Computational fluid dynamics multiscale modelling of bubbly flow. A critical study and new developments on volume of fluid, discrete element and two-fluid methods

The study and modelling of two-phase flow, even the simplest ones such as the bubbly flow, remains a challenge that requires exploring the physical phenomena from different spatial and temporal resolution levels. CFD (Computational Fluid Dynamics) is a widespread and promising tool for modelling, but nowadays, there is no single approach or method to predict the dynamics of these systems at the different resolution levels providing enough precision of the results. The inherent difficulties of the events occurring in this flow, mainly those related with the interface between phases, makes that low or intermediate resolution level approaches as system codes (RELAP, TRACE, ...) or 3D TFM (Two-Fluid Model) have significant issues to reproduce acceptable results, unless well-known scenarios and global values are considered. Instead, methods based on high resolution level such as Interfacial Tracking Method (ITM) or Volume Of Fluid (VOF) require a high computational effort that makes unfeasible its use in complex systems. In this thesis, an open-source simulation framework has been designed and developed using the OpenFOAM library to analyze the cases from microescale to macroscale levels. The different approaches and the information that is required in each one of them have been studied for bubbly flow. In the first part, the dynamics of single bubbles at a high resolution level have been examined through VOF. This technique has allowed to obtain accurate results related to the bubble formation, terminal velocity, path, wake and instabilities produced by the wake. However, this approach has been impractical for real scenarios with more than dozens of bubbles. Alternatively, this thesis proposes a CFD Discrete Element Method (CFD-DEM) technique, where each bubble is represented discretely. A novel solver for bubbly flow has been developed in this thesis. This includes a large number of improvements necessary to reproduce the bubble-bubble and bubble-wall interactions, turbulence, velocity seen by the bubbles, momentum and mass exchange term over the cells or bubble expansion, among others. But also new implementations as an algorithm to seed the bubbles in the system have been incorporated. As a result, this new solver gives more accurate results as the provided up to date. Following the decrease on resolution level, and therefore the required computational resources, a 3D TFM have been developed with a population balance equation solved with an implementation of the Quadrature Method Of Moments (QMOM). The solver is implemented with the same closure models as the CFD-DEM to analyze the effects involved with the lost of information due to the averaging of the instantaneous Navier-Stokes equation. The analysis of the results with CFD-DEM reveals the discrepancies found by considering averaged values and homogeneous flow in the models of the classical TFM formulation. Finally, for the lowest resolution level approach, the system code RELAP5/MOD3 is used for modelling the bubbly flow regime. The code has been modified to reproduce properly the two-phase flow characteristics in vertical pipes, comparing the performance of the calculation of the drag term based on drift-velocity and drag coefficient approaches.El estudio y modelado de flujos bifásicos, incluso los más simples como el bubbly flow, sigue siendo un reto que conlleva aproximarse a los fenómenos físicos que lo rigen desde diferentes niveles de resolución espacial y temporal. El uso de códigos CFD (Computational Fluid Dynamics) como herramienta de modelado está muy extendida y resulta prometedora, pero hoy por hoy, no existe una única aproximación o técnica de resolución que permita predecir la dinámica de estos sistemas en los diferentes niveles de resolución, y que ofrezca suficiente precisión en sus resultados. La dificultad intrínseca de los fenómenos que allí ocurren, sobre todo los ligados a la interfase entre ambas fases, hace que los códigos de bajo o medio nivel de resolución, como pueden ser los códigos de sistema (RELAP, TRACE, etc.) o los basados en aproximaciones 3D TFM (Two-Fluid Model) tengan serios problemas para ofrecer resultados aceptables, a no ser que se trate de escenarios muy conocidos y se busquen resultados globales. En cambio, códigos basados en alto nivel de resolución, como los que utilizan VOF (Volume Of Fluid), requirieren de un esfuerzo computacional tan elevado que no pueden ser aplicados a sistemas complejos. En esta tesis, mediante el uso de la librería OpenFOAM se ha creado un marco de simulación de código abierto para analizar los escenarios desde niveles de resolución de microescala a macroescala, analizando las diferentes aproximaciones, así como la información que es necesaria aportar en cada una de ellas, para el estudio del régimen de bubbly flow. En la primera parte se estudia la dinámica de burbujas individuales a un alto nivel de resolución mediante el uso del método VOF (Volume Of Fluid). Esta técnica ha permitido obtener resultados precisos como la formación de la burbuja, velocidad terminal, camino recorrido, estela producida por la burbuja e inestabilidades que produce en su camino. Pero esta aproximación resulta inviable para entornos reales con la participación de más de unas pocas decenas de burbujas. Como alternativa, se propone el uso de técnicas CFD-DEM (Discrete Element Methods) en la que se representa a las burbujas como partículas discretas. En esta tesis se ha desarrollado un nuevo solver para bubbly flow en el que se han añadido un gran número de nuevos modelos, como los necesarios para