between coarse and fine regions. The numerical results obtained are assessed and compared to the results of the direct numerical simulations (DNS) on the basis of first and second order statistics.Blending regularization and large-eddy simulation. From homogeneous isotropic turbulence to wind farm boundary layers
http://hdl.handle.net/2117/22034
Title: Blending regularization and large-eddy simulation. From homogeneous isotropic turbulence to wind farm boundary layers
Authors: Folch Flórez, David; Trias Miquel, Francesc Xavier; Gorobets, Andrei; Oliva Llena, Asensio
Abstract: 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.Thu, 13 Mar 2014 15:24:24 GMThttp://hdl.handle.net/2117/220342014-03-13T15:24:24ZFolch Flórez, David; Trias Miquel, Francesc Xavier; Gorobets, Andrei; Oliva Llena, AsensionoThe 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.Direct numerical simulation of viscoplastic-type non-Newtonian fluid flows in stenosed arteries
http://hdl.handle.net/2117/21943
Title: Direct numerical simulation of viscoplastic-type non-Newtonian fluid flows in stenosed arteries
Authors: Carmona Muñoz, Ángel; Lehmkuhl Barba, Oriol; Pérez Segarra, Carlos David; Oliva Llena, Asensio
Abstract: The aim of this work is to provide DNS solutions for turbulence flows of viscoplastic-type non-Newtonian fluids and thus contribute to gain insight into the underlying physics of the non-Newtonian turbulent flows. This knowledge may be useful, among many other things, for developing more accurate turbulence models which describe better the implicit physics of this subject. Nevertheless, from our point of view, few DNS solutions of viscoplastic-type non-Newtonian fluid flows have been provided with this objective, despite the growing presence of these kind of fluids in the field of CFD simulations.Fri, 07 Mar 2014 16:20:34 GMThttp://hdl.handle.net/2117/219432014-03-07T16:20:34ZCarmona Muñoz, Ángel; Lehmkuhl Barba, Oriol; Pérez Segarra, Carlos David; Oliva Llena, AsensionoThe aim of this work is to provide DNS solutions for turbulence flows of viscoplastic-type non-Newtonian fluids and thus contribute to gain insight into the underlying physics of the non-Newtonian turbulent flows. This knowledge may be useful, among many other things, for developing more accurate turbulence models which describe better the implicit physics of this subject. Nevertheless, from our point of view, few DNS solutions of viscoplastic-type non-Newtonian fluid flows have been provided with this objective, despite the growing presence of these kind of fluids in the field of CFD simulations.From extruded-2D to fully-3D geometries for DNS: a multigrid-based extension of the Poisson solver
http://hdl.handle.net/2117/21618
Title: From extruded-2D to fully-3D geometries for DNS: a multigrid-based extension of the Poisson solver
Authors: Gorobets, Andrei; Trias Miquel, Francesc Xavier; Soria Guerrero, Manel; Pérez Segarra, Carlos David; Oliva Llena, Asensio
Abstract: Direct numerical simulation (DNS) of incompressible flows is an essential tool for improving the understanding of the physics of turbulence and for the development of better turbulence models. The Poisson equation, the main bottleneck from a parallel point of view, usually also limits its applicability for complex geometries. In this context, efficient and scalable Poisson solvers on fully-3D geometries are of high interest.In our previous work, a scalable algorithm for Poisson equation was proposed. It performed well on both small clusters with poor network performance and supercomputers using efficiently up to a thousand of CPUs. This algorithm named Krylov-Schur-Fourier Decomposition (KSFD) can be used for problems in parallelepipedic 3D domains with structured meshes and obstacles can be placed inside the flow. However, since a FFT decomposition is applied in one direction, mesh is restricted to be uniform and obstacles to be 2D shapes extruded along this direction.The present work is devoted to extend the previous KSFD algorithm to eliminate these limitations. The extension is based on a two-level Multigrid (MG) method that uses KSFD as a solver for second level. The algorithm is applied for a DNS of a turbulent flow in a channel with wall-mounted cube. Illustrative results at Re τ = 590 (based on the cube height and the bulk velocity Re h = 7235) are shown.Mon, 17 Feb 2014 15:33:45 GMThttp://hdl.handle.net/2117/216182014-02-17T15:33:45ZGorobets, Andrei; Trias Miquel, Francesc Xavier; Soria Guerrero, Manel; Pérez Segarra, Carlos David; Oliva Llena, Asensionoparallel 3D Poisson solver, Schur complement method, FFT, multigrid, preconditioned conjugate gradient, wall-mounted cube, DNSDirect numerical simulation (DNS) of incompressible flows is an essential tool for improving the understanding of the physics of turbulence and for the development of better turbulence models. The Poisson equation, the main bottleneck from a parallel point of view, usually also limits its applicability for complex geometries. In this context, efficient and scalable Poisson solvers on fully-3D geometries are of high interest.In our previous work, a scalable algorithm for Poisson equation was proposed. It performed well on both small clusters with poor network performance and supercomputers using efficiently up to a thousand of CPUs. This algorithm named Krylov-Schur-Fourier Decomposition (KSFD) can be used for problems in parallelepipedic 3D domains with structured meshes and obstacles can be placed inside the flow. However, since a FFT decomposition is applied in one direction, mesh is restricted to be uniform and obstacles to be 2D shapes extruded along this direction.The present work is devoted to extend the previous KSFD algorithm to eliminate these limitations. The extension is based on a two-level Multigrid (MG) method that uses KSFD as a solver for second level. The algorithm is applied for a DNS of a turbulent flow in a channel with wall-mounted cube. Illustrative results at Re τ = 590 (based on the cube height and the bulk velocity Re h = 7235) are shown.DNS of turbulent natural convection flows on the MareNostrum supercomputer
