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Browsing Conference papers by Subject "CFD"
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Item Open Access 3D CFD Modeling Investigation of Potential Vortex Formation at the Intakes of Caruachi Powerhouse(INTERNATIONAL CONFERENCE ONHYDRAULICS OF DAMS AND RIVER STRUCTURES, TEHRAN, IRAN; 04/2004, 2004-04) Marcano, A.; Rojas-Solorzano, L.; Reyes, M.In this paper, the 3-D CFD simulation of the free-surface flow approaching the intakes of Caruachi Powerhouse is presented. The aim of the investigation is to determine whether or not vortex structures are likely to appear from the water surface through the intakes, as the result of the presence of cofferdams placed few meters upstream of the intakes. The presence of cofferdams was a note of concern with regard to the effects they might have on the turbine intakes once the hydroelectric central starts operating. In all the considered conditions, results did not show neither strong surface vortices in the proximities of the Power House intakes, nor air entrainment-entrapment towards the intakes, which reflects the safe operation of the turbines in the presence of the cofferdams. The latter added in decision taking on leaving the cofferdams submerged instead of removing them, which resulted in cost savings for the projectItem Open Access CFD modeling of slurry flows in horizontal pipes(Proceedings of FEDSM2008 8th Symposium on Applications in Computational Fluid Dynamics August 17-20, Jacksonville, FL, 2008-08-17) Hernández, Franz H.; Blanco, Armando J.; Rojas-Solorzano, L.Liquid-solid two-phase flows are found in numerous operations in the chemical, petroleum, pharmaceutical and many other industries. In numerous cases, the mixture or slurry that flows is composed by a suspension of solid particles (dispersed phase) transported by a liquid (continuum phase). However, the large number and range of variables encountered in slurry flows, in the case of pipelines, cause the flow behavior of these slurry systems to vary over a wide range of conditions, and consequently, different approaches have been used to describe the behavior of different flow regimes. Therefore, there are numerous studies of particular cases that cover limited ranges of conditions. In consequence, the experimental approach is necessarily limited by geometric and physical scale factors. For these reasons, Computational Fluid Dynamics, CFD, constitutes an ideal technique for predicting the general flow behavior of these systems. CFD models in this area can be divided in two different classes: Eulerian-Eulerian and Lagrangian-Eulerian models. Differences between these models are related to the way the solid phase flow is represented. Lagrangian-Eulerian models calculate the path and motion of each particle, while Eulerian-Eulerian models treat the particle phase as a continuum and average out motion on the scale of individual particles. This work focuses on the Eulerian-Eulerian approach for modeling the flow of a mixture of sand particles and water in a horizontal pipe. Homogeneous and heterogeneous flow regimes are considered. The k-ε model was used for modeling turbulent effects. Additionally, closure of solid-phase momentum equations requires a description for the solid-phase stress. Constitutive relations for the solid-phase stress considering the inelastic nature of particle collisions based on the Gas Kinetic Theory concepts have been used. Governing equations are solved numerically using the control volume-based finite element method. An unstructured non-uniform grid was chosen to discretize the entire computational domain. A second-order scheme in space and time was used. Numerical solutions in fully developed turbulent flow were found. Results show that flow predictions are very sensitive to the restitution coefficient and pseudo-viscosity of the solid phase. The mean pressure gradients from numerical solutions were compared with results obtained using the correlations of Einstein, Thomas and Krieger for homogeneous cases and with experimental data found in the open literature for heterogeneous cases. The solutions were found to be in good agreement with both correlations and experimental data. In addition, these numerical results were closer to experimental data than results obtained using other numerical models.Item Open Access Numerical study of smoke propagation in a simulated fire in a wagon within a subway tunnel(Proceedings of FEDSM2008 2008 ASME Fluids Engineering Conference August 10-14, 2008 Jacksonville, Florida USA, 2008-08-10) Vittori, Felipe; Rojas-Solorzano, L.; Blanco, Armando J.; Urbina, RafaelThis work deals with the numerical (CFD) analysis of the smoke propagation during fires within closed environments. It is evaluated the capacity of the emergency ventilation system in controlling the smoke propagation and minimizing the deadly impact of an eventual fire in a wagon within the Metro de Caracas subway tunnel on the passengers safety. For the study, it was chosen the tunnel section between Teatros and Nuevo Circo subway stations, which consists of two parallel independent twin tunnels, connected through a transverse passage. The tunnels are provided by a longitudinal ventilation system, integrated by a set of reversible fans located at both ends of the tunnels. Three stages were considered in the study: (a) Model set up; (b) Mesh sensitivity analysis; (c) Validation of the physical-numerical parameters to be used in the numerical model; and (d) Simulation of fire scenarios