contemplar los choques entre burbujas o con las paredes, la turbulencia, la velocidad vista por las burbujas, la distribución del intercambio de momento y masas con el fluido en las diferentes celdas por cada una de las burbujas o la expansión de la fase gaseosa entre otros. Pero también se han tenido que incluir nuevos algoritmos como el necesario para inyectar de forma adecuada la fase gaseosa en el sistema. Este nuevo solver ofrece resultados con un nivel de resolución superior a los desarrollados hasta la fecha. Siguiendo con la reducción del nivel de resolución, y por tanto los recursos computacionales necesarios, se efectúa el desarrollo de un solver tridimensional de TFM en el que se ha implementado el método QMOM (Quadrature Method Of Moments) para resolver la ecuación de balance poblacional. El solver se desarrolla con los mismos modelos de cierre que el CFD-DEM para analizar los efectos relacionados con la pérdida de información debido al promediado de las ecuaciones instantáneas de Navier-Stokes. El análisis de resultados de CFD-DEM permite determinar las discrepancias encontradas por considerar los valores promediados y el flujo homogéneo de los modelos clásicos de TFM. Por último, como aproximación de nivel de resolución más bajo, se investiga el uso uso de códigos de sistema, utilizando el código RELAP5/MOD3 para analizar el modelado del flujo en condiciones de bubbly flow. El código es modificado para reproducir correctamente el flujo bifásico en tuberías verticales, comparando el comportamiento de aproximaciones para el cálculo del término dL'estudi i modelatge de fluxos bifàsics, fins i tot els més simples com bubbly flow, segueix sent un repte que comporta aproximar-se als fenòmens físics que ho regeixen des de diferents nivells de resolució espacial i temporal. L'ús de codis CFD (Computational Fluid Dynamics) com a eina de modelatge està molt estesa i resulta prometedora, però ara per ara, no existeix una única aproximació o tècnica de resolució que permeta predir la dinàmica d'aquests sistemes en els diferents nivells de resolució, i que oferisca suficient precisió en els seus resultats. Les dificultat intrínseques dels fenòmens que allí ocorren, sobre tots els lligats a la interfase entre les dues fases, fa que els codis de baix o mig nivell de resolució, com poden ser els codis de sistema (RELAP,TRACE, etc.) o els basats en aproximacions 3D TFM (Two-Fluid Model) tinguen seriosos problemes per a oferir resultats acceptables , llevat que es tracte d'escenaris molt coneguts i se persegueixen resultats globals. En canvi, codis basats en alt nivell de resolució, com els que utilitzen VOF (Volume Of Fluid), requereixen d'un esforç computacional tan elevat que no poden ser aplicats a sistemes complexos. En aquesta tesi, mitjançant l'ús de la llibreria OpenFOAM s'ha creat un marc de simulació de codi obert per a analitzar els escenaris des de nivells de resolució de microescala a macroescala, analitzant les diferents aproximacions, així com la informació que és necessària aportar en cadascuna d'elles, per a l'estudi del règim de bubbly flow. En la primera part s'estudia la dinàmica de bambolles individuals a un alt nivell de resolució mitjançant l'ús del mètode VOF. Aquesta tècnica ha permès obtenir resultats precisos com la formació de la bambolla, velocitat terminal, camí recorregut, estela produida per la bambolla i inestabilitats que produeix en el seu camí. Però aquesta aproximació resulta inviable per a entorns reals amb la participació de més d'unes poques desenes de bambolles. Com a alternativa en aqueix cas es proposa l'ús de tècniques CFD-DEM (Discrete Element Methods) en la qual es representa a les bambolles com a partícules discretes. En aquesta tesi s'ha desenvolupat un nou solver per a bubbly flow en el qual s'han afegit un gran nombre de nous models, com els necessaris per a contemplar els xocs entre bambolles o amb les parets, la turbulència, la velocitat vista per les bambolles, la distribució de l'intercanvi de moment i masses amb el fluid en les diferents cel·les per cadascuna de les bambolles o els models d'expansió de la fase gasosa entre uns altres. Però també s'ha hagut d'incloure nous algoritmes com el necessari per a injectar de forma adequada la fase gasosa en el sistema. Aquest nou solver ofereix resultats amb un nivell de resolució superior als desenvolupat fins la data. Seguint amb la reducció del nivell de resolució, i per tant els recursos computacionals necessaris, s'efectua el desenvolupament d'un solver tridimensional de TFM en el qual s'ha implementat el mètode QMOM (Quadrature Method Of Moments) per a resoldre l'equació de balanç poblacional. El solver es desenvolupa amb els mateixos models de tancament que el CFD-DEM per a analitzar els efectes relacionats amb la pèrdua d'informació a causa del promitjat de les equacions instantànies de Navier-Stokes. L'anàlisi de resultats de CFD-DEM permet determinar les discrepàncies ocasionades per considerar els valors promitjats i el flux homogeni dels models clàssics de TFM. Finalment, com a aproximació de nivell de resolució més baix, s'analitza l'ús de codis de sistema, utilitzant el codi RELAP5/MOD3 per a analitzar el modelatge del fluxos en règim de bubbly flow. El codi és modificat per a reproduir correctament les característiques del flux bifàsic en canonades verticals, comparant el comportament d'aproximacions per al càlcul del terme de drag basades en velocitat de drift flux model i de les basades en coePeña Monferrer, C. (2017). Computational fluid dynamics multiscale modelling of bubbly flow. A critical study and new developments on volume of fluid, discrete element and two-fluid methods [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/90493TESI