http://hdl.handle.net/2117/21617
Title: DNS of turbulent natural convection flows on the MareNostrum supercomputer
Authors: Trias Miquel, Francesc Xavier; Gorobets, Andrei; Soria Guerrero, Manel; Oliva Llena, Asensio
Abstract: A code for the direct numerical simulation (DNS) of incompressible turbulent flows that provides a fairly good scalability for a wide range of computer architectures has been developed. The spatial discretization of the incompressible Navier-Stokes equations is carried out using a fourth-order symmetry-preserving discretization. Since the code is fully explicit, from a parallel point of view, the main bottleneck is the Poisson equation. In the previous version of the code, that was conceived for low cost PC clusters with poor network performance, a Direct Schur-Fourier Decomposition (DSFD) algorithm was used to solve the Poisson equation. Such method, that was very efficient for PC clusters, can not be efficiently used with an arbitrarily large number of processors, mainly due to the RAM requirements (that grows with the number of processors). To do so, a new version of the solver, named Krylov-Schur-Fourier Decomposition (KSFD), is presented here. Basically, it is based on the Direct Schur Decomposition (DSD) algorithm that is used as a preconditioner for a Krylov method (CG) after Fourier decomposition. Benchmark results illustrating the robustness and scalability of the method on the MareNostrum supercomputer are presented and discussed. Finally, illustrative DNS simulations of wall-bounded turbulent flows are also presented.Mon, 17 Feb 2014 15:22:24 GMThttp://hdl.handle.net/2117/216172014-02-17T15:22:24ZTrias Miquel, Francesc Xavier; Gorobets, Andrei; Soria Guerrero, Manel; Oliva Llena, AsensionoDirect numerical simulation, MareNostrum supercomputer, parallel Poisson solver, Schur complement method, conjugate gradient, natural convectionA code for the direct numerical simulation (DNS) of incompressible turbulent flows that provides a fairly good scalability for a wide range of computer architectures has been developed. The spatial discretization of the incompressible Navier-Stokes equations is carried out using a fourth-order symmetry-preserving discretization. Since the code is fully explicit, from a parallel point of view, the main bottleneck is the Poisson equation. In the previous version of the code, that was conceived for low cost PC clusters with poor network performance, a Direct Schur-Fourier Decomposition (DSFD) algorithm was used to solve the Poisson equation. Such method, that was very efficient for PC clusters, can not be efficiently used with an arbitrarily large number of processors, mainly due to the RAM requirements (that grows with the number of processors). To do so, a new version of the solver, named Krylov-Schur-Fourier Decomposition (KSFD), is presented here. Basically, it is based on the Direct Schur Decomposition (DSD) algorithm that is used as a preconditioner for a Krylov method (CG) after Fourier decomposition. Benchmark results illustrating the robustness and scalability of the method on the MareNostrum supercomputer are presented and discussed. Finally, illustrative DNS simulations of wall-bounded turbulent flows are also presented.Low-frequency unsteadiness in the vortex formation region of a circular cylinder
http://hdl.handle.net/2117/21548
Title: Low-frequency unsteadiness in the vortex formation region of a circular cylinder
Authors: Lehmkuhl Barba, Oriol; Rodríguez Pérez, Ivette María; Borrell Pol, Ricard; Oliva Llena, Asensio
Abstract: The presence of low-frequency fluctuations in the wake of bluff bodies have been observed in several investigations. Even though the flow past a circular cylinder at Re = 3900 (Re = U ref D/ν) has been the object of several experimental and numerical investigations, there is a large scattering in the average statistics in the near wake. In the present work, the flow dynamics of the near wake region behind a circular cylinder has been investigated by means of direct numerical simulations and statistics have been computed for more than 858 shedding cycles. The analysis of instantaneous velocity signals of several probes located in the vortex formation region, point out the existence of a low-frequency fluctuation at the non-dimensional frequency of f m = 0.0064. This large-scale almost periodic motion seems to be related with the modulation of the recirculation bubble which causes its shrinking and enlargement over the time. Two different configurations have been identified: (i) a high-energy mode with larger fluctuations in the shear-layer and in the vortex formation region (Mode H) and (ii) a low-energy mode with weaker fluctuations in the shear layer (Mode L). The influence of such a low-frequency in the wake topology has been studied not only by means of the phase-average flow field for each mode, but also by the analysis of the time-average first- and second-order statistics of each wake mode. The results are compared with the long-term averaged solution and with results in the existing literature.
