The Experts below are selected from a list of 5943 Experts worldwide ranked by ideXlab platform
E Rank - One of the best experts on this subject based on the ideXlab platform.
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a p version finite element approach for two and three dimensional problems of the j2 flow theory with non linear isotropic hardening
International Journal for Numerical Methods in Engineering, 2002Co-Authors: Alexander Düster, E RankAbstract:In this paper an implementation of a two- and three-dimensional p-version approach to the J 2 flow theory with non-linear isotropic hardening for small displacements and small strains is presented. Based on higher-order quadrilateral and hexahedral element formulations, a Newton-Raphson iteration scheme combined with a radial return algorithm is applied to find approximate solutions for the underlying physically non-linear model problem. Curved boundaries are taken care of with the Blending Function method, allowing an accurate representation of geometry with only a few p-elements. Numerical examples demonstrate, that the p-version supplies efficient and accurate approximations to this class of physically non-linear problems.
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the p version of the finite element method for three dimensional curved thin walled structures
International Journal for Numerical Methods in Engineering, 2001Co-Authors: Alexander Düster, H. Bröker, E RankAbstract:In this paper we present an implementation of a three-dimensional p-version for structural problems of solids with almost arbitrarily curved surfaces. Applying the Blending Function method, complex structures can often be modelled by a few p-elements, being the basis for a higher order approximation. Numerical examples will demonstrate, that the p-version with anisotropic Ansatz spaces allows to predict the structural behaviour of three-dimensional plates and shells with approximately the same amount of degrees of freedom as in the two-dimensional case, yet significantly more accurate due to the three-dimensional model. Furthermore, it is advantageous to compute complex structures exclusively with three-dimensional discretizations as no special elements are needed to model the transition from dimensionally reduced formulations like plates or shells to fully three-dimensional solid elements. Using the p-version with anisotropic Ansatz spaces the whole structure can be efficiently discretized with solid elements, even if the aspect ratio of the elements becomes very large. Copyright © 2001 John Wiley Sons, Ltd.
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The p-version of the finite element method for three-dimensional curved thin walled structures
International Journal for Numerical Methods in Biomedical Engineering, 2001Co-Authors: A Düster, H. Bröker, E RankAbstract:In this paper we present an implementation of a three-dimensional p-version for structural problems of solids with almost arbitrarily curved surfaces. Applying the Blending Function method, complex structures can often be modelled by a few p-elements, being the basis for a higher order approximation. Numerical examples will demonstrate, that the p-version with anisotropic Ansatz spaces allows to predict the structural behaviour of three-dimensional plates and shells with approximately the same amount of degrees of freedom as in the two-dimensional case, yet significantly more accurate due to the three-dimensional model. Furthermore, it is advantageous to compute complex structures exclusively with three-dimensional discretizations as no special elements are needed to model the transition from dimensionally reduced formulations like plates or shells to fully three-dimensional solid elements. Using the p-version with anisotropic Ansatz spaces the whole structure can be efficiently discretized with solid elements, even if the aspect ratio of the elements be comes very large .
H. A. Hassan - One of the best experts on this subject based on the ideXlab platform.
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compressible flow simulations using a new large eddy simulation reynolds averaged navier stokes model
AIAA Journal, 2011Co-Authors: Daniel A Gieseking, Jack R. Edwards, Jungil Choi, H. A. HassanAbstract:A new hybrid large-eddy simulation/Reynolds-averaged Navier–Stokes simulation (LES/RANS) method is presented in this work. In this approach, the resolved turbulence kinetic energy, ensemble-averaged modeled turbulence kinetic energy and turbulence frequency, and time-resolved turbulence frequency are used to form an estimate of an outer-layer turbulence length scale that is nearly Reynolds-number-independent. The ratio of this outer-layer scale with an inner-layer length scale (proportional to the wall distance) is used to construct a Blending Function that facilitates the shift between an unsteady RANS formulation near solid surfaces and a LES formulation away from the wall. The new model is tested through simulations of compressible flat-plate boundary layers over a widerangeofReynoldsnumbersandMach2.86 flowoverasmoothcompressionramp.Theresultsshowthatthenew modelpredicts mean andsecond-moment statistics that arein goodagreement withexperiment andare comparable with those obtained using an earlier model (Edwards, J. R., Choi, J-I., and Boles, J. A., “Hybrid Large-Eddy/ Reynolds-Averaged Navier–Stokes Simulation of a Mach-5 Compression Corner Interaction,” AIAA Journal, Vol. 464, 2008, pp. 977–991.) which required a case-by-case calibration of a model constant.