Development and validation of a one-dimensional solver in a CFD platform for boiling flows in bubbly regimes

This paper presents a new one-dimensional solver for two-phase flow simulations where boiling is involved. The solver has been implemented within the OpenFOAM® platform. The basic formulation follows the Eulerian description of the Navier–Stokes equations. Different closure equations for one-dimensional simulations are also included, as well as a subcooled boiling model in order to perform accurate computations of the mass and heat transfer between phases. In addition to the fluid, a domain is included in order to represent the solid structure, so the solver is able to solve conjugate heat transfer problems. Two different test cases are presented in this work, first a single-phase test case in order to verify the conjugate heat transfer, and then a case based on the Bartolomej international benchmark, which consists of a vertical pipe where the fluid runs upwards while it is heated. Transient calculation were performed, and the results were compared to the TRACE system code, and to the experimental data in the corresponding case. With this calculations, the capability of this new solver to simulate one-dimensional single-phase and two-phase flows including boiling is demonstrated. This work is a first step of a final objective, which consists in allowing a 1D–3D coupling within the CFD platform, avoiding external links

Development and validation of a one-dimensional solver in a CFD platform for boiling flows in bubbly regimes

[EN] This paper presents a new one-dimensional solver for two-phase flow simulations where boiling is involved. The solver has been implemented within the OpenFOAM® platform. The basic formulation follows the Eulerian description of the Navier¿Stokes equations. Different closure equations for one-dimensional simulations are also included, as well as a subcooled boiling model in order to perform accurate computations of the mass and heat transfer between phases. In addition to the fluid, a domain is included in order to represent the solid structure, so the solver is able to solve conjugate heat transfer problems. Two different test cases are presented in this work, first a single-phase test case in order to verify the conjugate heat transfer, and then a case based on the Bartolomej international benchmark, which consists of a vertical pipe where the fluid runs upwards while it is heated. Transient calculation were performed, and the results were compared to the TRACE system code, and to the experimental data in the corresponding case. With this calculations, the capability of this new solver to simulate one-dimensional single-phase and two-phase flows including boiling is demonstrated. This work is a first step of a final objective, which consists in allowing a 1D¿3D coupling within the CFD platform, avoiding external links.This work has been partially supported by the Spanish Agencia Estatal de Investigacion [grant number BES-2013-064783], and the Spanish Ministerio de Economia Industria y Competitividad [project NUC-MULTPHYS ENE2012-34585].Gomez-Zarzuela-Quel, C.; Chiva Vicent, S.; Peña-Monferrer, C.; Miró Herrero, R. (2021). Development and validation of a one-dimensional solver in a CFD platform for boiling flows in bubbly regimes. Progress in Nuclear Energy. 134:1-16. https://doi.org/10.1016/j.pnucene.2021.103680S11613