Description: Electronic version of an article published as "Physics of fluids", vol. 25, no 8, 2013. DOI: http://dx.doi.org/10.1063/1.4818641.Wed, 12 Feb 2014 15:33:46 GMThttp://hdl.handle.net/2117/215482014-02-12T15:33:46ZLehmkuhl Barba, Oriol; Rodríguez Pérez, Ivette María; Borrell Pol, Ricard; Oliva Llena, AsensionoBubbles, Computational fluid dynamics, External flows, Fflow instability, Flow simulation, Fluctuations, Numerical analysis, Shear flow, Two-phase flow, Vortices, WakesThe presence of low-frequency fluctuations in the wake of bluff bodies have been observed in several investigations. Even though the flow past a circular cylinder at Re = 3900 (Re = U ref D/ν) has been the object of several experimental and numerical investigations, there is a large scattering in the average statistics in the near wake. In the present work, the flow dynamics of the near wake region behind a circular cylinder has been investigated by means of direct numerical simulations and statistics have been computed for more than 858 shedding cycles. The analysis of instantaneous velocity signals of several probes located in the vortex formation region, point out the existence of a low-frequency fluctuation at the non-dimensional frequency of f m = 0.0064. This large-scale almost periodic motion seems to be related with the modulation of the recirculation bubble which causes its shrinking and enlargement over the time. Two different configurations have been identified: (i) a high-energy mode with larger fluctuations in the shear-layer and in the vortex formation region (Mode H) and (ii) a low-energy mode with weaker fluctuations in the shear layer (Mode L). The influence of such a low-frequency in the wake topology has been studied not only by means of the phase-average flow field for each mode, but also by the analysis of the time-average first- and second-order statistics of each wake mode. The results are compared with the long-term averaged solution and with results in the existing literature.Flow past a NACA0012 airfoil: from laminar separation bubbles to fully stalled regime
http://hdl.handle.net/2117/21439
Title: Flow past a NACA0012 airfoil: from laminar separation bubbles to fully stalled regime
Authors: Rodríguez Pérez, Ivette María; Lehmkuhl Barba, Oriol; Borrell Pol, Ricard; Oliva Llena, AsensioMon, 03 Feb 2014 15:40:09 GMThttp://hdl.handle.net/2117/214392014-02-03T15:40:09ZRodríguez Pérez, Ivette María; Lehmkuhl Barba, Oriol; Borrell Pol, Ricard; Oliva Llena, AsensionoDirect numerical simulation of a NACA0012 in full stall
http://hdl.handle.net/2117/21438
Title: Direct numerical simulation of a NACA0012 in full stall
Authors: Rodríguez Pérez, Ivette María; Lehmkuhl Barba, Oriol; Borrell Pol, Ricard; Oliva Llena, Asensio
Abstract: This work aims at investigating the mechanisms of separation and the transition to turbulence in the separated shear-layer of aerodynamic profiles, while at the same time to gain insight into coherent structures
formed in the separated zone at low-to-moderate Reynolds numbers. To do this, direct numerical simulations of the flow past a NACA0012 airfoil at Reynolds numbers Re = 50,000 (based on the free-stream velocity and the airfoil chord) and angles of attack AOA = 9.25 and AOA = 12 have been carried out. At low-to-moderate Reynolds numbers, NACA0012 exhibits a combination of leading-edge/trailing-edge stall which causes the massive separation of the flow on the suction side of the airfoil. The
initially laminar shear layer undergoes transition to turbulence and vortices formed are shed forming a von Kármán like vortex street in the airfoil wake. The main characteristics of this flow together with its
main features, including power spectra of a set of selected monitoring probes at different positions on the suction side and in the wake of the airfoil are provided and discussed in detail.Mon, 03 Feb 2014 15:29:47 GMThttp://hdl.handle.net/2117/214382014-02-03T15:29:47ZRodríguez Pérez, Ivette María; Lehmkuhl Barba, Oriol; Borrell Pol, Ricard; Oliva Llena, AsensionoThis work aims at investigating the mechanisms of separation and the transition to turbulence in the separated shear-layer of aerodynamic profiles, while at the same time to gain insight into coherent structures