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Blending Functions in Hybrid Large-Eddy/Reynolds-Averaged Navier-Stokes Simulations
AIAA Journal, 2004Co-Authors: Xudong Xiao, Jack R. Edwards, H. A. HassanAbstract:Several Blending Functions for use in hybrid large-eddy simulation (LES)/Reynolds-averaged Navier-Stokes (RANS) simulations of shock-separated flows are tested. The Blending Functions shift the turbulence closure from a k-ζ turbulence model near solid surfaces to a k − ∆ subgrid closure away from the wall. Three distinct forms for the Blending Function are developed: one that depends on the ratio of the von Karman length scale and the Taylor microscale, another that depends on the ratio of the RANS eddy viscosity to the subgrid eddy viscosity, and a third which replaces the von Karman length scale in the first form with the distance to the nearest wall. Comparisons are made for two cases: Mach 2.79 flow over a 20-deg compression corner and Mach 2.88 flow over a 25-deg compression/expansion corner. Inflow boundary conditions for all calculations employ the rescaling/reintroducing procedure developed by Xiao et al. (Xiao, X., Edwards, J. R., Hassan, H. A., and Baurle, R. A., "Inflow Boundary Conditions for Hybrid Large Eddy/Reynolds Averaged Navier-Stokes Simulations," AIAA Journal ,V ol. 41, No. 8, 2003, pp. 1481-1489) for hybrid LES/RANS simulations of wall-bounded flows. In general, the Blending Function based on the von Karman length scale gives the best results when compared with measured data. The skin friction predictions show the highest sensitivity to the various Blending Functions.
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Investigation of Flow Dependent Blending Functions in Hybrid LES/RANS Simulations
33rd AIAA Fluid Dynamics Conference and Exhibit, 2003Co-Authors: Xudong Xiao, Jack R. Edwards, H. A. HassanAbstract:A Blending Function that depends on the ratio of the von Karman length scale and the Taylor microscale in conjunction with k‐ turbulence model is used to study two Mach 3 shock wave/boundary layer interactions on 20 degree compression ramp and 25 degree compression-expansion ramp. The results are compared with two other Blending Functions: one in which the von Karman length scale is replaced by the distance to the nearest wall while the other employs a grid dependent expression proportional to the ratio of the RANS eddy viscosity to that of the LES. The results were further compared with a calculation where the underlying RANS model is that of Menter’s shear stress (SST) model with a Blending Function that depends on the distance to the wall. All inflow boundaries employ the rescaling-reintroducing procedure developed by Xiao et al. for hybrid LES/RANS approaches. In general, the Blending Function based on the von Karman length scale gives the best results when compared with available measurements. The skin friction measurements showed the highest sensitivity to the various Blending Functions and the two RANS models employed.
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hybrid simulation approach for cavity flows Blending algorithm and boundary treatment issues
Computational Fluid Dynamics Conference, 2003Co-Authors: R A Baurle, Jack R. Edwards, H. A. HassanAbstract:The maturation of high-performance computer architectures and computational algorithms has prompted the development of a new generation of models that attempt to combine the robustness and efficiency offered by the Reynolds averaged Navier-Stokes equations with the higher level of modeling offered by the equations developed for large eddy simulation. The application of a new hybrid approach is discussed, where the transition between these equation sets is controlled by a Blending Function that depends on local turbulent flow properties, as well as the local mesh spacing. The utilization of local turbulence properties provides added control in specifying the regions of the flow intended for each equation set, removing much of the burden from the grid-generation process. Moreover, the model framework allows for the combination of existing closure model equations, avoiding the difficulty of formulating a single set of closure coefficients that perform well in both Reynolds averaged and large eddy simulation modes. Simple modifications to common second-order accurate Reynolds averaged Navier-Stokes algorithms are proposed to enhance the capturing of large eddy motions
Alexander Düster - One of the best experts on this subject based on the ideXlab platform.