CFD Turbulence Study of PWR Spacer-Grids in a Rod Bundle

Nuclear fuel bundles include spacers essentially for mechanical stability and to influence the flow dynamics and heat transfer phenomena along the fuel rods. This work presents the analysis of the turbulence effects of a split-type and swirl-type spacer-grid geometries on single phase in a PWR (pressurized water reactor) rod bundle. Various computational fluid dynamics (CFD) calculations have been performed and the results validated with the experiments of the OECD/NEA-KAERI rod bundle CFD blind benchmark exercise on turbulent mixing in a rod bundle with spacers at the MATiS-H facility. Simulation of turbulent phenomena downstream of the spacer-grid presents high complexity issues; a wide range of length scales are present in the domain increasing the difficulty of defining in detail the transient nature of turbulent flowwith ordinary turbulence models. This paper contains a complete description of the procedure to obtain a validated CFD model for the simulation of the spacer-grids. Calculations were performed with the commercial code ANSYS CFX using large eddy simulation (LES) turbulence model and the CFD modeling procedure validated by comparison with measurements to determine their suitability in the prediction of the turbulence phenomena.The authors sincerely thank the Consejo de Seguridad Nuclear (CSN) (Spanish Nuclear Safety Council) and the "Plan Nacional de I+D+i" Project EXPERTISER ENE2010-21368-C02-01 and ENE2010-21368-C02-02 for funding the project.Peña Monferrer, C.; Muñoz-Cobo González, JL.; Chiva Vicent, S. (2014). CFD Turbulence Study of PWR Spacer-Grids in a Rod Bundle. Science and Technology of Nuclear Installations. 2014:1-15. doi:10.1155/2014/635651S1152014Pioro, I. L., Groeneveld, D. C., Doerffer, S. S., Guo, Y., Cheng, S. C., & Vasić, A. (2002). Effects of flow obstacles on the critical heat flux in a vertical tube cooled with upward flow of R-134a. International Journal of Heat and Mass Transfer, 45(22), 4417-4433. doi:10.1016/s0017-9310(02)00150-3Yang, S. K., & Chung, M. K. (1998). Turbulent Flow Through Spacer Grids in Rod Bundles. Journal of Fluids Engineering, 120(4), 786-791. doi:10.1115/1.2820739Caraghiaur, D., Anglart, H., & Frid, W. (2009). Experimental investigation of turbulent flow through spacer grids in fuel rod bundles. Nuclear Engineering and Design, 239(10), 2013-2021. doi:10.1016/j.nucengdes.2009.05.029Dominguez-Ontiveros, E. E., Hassan, Y. A., Conner, M. E., & Karoutas, Z. (2012). Experimental benchmark data for PWR rod bundle with spacer-grids. Nuclear Engineering and Design, 253, 396-405. doi:10.1016/j.nucengdes.2012.09.003Nematollahi, M. R., & Nazifi, M. (2008). Enhancement of heat transfer in a typical pressurized water reactor by different mixing vanes on spacer grids. Energy Conversion and Management, 49(7), 1981-1988. doi:10.1016/j.enconman.2007.12.016Pazirandeh, A., Ghaseminejad, S., & Ghaseminejad, M. (2011). Effects of various spacer grid modeling on the neutronic parameters of the VVER-1000 reactor. Annals of Nuclear Energy, 38(9), 1978-1986. doi:10.1016/j.anucene.2011.04.020Jayanti, S., & Rajesh Reddy, K. (2013). Effect of spacer grids on CHF in nuclear rod bundles. Nuclear Engineering and Design, 261, 66-75. doi:10.1016/j.nucengdes.2013.03.044SMAGORINSKY, J. (1963). GENERAL CIRCULATION EXPERIMENTS WITH THE PRIMITIVE EQUATIONS. Monthly Weather Review, 91(3), 99-164. doi:10.1175/1520-0493(1963)0912.3.co;2Germano, M., Piomelli, U., Moin, P., & Cabot, W. H. (1991). A dynamic subgrid‐scale eddy viscosity model. Physics of Fluids A: Fluid Dynamics, 3(7), 1760-1765. doi:10.1063/1.857955Deardorff, J. W. (1970). A numerical study of three-dimensional turbulent channel flow at large Reynolds numbers. Journal of Fluid Mechanics, 41(2), 453-480. doi:10.1017/s0022112070000691Deardorff, J. W. (1973). The Use of Subgrid Transport Equations in a Three-Dimensional Model of Atmospheric Turbulence. Journal of Fluids Engineering, 95(3), 429-438. doi:10.1115/1.3447047Ciofalo, M. (1994). Large-Eddy Simulation: A Critical Survey of Models and Applications. Advances in Heat Transfer, 321-419. doi:10.1016/s0065-2717(08)70196-5Lesieur, M., & Metais, O. (1996). New Trends in Large-Eddy Simulations of Turbulence. Annual Review of Fluid Mechanics, 28(1), 45-82. doi:10.1146/annurev.fl.28.010196.000401Guermond, J.-L., Oden, J. T., & Prudhomme, S. (2004). Mathematical Perspectives on Large Eddy Simulation Models for Turbulent Flows. Journal of Mathematical Fluid Mechanics, 6(2), 194-248. doi:10.1007/s00021-003-0091-5Lee, J. R., Kim, J., & Song, C.-H. (2014). Synthesis of the turbulent mixing in a rod bundle with vaned spacer grids based on the OECD-KAERI CFD benchmark exercise. Nuclear Engineering and Design, 279, 3-18. doi:10.1016/j.nucengdes.2014.03.008Meyers, J., Geurts, B. J., & Sagaut, P. (Eds.). (2008). Quality and Reliability of Large-Eddy Simulations. Ercoftac Series. doi:10.1007/978-1-4020-8578-

