DSpace Collection:
http://hdl.handle.net/2117/3191
2014-11-22T10:16:36ZFrom 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.2014-02-17T15:33:45ZDNS 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.2014-02-17T15:22:24ZLow-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/10.1063/1.4818641.2014-02-12T15:33:46ZDirect 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.2014-02-03T15:29:47ZLimits 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.2014-02-03T14:56:31ZHeat 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.2014-02-03T14:32:57ZSymmetry-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.2013-12-31T10:28:23ZImproved semi-analytical method for air curtains prediction
http://hdl.handle.net/2117/21065
Title: Improved semi-analytical method for air curtains prediction
Authors: Giráldez García, Héctor; Pérez Segarra, Carlos David; Rodríguez Pérez, Ivette María; Oliva Llena, Asensio
Abstract: The present study is devoted to the analysis and prediction of the efficiency of air curtains. The attention is focused on improving existing semi-analytical methods with the information from CFD simulations and experimental measurements. The goal is to obtain an accurate simplified model which describes the three-dimensional behaviour of the air jet without requiring large time consuming calculations. The interaction of the air curtain with other agents, e.g. pedestrians and flying insects, is also revised. The model is validated against both experimental data and advanced LES calculations carried out by the authors. Furthermore, a parametric study shows the air curtain behaviour for different configurations.2013-12-19T15:33:29ZA simple approach to discretize the viscous term with spatially varying (eddy-)viscosity
http://hdl.handle.net/2117/20836
Title: A simple approach to discretize the viscous term with spatially varying (eddy-)viscosity
Authors: Trias Miquel, Francesc Xavier; Gorobets, Andrei; Oliva Llena, Asensio
Abstract: A simple approach to discretize the viscous dissipation term in the incompressible Navier–Stokes equations with spatially varying viscosity is presented. Unlike the case where the viscosity remains constant, its discretization may be quite cumbersome especially for high-order staggered formulations. To circumvent this problem, we propose an alternative form of the viscous term whose discretization is straightforward. Notice that this approach is also suitable for eddy-viscosity models for Large-Eddy Simulation. Moreover, since it is based on already available operators, it can be easily implemented on any structured or unstructured code. The (supra)convergence of the method is numerically shown on both a fourth-order Cartesian staggered and an unstructured collocated formulation.2013-11-28T14:02:15ZSymmetry-preserving regularization of wall-bounded turbulent flows
http://hdl.handle.net/2117/20783
Title: Symmetry-preserving regularization of wall-bounded turbulent flows
Authors: Trias Miquel, Francesc Xavier; Gorobets, Andrei; Verstappen, Roel; Oliva Llena, Asensio
Abstract: The incompressible Navier-Stokes equations constitute an excellent mathematical modelization of turbulence. Unfortunately, attempts at performing direct simulations are limited to relatively low-Reynolds numbers because of the almost numberless small scales produced by the non-linear convective term. Alternatively, a dynamically less complex formulation is proposed here. Namely, regularizations of the Navier-Stokes equations that preserve the symmetry and conservation properties exactly. To do so, both convective and diffusive term are altered in the same vein. In this way, the convective production of small scales is effectively restrained whereas the modified diffusive term introduces an hyper-viscosity effect and consequently enhances the destruction of small scales. In practice, the only additional ingredient is a self-adjoint linear filter whose local filter length is determined from the requirement that vortex-stretching must stop at the smallest grid scale. To do so, a new criterion based on the invariants of the local strain tensor is proposed here. Altogether, the proposed method constitutes a parameter-free turbulence model.2013-11-26T15:19:06ZModular object-oriented methodology for the resolution of molten salt storage tanks for CSP plants
http://hdl.handle.net/2117/19718
Title: Modular object-oriented methodology for the resolution of molten salt storage tanks for CSP plants
Authors: Rodríguez Pérez, Ivette María; Pérez Segarra, Carlos David; Lehmkuhl Barba, Oriol; Oliva Llena, Asensio
Abstract: Two-tank molten salt storages are the most widespread thermal energy storage technology within concentrated solar power plants. In spite of this, there are design aspects such as thermal losses control, optimisation of the storage or how these devices scale up with the increase in power capacity of the plant which still should be considered. In this sense, numerical modelling of these systems can be a powerful tool for reducing their cost. The present work aims at modelling molten salt tanks by proposing a parallel modular object-oriented methodology which considers the different elements of the storage (e.g. tank walls, insulation material, tank foundation, molten salt storage media, etc.) as independent systems. Each of these elements can be solved independently and using different levels of modelling (from global to fully three-dimensional models), while at the same time they are linked to each other through their boundary conditions. The mathematical models used, together with some illustrative examples of the application of the proposed methodology, are presented and discussed in detail.2013-06-28T14:10:19ZNumerical simulation of wind flow around a parabolic trough solar collector
http://hdl.handle.net/2117/18780
Title: Numerical simulation of wind flow around a parabolic trough solar collector
Authors: Amine Hachicha, Ahmed; Rodríguez Pérez, Ivette María; Castro González, Jesús; Oliva Llena, Asensio
Abstract: The use of parabolic trough solar technology in solar power plants has been increased in recent years.