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axisymmetric pressure boundary loading for finite deformation analysis using p fem
Computer Methods in Applied Mechanics and Engineering, 2007Co-Authors: Zohar Yosibash, Ulrich Heisserer, Ernst Rank, Stefan Hartmann, Alexander Düster, Mordechai SzantoAbstract:Follower loads, i.e. loads which depend on the boundary displacements by definition, frequently occur in finite deformation boundary-value problems. Restricting to axisymmetrical applications, we provide analytical and numerical solutions for a set of problems in compressible Neo-Hookean materials so to serve as benchmark problems for verifying the accuracy and efficiency of various FE methods for follower load applications. Thereafter, the weak formulation for the follower-load in 3-D domain is reduced to an axisymmetrical setting, and, subsequently, consistently linearized in the framework of p-FEMs, exploiting the Blending Function mapping techniques. The set of axisymmetric benchmark solutions is compared to numerical experiments, in which the results obtained by a p-FEM code are compared to these obtained by a state-of-the-art commercial h-FEM code and to the ‘‘exact’’ results. These demonstrate the efficiency and accuracy of p-FEMs when applied to problems in finite deformations with follower loads. � 2006 Elsevier B.V. All rights reserved.
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a p version finite element approach for two and three dimensional problems of the j2 flow theory with non linear isotropic hardening
International Journal for Numerical Methods in Engineering, 2002Co-Authors: Alexander Düster, E RankAbstract:In this paper an implementation of a two- and three-dimensional p-version approach to the J 2 flow theory with non-linear isotropic hardening for small displacements and small strains is presented. Based on higher-order quadrilateral and hexahedral element formulations, a Newton-Raphson iteration scheme combined with a radial return algorithm is applied to find approximate solutions for the underlying physically non-linear model problem. Curved boundaries are taken care of with the Blending Function method, allowing an accurate representation of geometry with only a few p-elements. Numerical examples demonstrate, that the p-version supplies efficient and accurate approximations to this class of physically non-linear problems.
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the p version of the finite element method for three dimensional curved thin walled structures
International Journal for Numerical Methods in Engineering, 2001Co-Authors: Alexander Düster, H. Bröker, E RankAbstract:In this paper we present an implementation of a three-dimensional p-version for structural problems of solids with almost arbitrarily curved surfaces. Applying the Blending Function method, complex structures can often be modelled by a few p-elements, being the basis for a higher order approximation. Numerical examples will demonstrate, that the p-version with anisotropic Ansatz spaces allows to predict the structural behaviour of three-dimensional plates and shells with approximately the same amount of degrees of freedom as in the two-dimensional case, yet significantly more accurate due to the three-dimensional model. Furthermore, it is advantageous to compute complex structures exclusively with three-dimensional discretizations as no special elements are needed to model the transition from dimensionally reduced formulations like plates or shells to fully three-dimensional solid elements. Using the p-version with anisotropic Ansatz spaces the whole structure can be efficiently discretized with solid elements, even if the aspect ratio of the elements becomes very large. Copyright © 2001 John Wiley Sons, Ltd.
Jack R. Edwards - One of the best experts on this subject based on the ideXlab platform.