Improving the User Experience of the rCUDA Remote GPU Virtualization Framework

Graphics processing units (GPUs) are being increasingly embraced by the high-performance computing community as an effective way to reduce execution time by accelerating parts of their applications. remote CUDA (rCUDA) was recently introduced as a software solution to address the high acquisition costs and energy consumption of GPUs that constrain further adoption of this technology. Specifically, rCUDA is a middleware that allows a reduced number of GPUs to be transparently shared among the nodes in a cluster. Although the initial prototype versions of rCUDA demonstrated its functionality, they also revealed concerns with respect to usability, performance, and support for new CUDA features. In response, in this paper, we present a new rCUDA version that (1) improves usability by including a new component that allows an automatic transformation of any CUDA source code so that it conforms to the needs of the rCUDA framework, (2) consistently features low overhead when using remote GPUs thanks to an improved new communication architecture, and (3) supports multithreaded applications and CUDA libraries. As a result, for any CUDA-compatible program, rCUDA now allows the use of remote GPUs within a cluster with low overhead, so that a single application running in one node can use all GPUs available across the cluster, thereby extending the single-node capability of CUDA. Copyright © 2014 John Wiley & Sons, Ltd.This work was funded by the Generalitat Valenciana under Grant PROMETEOII/2013/009 of the PROMETEO program phase II. The author from Argonne National Laboratory was supported by the US Department of Energy, Office of Science, under Contract No. DE-AC02-06CH11357. The authors are also grateful for the generous support provided by Mellanox Technologies.Reaño González, C.; Silla Jiménez, F.; Castello Gimeno, A.; Peña Monferrer, AJ.; Mayo Gual, R.; Quintana Ortí, ES.; Duato Marín, JF. (2015). Improving the User Experience of the rCUDA Remote GPU Virtualization Framework. Concurrency and Computation: Practice and Experience. 27(14):3746-3770. https://doi.org/10.1002/cpe.3409S374637702714NVIDIA NVIDIA industry cases http://www.nvidia.es/object/tesla-case-studiesFigueiredo, R., Dinda, P. A., & Fortes, J. (2005). Guest Editors’ Introduction: Resource Virtualization Renaissance. Computer, 38(5), 28-31. doi:10.1109/mc.2005.159Duato J Igual FD Mayo R Peña AJ Quintana-Ortí ES Silla F An efficient implementation of GPU virtualization in high performance clusters Euro-Par 2009 Workshops, ser. LNCS, 6043 Delft, Netherlands, 385 394Duato J Peña AJ Silla F Mayo R Quintana-Ortí ES Performance of CUDA virtualized remote GPUs in high performance clusters International Conference on Parallel Processing, Taipei, Taiwan 2011 365 374Duato J Peña AJ Silla F Fernández JC Mayo R Quintana-Ortí ES Enabling CUDA acceleration within virtual machines using rCUDA International Conference on High Performance Computing, Bangalore, India 2011 