Such devices are located in open terrain and can be the subject of strong winds. As a result, the stability of these devices to track accurately the sun and the convection heat transfer from the receiver tube could be affected. In this paper, a detailed numerical aerodynamic and heat transfer model based on Large Eddy Simulations (LES) modelling for these equipments is presented. First, the model is verified on a circular cylinder in a cross-flow. The drag forces and the heat transfer coefficients are then validated with available experimental measurements. After that, simulations are performed on an Eurotrough solar collector to study the fluid flow and heat transfer around the solar collector and its receiver. Computations are carried out for a Reynolds number of Re W = 3.6 x 10(5) (based on the aperture) and for various pitch angles (h=0,45,90, 135, 80, 270). The aerodynamic coefficients are calculated around the solar collector
and validated with measurements performed in wind tunnel tests. Instantaneous velocity field is also studied and compared to aerodynamic coefficients for different pitch angles. The time-averaged flow is characterised by the formation of several recirculation regions around the solar collector and the receiver
tube depending on the pitch angle. The study also presents a comparative study of the heat transfer coefficients around the heat collector element with the circular cylinder in a cross-flow and the effect of the pitch angle on the Nusselt number.2013-04-12T13:17:16ZParallel algorithms for Sn transport sweeps on unstructured meshes
http://hdl.handle.net/2117/17446
Title: Parallel algorithms for Sn transport sweeps on unstructured meshes
Authors: Colomer, Guillem; Borrell Pol, Ricard; Trias Miquel, Francesc Xavier; Rodríguez Pérez, Ivette María
Abstract: The Boltzmann Transport Equation is solved on unstructured meshes using the Discrete
Ordinates Method. The flux for each ordinate is swept across the computational grid,
within a source iteration loop that accounts for the coupling between the different
ordinates. In this paper, a spatial domain decomposition strategy is used to divide the work among the available CPUs. The sequential nature of the sweep process makes the parallelization of the overall algorithm the most challenging aspect. Several parallel sweep algorithms, which represent different options of interleaving communications and calculations in the solution process, are analysed. The option of grouping messages by means of buffering is also considered. One of the heuristics proposed consistently stands out as
the best option in all the situations analyzed, which include different geometries and different sizes of the ordinate set. With this algorithm, good scalability results have been achieved regarding both weak and strong speedup tests with up to 2560 CPUs.2013-01-21T16:36:47ZAssessment of the symmetry-preserving regularization model on complex flows using unstructured grids
http://hdl.handle.net/2117/15762
Title: Assessment of the symmetry-preserving regularization model on complex flows using unstructured grids
Authors: Lehmkuhl Barba, Oriol; Borrell Pol, Ricard; Rodríguez Pérez, Ivette María; Pérez Segarra, Carlos David; Oliva Llena, Asensio
Abstract: The main objective of the present paper is the assessment of symmetry-preserving regularization models on unstructured meshes. Three different test cases have been studied: the impinging jet flow, the flow past a circular cylinder and a simplified Ahmed car. The properties of the filters and their performance on general unstructured meshes have also been considered. A detailed analysis considering the Gaussian and the Helmholtz differential filters is presented.2012-04-25T13:27:10ZObject-oriented simulation of reciprocating compressors: Numerical verification and experimental comparison
http://hdl.handle.net/2117/15021
Title: Object-oriented simulation of reciprocating compressors: Numerical verification and experimental comparison
Authors: Mohan Damle, Rashmin; Rigola Serrano, Joaquim; Pérez Segarra, Carlos David; Castro González, Jesús; Oliva Llena, Asensio
Abstract: Numerical simulation of reciprocating compressors is important for the design, development, improvement and optimization of the elements constituting the compressor circuit. In this work, an object-oriented unstructured modular numerical simulation of reciprocating compressors is presented. Pressure correction approach is applied for the resolution of tubes, chambers and compression chambers, while valve dynamics are modelled assuming a spring-mass system having single degree of freedom. The modular approach offers advantages of handling complex circuitry (e.g. parallel paths, multiple compressor chambers, etc.), coupling different simulation models for each element and adaptability to different configurations without changing the program. The code has been verified with some basic tests for assuring asymptotic behaviour to guarantee error free code and physically
realistic results. Cases with different compressor configurations and working fluids
(R134a, R600a and R744) have also been worked out. Numerical results are compared with experimental data and illustrative cases of multi-stage compression are also presented.2012-02-08T13:39:07Z