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compressible flow simulations using a new large eddy simulation reynolds averaged navier stokes model
AIAA Journal, 2011Co-Authors: Daniel A Gieseking, Jack R. Edwards, Jungil Choi, H. A. HassanAbstract:A new hybrid large-eddy simulation/Reynolds-averaged Navier–Stokes simulation (LES/RANS) method is presented in this work. In this approach, the resolved turbulence kinetic energy, ensemble-averaged modeled turbulence kinetic energy and turbulence frequency, and time-resolved turbulence frequency are used to form an estimate of an outer-layer turbulence length scale that is nearly Reynolds-number-independent. The ratio of this outer-layer scale with an inner-layer length scale (proportional to the wall distance) is used to construct a Blending Function that facilitates the shift between an unsteady RANS formulation near solid surfaces and a LES formulation away from the wall. The new model is tested through simulations of compressible flat-plate boundary layers over a widerangeofReynoldsnumbersandMach2.86 flowoverasmoothcompressionramp.Theresultsshowthatthenew modelpredicts mean andsecond-moment statistics that arein goodagreement withexperiment andare comparable with those obtained using an earlier model (Edwards, J. R., Choi, J-I., and Boles, J. A., “Hybrid Large-Eddy/ Reynolds-Averaged Navier–Stokes Simulation of a Mach-5 Compression Corner Interaction,” AIAA Journal, Vol. 464, 2008, pp. 977–991.) which required a case-by-case calibration of a model constant.
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large eddy reynolds averaged navier stokes simulations of sonic injection into mach 2 crossflow
AIAA Journal, 2010Co-Authors: Joh A Oles, Jack R. Edwards, Robe A AurleAbstract:Computational predictions of transverse injection of air, helium, and ethylene into a Mach 1.98 crossflow of air are presented. A hybrid large-eddy simulation/Reynolds-averaged Navier―Stokes turbulence model is used. A Blending Function, dependent on modeled turbulence variables, is used to shift the turbulence closure from the Menter t-ω model near solid surfaces to a Smagorinsky subgrid model in the outer part of the incoming boundary layer and in the jet mixing zone. The results show reasonably good agreement with time-averaged Mie-scattering images of the plume structure for both helium and air injection and with experimental surface pressure distributions, even though the penetration of the jet into the crossflow is slightly overpredicted. Predictions of ethylene mole fraction at several transverse stations within the plume are in good agreement with time-averaged Raman-scattering mole-fraction data. The model results are used to examine the validity of the commonly used assumption of the constant turbulent Schmidt number in the intense mixing zone downstream of the injection location. The assumption of a constant turbulent Schmidt is shown to be inadequate for jet mixing dominated by large-scale entrainment.
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Blending Functions in Hybrid Large-Eddy/Reynolds-Averaged Navier-Stokes Simulations
AIAA Journal, 2004Co-Authors: Xudong Xiao, Jack R. Edwards, H. A. HassanAbstract:Several Blending Functions for use in hybrid large-eddy simulation (LES)/Reynolds-averaged Navier-Stokes (RANS) simulations of shock-separated flows are tested. The Blending Functions shift the turbulence closure from a k-ζ turbulence model near solid surfaces to a k − ∆ subgrid closure away from the wall. Three distinct forms for the Blending Function are developed: one that depends on the ratio of the von Karman length scale and the Taylor microscale, another that depends on the ratio of the RANS eddy viscosity to the subgrid eddy viscosity, and a third which replaces the von Karman length scale in the first form with the distance to the nearest wall. Comparisons are made for two cases: Mach 2.79 flow over a 20-deg compression corner and Mach 2.88 flow over a 25-deg compression/expansion corner. Inflow boundary conditions for all calculations employ the rescaling/reintroducing procedure developed by Xiao et al. (Xiao, X., Edwards, J. R., Hassan, H. A., and Baurle, R. A., "Inflow Boundary Conditions for Hybrid Large Eddy/Reynolds Averaged Navier-Stokes Simulations," AIAA Journal ,V ol. 41, No. 8, 2003, pp. 1481-1489) for hybrid LES/RANS simulations of wall-bounded flows. In general, the Blending Function based on the von Karman length scale gives the best results when compared with measured data. The skin friction predictions show the highest sensitivity to the various Blending Functions.