formed in the separated zone at low-to-moderate Reynolds numbers. To do this, direct numerical simulations of the flow past a NACA0012 airfoil at Reynolds numbers Re = 50,000 (based on the free-stream velocity and the airfoil chord) and angles of attack AOA = 9.25 and AOA = 12 have been carried out. At low-to-moderate Reynolds numbers, NACA0012 exhibits a combination of leading-edge/trailing-edge stall which causes the massive separation of the flow on the suction side of the airfoil. The
initially laminar shear layer undergoes transition to turbulence and vortices formed are shed forming a von Kármán like vortex street in the airfoil wake. The main characteristics of this flow together with its
main features, including power spectra of a set of selected monitoring probes at different positions on the suction side and in the wake of the airfoil are provided and discussed in detail.Limits of the Oberbeck–Boussinesq approximation in a tall differentially heated cavity filled with water
http://hdl.handle.net/2117/21437
Title: Limits of the Oberbeck–Boussinesq approximation in a tall differentially heated cavity filled with water
Authors: Kizildag, Deniz; Rodríguez Pérez, Ivette María; Oliva Llena, Asensio; Lehmkuhl Barba, Oriol
Abstract: The present work assesses the limits of the Oberbeck–Boussinesq (OB) approximation for the resolution of turbulent fluid flow and heat transfer inside a tall differentially heated cavity of aspect ratio G = 6.67 filled with water (Pr = 3.27, Ra = 2.12e11). The cavity models the integrated solar collector-storage element installed on an advanced façade. The implications of the Oberbeck–Boussinesq approximation is submitted to investigation by means of direct numerical simulations (DNS) carried out for a wide range of temperature differences. Non-Oberbeck–Boussinesq (NOB) effects are found to be relevant, especially beyond the temperature difference of 30 °C, in the estimation of heat transfer, stratification, and flow configuration.Mon, 03 Feb 2014 14:56:31 GMThttp://hdl.handle.net/2117/214372014-02-03T14:56:31ZKizildag, Deniz; Rodríguez Pérez, Ivette María; Oliva Llena, Asensio; Lehmkuhl Barba, OriolnoNon-Oberbeck–Boussinesq effects, Turbulent natural convection, Differentially heated cavityThe present work assesses the limits of the Oberbeck–Boussinesq (OB) approximation for the resolution of turbulent fluid flow and heat transfer inside a tall differentially heated cavity of aspect ratio G = 6.67 filled with water (Pr = 3.27, Ra = 2.12e11). The cavity models the integrated solar collector-storage element installed on an advanced façade. The implications of the Oberbeck–Boussinesq approximation is submitted to investigation by means of direct numerical simulations (DNS) carried out for a wide range of temperature differences. Non-Oberbeck–Boussinesq (NOB) effects are found to be relevant, especially beyond the temperature difference of 30 °C, in the estimation of heat transfer, stratification, and flow configuration.Heat transfer analysis and numerical simulation of a parabolic trough solar collector
http://hdl.handle.net/2117/21435
Title: Heat transfer analysis and numerical simulation of a parabolic trough solar collector
Authors: Amine Hachicha, Ahmed; Rodríguez Pérez, Ivette María; Capdevila Paramio, Roser; Oliva Llena, Asensio
Abstract: Parabolic trough solar collector is the most proven industry-scale solar generation technology today available. The thermal performance of such devices is of major interest for optimising the solar field output and increase the efficiency of power plants. In this paper, a detailed numerical heat transfer model based on the finite volume method for these equipment is presented. In the model, the different elements of the receiver are discretised into several segments in both axial and azimuthal directions and energy balances are applied for each control volume. An optical model is also developed for calculating the non-uniform solar flux distribution around the receiver. This model is based on finite volume method and ray trace techniques and takes into account the finite size of the Sun. The solar heat flux is determined as a pre-processing task and coupled to the energy balance model as a boundary condition for the outer surface of the receiver. The set of algebraic equations