1 10Shi, L., Chen, H., Sun, J., & Li, K. (2012). vCUDA: GPU-Accelerated High-Performance Computing in Virtual Machines. IEEE Transactions on Computers, 61(6), 804-816. doi:10.1109/tc.2011.112Gupta V Gavrilovska A Schwan K Kharche H Tolia N Talwar V Ranganathan P GViM: GPU-accelerated virtual machines 3rd Workshop on System-Level Virtualization for High Performance Computing, Nuremberg, Germany 2009 17 24Giunta G Montella R Agrillo G Coviello G A GPGPU transparent virtualization component for high performance computing clouds Euro-Par 2010 - Parallel Processing, 6271 Ischia, Italy, 379 391Zillians VGPU http://www.zillians.com/vgpuLiang TY Chang YW GridCuda: a grid-enabled CUDA programming toolkit Proceedings of the 25th IEEE International Conference on Advanced Information Networking and Applications Workshops (WAINA), Biopolis, Singapore 2011 141 146Barak A Ben-Nun T Levy E Shiloh A Apackage for OpenCL based heterogeneous computing on clusters with many GPU devices Workshop on Parallel Programming and Applications on Accelerator Clusters, Heraklion, Crete, Greece 2010 1 7Xiao S Balaji P Zhu Q Thakur R Coghlan S Lin H Wen G Hong J Feng W-C VOCL: an optimized environment for transparent virtualization of graphics processing units Proceedings of InPar, San Jose, California, USA 2012 1 12Kim J Seo S Lee J Nah J Jo G Lee J SnuCL: an OpenCL framework for heterogeneous CPU/GPU clusters Proceedings of the 26th International Conference on Supercomputing, Venice, Italy 2012 341 352NVIDIA The NVIDIA CUDA Compiler Driver NVCC Version 5, NVIDIA 2012Quinlan D Panas T Liao C ROSE http://rosecompiler.org/Free Software Foundation, Inc. GCC, the GNU Compiler Collection http://gcc.gnu.org/LLVM Clang: a C language family frontend for LLVM http://clang.llvm.org/Martinez G Feng W Gardner M CU2CL: a CUDA-to-OpenCL Translator for Multi- and Many-core Architectures http://eprints.cs.vt.edu/archive/00001161/01/CU2CL.pdfLLVM The LLVM compiler infrastructure http://llvm.org/Reaño C Peña AJ Silla F Duato J Mayo R Quintana-Orti ES CU2rCU: towards the complete rCUDA remote GPU virtualization and sharing solution Proceedings of the 19th International Conference on High Performance Computing (HiPC), Pune, India 2012 1 10NVIDIA The NVIDIA GPU Computing SDK Version 4, NVIDIA 2011Sandia National Labs LAMMPS molecular dynamics simulator http://lammps.sandia.gov/Citrix Systems, Inc. Xen http://xen.org/Peña AJ Virtualization of accelerators in high performance clusters Ph.D. Thesis, 2013NVIDIA CUDA profiler user's guide version 5, NVIDIA 2012Igual, F. D., Chan, E., Quintana-Ortí, E. S., Quintana-Ortí, G., van de Geijn, R. A., & Van Zee, F. G. (2012). The FLAME approach: From dense linear algebra algorithms to high-performance multi-accelerator implementations. Journal of Parallel and Distributed Computing, 72(9), 1134-1143. doi:10.1016/j.jpdc.2011.10.014Slurm workload manager http://slurm.schedmd.co

simulación de un elemento combustible PWR simplificado mediante los códigos acoplados CFD-Neutrónicos ANSYS CFX 12.1 y PARCS