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Investigation of Flow Dependent Blending Functions in Hybrid LES/RANS Simulations
33rd AIAA Fluid Dynamics Conference and Exhibit, 2003Co-Authors: Xudong Xiao, Jack R. Edwards, H. A. HassanAbstract:A Blending Function that depends on the ratio of the von Karman length scale and the Taylor microscale in conjunction with k‐ turbulence model is used to study two Mach 3 shock wave/boundary layer interactions on 20 degree compression ramp and 25 degree compression-expansion ramp. The results are compared with two other Blending Functions: one in which the von Karman length scale is replaced by the distance to the nearest wall while the other employs a grid dependent expression proportional to the ratio of the RANS eddy viscosity to that of the LES. The results were further compared with a calculation where the underlying RANS model is that of Menter’s shear stress (SST) model with a Blending Function that depends on the distance to the wall. All inflow boundaries employ the rescaling-reintroducing procedure developed by Xiao et al. for hybrid LES/RANS approaches. In general, the Blending Function based on the von Karman length scale gives the best results when compared with available measurements. The skin friction measurements showed the highest sensitivity to the various Blending Functions and the two RANS models employed.
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hybrid simulation approach for cavity flows Blending algorithm and boundary treatment issues
Computational Fluid Dynamics Conference, 2003Co-Authors: R A Baurle, Jack R. Edwards, H. A. HassanAbstract:The maturation of high-performance computer architectures and computational algorithms has prompted the development of a new generation of models that attempt to combine the robustness and efficiency offered by the Reynolds averaged Navier-Stokes equations with the higher level of modeling offered by the equations developed for large eddy simulation. The application of a new hybrid approach is discussed, where the transition between these equation sets is controlled by a Blending Function that depends on local turbulent flow properties, as well as the local mesh spacing. The utilization of local turbulence properties provides added control in specifying the regions of the flow intended for each equation set, removing much of the burden from the grid-generation process. Moreover, the model framework allows for the combination of existing closure model equations, avoiding the difficulty of formulating a single set of closure coefficients that perform well in both Reynolds averaged and large eddy simulation modes. Simple modifications to common second-order accurate Reynolds averaged Navier-Stokes algorithms are proposed to enhance the capturing of large eddy motions
Stefano Rolfo - One of the best experts on this subject based on the ideXlab platform.
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LES and Hybrid RANS/LES turbulence modelling in unstructured finite volume code and applications to nuclear reactor fuel bundle
2010Co-Authors: Stefano RolfoAbstract:Rod bundle is a typical constitutive element of a very wide range of nuclear reactor designs. This thesis describes the investigation of such geometry with wall-resolved Large Eddy Simulation (LES). In order to alleviate the mesh constraint, imposed by the near wall resolution, the usage of embedded refinements and polyhedral meshes is analysed firstly with a inviscid laminar case (Taylor Green vortices) and secondly with a fully turbulent case (channel flow only with embedded refinement). The inviscid test case shows that the addition of embedded refinements decreases the conservation properties of the code. Indeed the accuracy decreases from second order in a structured conformal mesh, to something in between first and second order depending on the quality of the unstructured mesh. Better results are obtained when the interface between refined and coarse areas presents a more regular and structured pattern, reducing the generation of skewed and stretched cells. The channel flow simulation shows that the Reynolds stresses, of some embedded refined meshes, are affected by spurious oscillations. Surprisingly this effect is present in the unstructured meshes with the best orthogonal properties. Indeed analysis of Reynolds stress budgets shows that terms, where the gradient in the wall normal direction is dominant, have a largely oscillatory behaviour. The cause of the problem is attributed to the convective term and in particular in the method used for the gradient reconstruction. As a consequence of these contradictory signs between the inviscid and the fully turbulent cases, the rod bundle test case is analysed using a conventional body fitted multiblock mesh. Two different Reynolds numbers are investigated reporting Reynolds stresses and budgets. The flow is characterised by an energetic and almost periodic azimuthal flow pulsation in the gap region between adjacent sub-channels, which makes turbulent quantities largely different from those in plane channel and pipes and enhances mixing. Experiments found that a constant Strouhal number, with the variation of the Reynolds number, characterises the phenomenon. The frequency analysis finds that present simulations are distinguished by three dominant frequencies, the first in agreement with the experimental value and two higher ones, which might be due to the correlation of the azimuthal velocity in the streamwise direction. Several passive temperature fields are added at the simulations in order to study the effects of the variation of the Prandtl number and the change in boundary conditions (Neumann and Dirichlet). A simplified case where an imbalance of the scalar between adjacent sub-channels is also investigated in order to evaluate the variation of the heat fluxes with respect to the homogeneous case. An alternative solution, to reduce the mesh constraint imposed by the wall, is to hybridize LES with RANS. The main achievement of this work is to integrate the heat transfer modelling to the already existing model for the dynamic part. Further investigations of the Blending Function, used to merge the two velocity fields, are carried out in conjunction with a study of the model dependency on the mesh resolution. The validation is performed on a fully developed channel flow at different Reynolds numbers and with constant wall heat flux. On coarse meshes the model shows an improvement of the results for both thermal and hydraulic parts with respect to a standard LES. On refined meshes, suitable for wall-resolved LES, the model suffers from a problem of double counting of modelled Reynolds stresses and heat fluxes because the RANS contribution does not naturally disappear as the mesh resolution increases.