are solved simultaneously using direct solvers. The model is thoroughly validated with results from the literature. First, the optical model is compared with known analytical solutions. After that, the performance of the overall model is tested against experimental measurements from Sandia National Laboratories and other un-irradiated receivers experiments. In all cases, results obtained shown a good agreement with experimental and analytical results.Mon, 03 Feb 2014 14:32:57 GMThttp://hdl.handle.net/2117/214352014-02-03T14:32:57ZAmine Hachicha, Ahmed; Rodríguez Pérez, Ivette María; Capdevila Paramio, Roser; Oliva Llena, AsensionoParabolic trough, CSP, Numerical model, Heat transfer analysis, Optical modelParabolic trough solar collector is the most proven industry-scale solar generation technology today available. The thermal performance of such devices is of major interest for optimising the solar field output and increase the efficiency of power plants. In this paper, a detailed numerical heat transfer model based on the finite volume method for these equipment is presented. In the model, the different elements of the receiver are discretised into several segments in both axial and azimuthal directions and energy balances are applied for each control volume. An optical model is also developed for calculating the non-uniform solar flux distribution around the receiver. This model is based on finite volume method and ray trace techniques and takes into account the finite size of the Sun. The solar heat flux is determined as a pre-processing task and coupled to the energy balance model as a boundary condition for the outer surface of the receiver. The set of algebraic equations are solved simultaneously using direct solvers. The model is thoroughly validated with results from the literature. First, the optical model is compared with known analytical solutions. After that, the performance of the overall model is tested against experimental measurements from Sandia National Laboratories and other un-irradiated receivers experiments. In all cases, results obtained shown a good agreement with experimental and analytical results.Direct and large-eddy simulation of non-oberbeck-boussinesq effects in a turbulent differentially heated cavity
http://hdl.handle.net/2117/21153
Title: Direct and large-eddy simulation of non-oberbeck-boussinesq effects in a turbulent differentially heated cavity
Authors: Kizildag, Deniz; Trias Miquel, Francesc Xavier; Rodríguez Pérez, Ivette María; Oliva Llena, AsensioTue, 07 Jan 2014 15:44:12 GMThttp://hdl.handle.net/2117/211532014-01-07T15:44:12ZKizildag, Deniz; Trias Miquel, Francesc Xavier; Rodríguez Pérez, Ivette María; Oliva Llena, AsensionoOn the large-eddy simulations of the flow past a cylinder at critical Reynolds numbers
http://hdl.handle.net/2117/21152
Title: On the large-eddy simulations of the flow past a cylinder at critical Reynolds numbers
Authors: Lehmkuhl Barba, Oriol; Rodríguez Pérez, Ivette María; Chiva Segura, Jorge; Borrell Pol, RicardTue, 07 Jan 2014 15:19:56 GMThttp://hdl.handle.net/2117/211522014-01-07T15:19:56ZLehmkuhl Barba, Oriol; Rodríguez Pérez, Ivette María; Chiva Segura, Jorge; Borrell Pol, RicardnoSymmetry-preserving discretization of Navier-Stokes equations on collocated unstructured meshes
http://hdl.handle.net/2117/21111
Title: Symmetry-preserving discretization of Navier-Stokes equations on collocated unstructured meshes
Authors: Trias Miquel, Francesc Xavier; Lehmkuhl Barba, Oriol; Oliva Llena, Asensio; Pérez Segarra, Carlos David; Verstappen, R.W.C.P.
Abstract: A fully-conservative discretization is presented in this paper. The same principles followed by Verstappen and Veldman (2003) [3] 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.Tue, 31 Dec 2013 10:28:23 GMThttp://hdl.handle.net/2117/211112013-12-31T10:28:23ZTrias Miquel, Francesc Xavier; Lehmkuhl Barba, Oriol; Oliva Llena, Asensio; Pérez Segarra, Carlos David; Verstappen, R.W.C.P.noSymmetry-preserving discretization, Collocated formulation, Unstructured grid, Checkerboard, Regularization, Differentially heated cavityA fully-conservative discretization is presented in this paper. The same principles followed by Verstappen and Veldman (2003) [3] 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.