in Metro de Caracas subway stations. Stages (b)-(c), aimed to testing and calibrating the CFD tool (ANSYS-CFX10TM), focused on reproducing experimental data from Vauquelin and Mégret [1], who studied the smoke propagation in a fire within a 1:20 scale road tunnel. Stage (d) critical scenarios were established via a preliminary discussion with safety experts from Metro de Caracas, in order to reduce the computer memory and the number of simulations to be performed. The analyses assessed the reliability of escape routes and alternative paths for the evacuation of passengers. Additionally, the smoke front movement was particularly computed, as a function of time, in order to determine the possible presence of the ¨backlayering¨ phenomenon [5]. Results demonstrate the strengths and weaknesses of the current ventilation system in the event of a fire in the subway tunnel, and suggest new strategies to address this potentially lethal event to minimize the risks for passengersItem Open Access Numerical study of turbulence models in the computation of blood flow in cannulas(Proceedings of FEDSM2008 2008 ASME Fluids Engineering Conference August 10-14, 2008, Jacksonville, FL, USA, 2008-08-10) Salazar, Felix A.; Rojas-Solorzano, L.; Antaki, James F.In recent years, CFD has become an increasingly used tool in the design of blood-based devices. However, the estimation of red blood cells damage (hemolysis) remains a very important challenge due to the complex rheology of blood and the turbulence present in most pumping devices. The objective of this study was to identify an appropriate turbulence model suitable for predicting hemolysis in Hemodialysis cannula. Several modern turbulence models were evaluated in comparison to Direct Numerical Simulation (DNS), which was used as the gold standard. The fluid dynamics in the cannula was modeled as a coaxial jet in which Reynolds’ number approached 2800. Based on comparison of velocity and stress time-averaged profiles, the Shear Stress Transport (SST) model with Gamma-Theta transition was identified as an optimal compromise between accuracy and computational cost.Item Open Access Two-dimensional numerical simulation of saltating particles using granular kinetic theory(Proceedings of FEDSM2007 2007 ASME/JSME Fluids Engineering Division Summer Meeting July 30-August 2, 2007, San Diego, California, USA, 2007-06-30) Marval, Juan P.; Rojas-Solorzano, L.Most granular flows at environmental conditions are unsteady and exhibit a complex physical behavior. Dune formation and migration in the desert are controlled not only by the flow of saltating particles over the sand bed, but also by turbulent atmospheric airflow. In fact, sediments are transported by the atmospheric airflow within a thin layer only a few centimeters above the sandy surface. These jumping particles reach a maximum sediment mass flux level at a certain delay time (known as the “saturation time”) after the initial movement by sliding and rolling begins. Unlike sediment transport in water where the particles are lifted by the turbulent suspension, the saltating particles are kept alive in the layer mainly due to particle-particle and particle-bed collisions. In order to model this Aeolian transport of sand, Jenkins and Pasini [1] proposed a two-fluid model (one-dimensional and steady state) using Granular Kinetic Theory (GKT) to describe the solid-phase stress. The present work extends the original idea of Jenkins and Pasini [1] by using a more robust model of GKT for the kinetic/collisional contributions to the solid-phase stress tensor, together with a friction model activated for sustained contacts between particles. In addition, a standard k-ε turbulence model for the air and a drag model for the interaction between the phases are employed. A rectangular 2D geometry was chosen with a logarithmic profile for the inlet air velocity, along with an initial amount of sand at rest in the lower part of the simulation domain, resembling the particle saltating flow commonly seen in the vertical middle plane within saltation wind tunnels. This model is validated with experimental data from Liu and Dong [2] and the results given by Pasini and Jenkins [1]. A good estimation for the particle erosion and mass flux in the saltation layer is predicted, even though the profiles of mass flux and concentration within the transport layer are very thin and lowerItem Open Access Verification and validation of computational fluid dynamics simulations of compound channel(2010) Filonovich, M. S.; Azevedo, R.; Rojas-Solorzano, L.; Leal, J. B.In this study the verification and validation of a 2nd order turbulence closure model is performed for an experimental compound channel flow, where the velocity field was measured by a Laser Doppler Velocimeter. Detailed Explicit Algebraic Reynolds Stress Model (EARSM) simulation is reported. The Grid Convergence Index (GCI) approach proposed by Roache (1998) was adopted to evaluate the uncertainty associated to grid resolution. The velocity components, the turbulent kinetic energy (TKE) and the dissipation rate were used as variables of interest. The GCI results present low values for the streamwise velocity, TKE and dissipation rate, but higher values in what concerns vertical and spanwise velocities. This indicates that the mean primary flow has converged but the secondary flow field still depends on grid resolution. Based on GCI values distribution, the mesh was locally refined. Comparison of numerical and experimental results shows good agreement