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les and hybrid rans les turbulence modelling in unstructured finite volume code and applications to nuclear reactor fuel bundle
[Thesis]. Manchester UK: The University of Manchester; 2010., 2010Co-Authors: Stefano RolfoAbstract:Rod bundle is a typical constitutive element of a very wide range of nuclear reactor designs. This thesis describes the investigation of such geometry with wall-resolved Large Eddy Simulation (LES). In order to alleviate the mesh constraint, imposed by the near wall resolution, the usage of embedded refinements and polyhedral meshes is analysed firstly with a inviscid laminar case (Taylor Green vortices) and secondly with a fully turbulent case (channel flow only with embedded refinement). The inviscid test case shows that the addition of embedded refinements decreases the conservation properties of the code. Indeed the accuracy decreases from second order in a structured conformal mesh, to something in between first and second order depending on the quality of the unstructured mesh. Better results are obtained when the interface between refined and coarse areas presents a more regular and structured pattern, reducing the generation of skewed and stretched cells. The channel flow simulation shows that the Reynolds stresses, of some embedded refined meshes, are affected by spurious oscillations. Surprisingly this effect is present in the unstructured meshes with the best orthogonal properties. Indeed analysis of Reynolds stress budgets shows that terms, where the gradient in the wall normal direction is dominant, have a largely oscillatory behaviour. The cause of the problem is attributed to the convective term and in particular in the method used for the gradient reconstruction. As a consequence of these contradictory signs between the inviscid and the fully turbulent cases, the rod bundle test case is analysed using a conventional body fitted multiblock mesh. Two different Reynolds numbers are investigated reporting Reynolds stresses and budgets. The flow is characterised by an energetic and almost periodic azimuthal flow pulsation in the gap region between adjacent sub-channels, which makes turbulent quantities largely different from those in plane channel and pipes and enhances mixing. Experiments found that a constant Strouhal number, with the variation of the Reynolds number, characterises the phenomenon. The frequency analysis finds that present simulations are distinguished by three dominant frequencies, the first in agreement with the experimental value and two higher ones, which might be due to the correlation of the azimuthal velocity in the streamwise direction. Several passive temperature fields are added at the simulations in order to study the effects of the variation of the Prandtl number and the change in boundary conditions (Neumann and Dirichlet). A simplified case where an imbalance of the scalar between adjacent sub-channels is also investigated in order to evaluate the variation of the heat fluxes with respect to the homogeneous case. An alternative solution, to reduce the mesh constraint imposed by the wall, is to hybridize LES with RANS. The main achievement of this work is to integrate the heat transfer modelling to the already existing model for the dynamic part. Further investigations of the Blending Function, used to merge the two velocity fields, are carried out in conjunction with a study of the model dependency on the mesh resolution. The validation is performed on a fully developed channel flow at different Reynolds numbers and with constant wall heat flux. On coarse meshes the model shows an improvement of the results for both thermal and hydraulic parts with respect to a standard LES. On refined meshes, suitable for wall-resolved LES, the model suffers from a problem of double counting of modelled Reynolds stresses and heat fluxes because the RANS contribution does not naturally disappear as the mesh resolution increases.