Se ha desarrollado una nueva herramienta computacional para los cálculos de reactores nucleares basada en el acople entre el código de transporte neutrónico PARCS y e l código comercial de dinámica de fluidos computacional (CFD) ANSYS CFX 12.1. En esta cont ribución se presentan los primeros resultados de la aplicación de esta nueva metodología para el acople de códigos CFD con códigos neutrónicos. C on esta nueva herramienta de simulación se abren nuevas posibilidades en el diseño de elementos combustibles, ya que contribuye a un mejor entendimiento y una mejor simulación de los procesos de transferencia de calor y fenómenos específicos de dinámica de fluidos como el 'crossflow'. La simulación de transitorios de inserción de barra de control, dilución de boro o inyección de agua fría se pueden llevar a cabo con un nivel de precisión que no es posible alcanzar con las metodologías actuales basadas en el uso de códigos de sistema.Peña Monferrer, C.; Chiva, S.; Miró Herrero, R.; Barrachina Celda, TM.; Pellacani, F.; Macian Juan, R. (2011). simulación de un elemento combustible PWR simplificado mediante los códigos acoplados CFD-Neutrónicos ANSYS CFX 12.1 y PARCS. Grupo Senda. http://hdl.handle.net/10251/34068S10

Boosting the performance of remote GPU virtualization using InfiniBand Connect-IB and PCIe 3.0

© 2014 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.[EN] A clear trend has emerged involving the acceleration of scientific applications by using GPUs. However, the capabilities of these devices are still generally underutilized. Remote GPU virtualization techniques can help increase GPU utilization rates, while reducing acquisition and maintenance costs. The overhead of using a remote GPU instead of a local one is introduced mainly by the difference in performance between the internode network and the intranode PCIe link. In this paper we show how using the new InfiniBand Connect-IB network adapters (attaining similar throughput to that of the most recently emerged GPUs) boosts the performance of remote GPU virtualization, reducing the overhead to a mere 0.19% in the application tested.This work was funded by the Generalitat Valenciana under Grant PROMETEOII/2013/009 of the PROMETEO program phase II. This material is based upon work supported by the U. S. Department of Energy, Office of Science, Advanced Scientific Computing Research (SC-21), under Contract No. DE-AC02-06CH11357. Authors from the Universitat Politècnica de València and Universitat Jaume I are grateful for the generous support provided by Mellanox Technologies.Reaño González, C.; Silla Jiménez, F.; Peña Monferrer, AJ.; Shainer, G.; Schultz, S.; Castelló Gimeno, A.; Quintana Orti, ES.... (2014). Boosting the performance of remote GPU virtualization using InfiniBand Connect-IB and PCIe 3.0. En 2014 IEEE International Conference on Cluster Computing (CLUSTER). IEEE. 266-267. doi:10.1109/CLUSTER.2014.6968737S26626

An Eulerian-Lagrangian open source solver for bubbly flow in vertical pipes

Air-water two-phase flow is present in natural and industrial processes of different nature as nuclear reactors. An accurate local prediction of the boiling flow could support safety and operation analyses of nuclear reactors. A new Eulerian-Lagrangian approach is investigated in this contribution. A new solver has been developed and implemented in the framework of the open source package OpenFOAM R and based on the PIMPLE algorithm coupled with the Lagrangian equation of motion has been implemented for computing incompressible bubbly flows. Each bubble is divided in equivolumetric volumes and tracked into the Eulerian mesh for an appropriate assignment of the effect of the bubble in the cell without resolving the interface. The coupling between phases is done considering in the momentum equation the interfacial forces and bubble induced contribution along the bubble path during an Eulerian time step. The bouncing of the bubbles between themselves and the wall is modeled with a dynamic soft sphere model. The computational results obtained for different flow conditions are validated with the recently released experimental data on upward pipe flow. The test section used is a 52 mm pipe of 5500 mm of length maintained under adiabatic conditions with air and water circulating fluids working with inlet velocity ranges of 0-2 m/s and 0-0.3 m/s for the continuous and dispersed phase respectively. Averaged results of radial distribution for void fraction, chord length, turbulence kinetic energy, dispersed and continuous velocity profiles show a good agreement among different flow conditions.Peña Monferrer, C.; Muñoz-Cobo González, JL.; Monrós Andreu, G.; Martinez Cuenca, R.; Chiva Vicent, S. (2014). An Eulerian-Lagrangian open source solver for bubbly flow in vertical pipes. Sociedad Nuclear Española. http://hdl.handle.net/10251/71943

Estudio del flujo bifásico aire-líquido en tuberías verticales: Base de datos experimental para la validación de programas CFD

El estudio del comportamiento del flujo multifásico y su modelado ha sufrido un gran cambio con la introducción de los programas CFD, y el avance en los sistemas de medida. La resolución temporal y espacial de los programas de cálculo y la instrumentación se ha incrementado notablemente. Así, se hace indispensable disponer de nuevos resultados experimentales que permitan desarrollar nuevos modelos adaptados a estas capacidades, y a su vez validar las actuales herramientas de cálculo, especialmente los códigos CFD. La Universitat Jaume I de Castelló y la Universidad Politécnica de Valencia poseen una larga trayectoria en la experimentación y modelado de sistemas bifásicos. Fruto de ello se ha desarrollado en la UJI una instalación experimental, un loop vertical de 6 metros para estudiar el flujo bifásico en tuberías, con abundante instrumentación como sondas de conductividas, LDA y cámaras rápidas. En este trabajo se presentarán los resultados para tuberías verticales de 52 mm de diámetro en régimen de bubbly y slug. La instalación experimental está totalmente automatizada y es capaz de medir distribución de fracción de huecos, velocidad de la fase gas y líquido, turbulencia de la fase líquida, densidad de área interfacial y distribución de población de burbujas y tamaños. Se han desarrollado las sondas de conductividad de 4 sensores y el software para el procesado e la señal obtenida. Se han ensayado en el rango de velocidad del líquido de 0.5 a 3 m/s y fracciones de gas medias de 5 a 30%, así como con temperaturas y tensiones interraciales del fluido distintas. Se han tomado medidas en tres puertos axiales y en cada uno de ellos 15 puntos radiales. Con todo ello se ha creado una base de datos detallada que permite a los investigadores desarrollar nuevos modelos o validar los códigos actuales con garantías. En ella se estudia el flujo en régimen bubbly, la transición hacia slug y el inicio del slug. Cabe destacar el estudio realizado mediante LDA de la fase líquido , lo que permite obtener distribuciones radiales de talladas de velocidad y turbulencia.Monrós, G.; Martínez, R.; Torro, S.; Chiva Vicent, S.; Peña Monferrer, C.; Muñoz-Cobo González, JL. (2014). Estudio del flujo bifásico aire-líquido en tuberías verticales: Base de datos experimental para la validación de programas CFD. Sociedad Nuclear Española. http://hdl.handle.net/10251/71303

CFD-Neutronic coupled calculation of a PWR fuel assembly considering pressure drop and turbulence produced by spacer grids

[EN] A computational code system called Coupled Solver ANSYS CFX/PARCS (CSAP) based on coupling the 3D neutron diffusion code PARCS v2.7 and the ANSYS CFX 13.0 Computational Fluid Dynamics (CFD) code has been developed as a tool for nuclear reactor systems simulations. This paper presents the coupling methodology between the CFD and the neutronic code. The methodology to simulate a 3D-neutronic problem coupled with 1D thermalhydraulics is already a mature technology, being part of the regular calculations performed to analyze different kinds of Reactivity Insertion Accidents (RIA) and asymmetric transients in Nuclear Power Plants, with state-of-the-art coupled codes like TRAC-B/NEM, RELAP5/PARCS, TRACE/PARCS, RELAP3D, RETRAN3D, etc. The transport of neutrons depends on several parameters, like fuel temperature, moderator temperature and density, boron concentration and fuel rod insertion. These data are calculated by the CFD code with high local resolution and used as input to the neutronic code to calculate a 3D nodal power distribution that will be returned and remapped to the CFD code control volumes (cells). Since two different nodalizations are used to discretize the same system, an averaging and interpolating procedure is needed to realize an effective data exchange. These procedures have been developed by means of the ANSYS CFX “User Fortran” interface; a library with several subroutines has been developed for calculation and synchronization purposes. The data exchange is realized by means of the Parallel Virtual Machine (PVM) software package. In this contribution, steady-state and transient results of a quarter of PWR fuel assembly with cold water injection are presented and compared with obtained results from a RELAP5/PARCS v2.7 coupled calculation. A simplified model for the spacers has been included. A methodology has been introduced to take into account the pressure drop and the turbulence enhancement produced by the spacers.This research was supported by the “Plan Nacional de I+D+i” Project EXPERTISER ENE2010-21368-C02-01 and ENE2010-21368-C02-02.Peña-Monferrer, C.; Pellacani, F.; Chiva Vicent, S.; Barrachina Celda, TM.; Miró Herrero, R.; Macián-Juan, R. (2012). CFD-Neutronic coupled calculation of a PWR fuel assembly considering pressure drop and turbulence produced by spacer grids. Nuclear Espana. 52-61. http://hdl.handle.net/10251/48423S526