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Flow Separation (flow + separation)
Selected AbstractsLarge eddy simulation of passive scalar in complex turbulence with flow impingement and flow separationHEAT TRANSFER - ASIAN RESEARCH (FORMERLY HEAT TRANSFER-JAPANESE RESEARCH), Issue 5 2001Ken-ichi Abe Abstract In order to reveal unknown characteristics of complex turbulent passive scalar fields, large eddy simulations in forced convection regimes have been performed under several strain conditions, including flow impingement and flow separation. By using the simulation results, relations between the dynamic and scalar fields are carefully examined. It is then confirmed that the scalar is transported by a large vortex structure near the examined regions wherever the mean shear vanishes, although in the high-shear regions, the scalar transport is governed by a coherent structure due to the high shear strain. In addition, a priori explorations are attempted by processing the data, focusing on the derivation of a possible direction for modeling algebraically the passive scalar transport in a complex strain field. The a priori tests suggest that an expanded form of the GGDH model introducing a quadratic product of the Reynolds stresses is promising for general flow cases. © 2001 Scripta Technica, Heat Trans Asian Res, 30(5): 402,418, 2001 [source] An updated interactive boundary layer method for high Reynolds number flowsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 2 2006F. Álvarez Abstract The quasi-simultaneous interactive boundary layer (IBL) method is improved with the iterative correction of an inviscid operator. The updated interactive boundary layer method (UIBL) presented in this work, uses the Hess,Smith panel method (HSPM) as an inviscid operator to update the outer flow calculation and the inviscid velocity in the interaction law (IL). The discretization of the Hilbert integral (HI) from the original method is modified to reduce the error introduced by the calculation of the HI in a restricted domain. The method is tested on a flat plate with a small indentation for two-dimensional, steady, incompressible and laminar flow. The UIBL method is capable to predict the flow separation and reattachment with good accuracy. The accuracy of the results is competitive with the numerical solution of the Navier,Stokes equations (NSE). Copyright © 2005 John Wiley & Sons, Ltd. [source] Numerical analysis of turbulent flow separation in a rectangular duct with a sharp 180-degree turn by algebraic Reynolds stress modelINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 12 2008Hitoshi Sugiyama Abstract Turbulent flow in a rectangular duct with a sharp 180-degree turn is difficult to predict numerically because the flow behavior is influenced by several types of forces, including centrifugal force, pressure-driven force, and shear stress generated by anisotropic turbulence. In particular, this type of flow is characterized by a large-scale separated flow, and it is difficult to predict the reattachment point of a separated flow. Numerical analysis has been performed for a turbulent flow in a rectangular duct with a sharp 180-degree turn using the algebraic Reynolds stress model. A boundary-fitted coordinate system is introduced as a method for coordinate transformation to set the boundary conditions next to complicated shapes. The calculated results are compared with the experimental data, as measured by a laser-Doppler anemometer, in order to examine the validity of the proposed numerical method and turbulent model. In addition, the possibility of improving the wall function method in the separated flow region is examined by replacing the log-law velocity profile for a smooth wall with that for a rough wall. The analysis results indicated that the proposed algebraic Reynolds stress model can be used to reasonably predict the turbulent flow in a rectangular duct with a sharp 180-degree turn. In particular, the calculated reattachment point of a separated flow, which is difficult to predict in a turbulent flow, agrees well with the experimental results. In addition, the calculation results suggest that the wall function method using the log-law velocity profile for a rough wall over a separated flow region has some potential for improving the prediction accuracy. Copyright © 2007 John Wiley & Sons, Ltd. [source] A simplified v2,f model for near-wall turbulenceINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 12 2007M. M. Rahman Abstract A simplified version of the v2,f model is proposed that accounts for the distinct effects of low-Reynolds number and near-wall turbulence. It incorporates modified C,(1,2) coefficients to amplify the level of dissipation in non-equilibrium flow regions, thus reducing the kinetic energy and length scale magnitudes to improve prediction of adverse pressure gradient flows, involving flow separation and reattachment. Unlike the conventional v2,f, it requires one additional equation (i.e. the elliptic equation for the elliptic relaxation parameter fµ) to be solved in conjunction with the k,, model. The scaling is evaluated from k in collaboration with an anisotropic coefficient Cv and fµ. Consequently, the model needs no boundary condition on and avoids free stream sensitivity. The model is validated against a few flow cases, yielding predictions in good agreement with the direct numerical simulation (DNS) and experimental data. Copyright © 2007 John Wiley & Sons, Ltd. [source] Numerical study of particulate suspension flow through wavy-walled channelsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 3 2006R. Usha Abstract The particulate suspension flow in a channel whose walls describe a travelling wave motion is examined numerically. A perturbation method is employed and the primitive variables are expanded in a series with the wall amplitude as the perturbation parameter. The boundary conditions are applied at the mean surface of the channel and the first-order perturbation quantities are numerically determined by solving the governing system of ordinary differential equations by shooting technique. The present approach does not impose any restriction on the Reynolds number of the flow and the wave number and frequency of the wavy-walled channel, although it is limited by the linear analysis. The wall shear stress and the positions of flow separation and reattachment points are computed and the influence of the volume fraction density of the particles is examined. The variations of velocity and pressure of the particulate suspension flow with frequency of excitation are also presented. Copyright © 2005 John Wiley & Sons, Ltd. [source] Reduced-order controllers for control of flow past an airfoilINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 5 2006S. S. Ravindran Abstract Reduced-order controller design by means of reduced-order model for control of a wake flow is presented. Reduced-order model is derived by combining the Galerkin projection with proper orthogonal decomposition (POD) or with other related reduced-order approaches such as singular value decomposition or reduced-basis method. In the present investigation, we discuss the applicability of the reduced-order approaches for fast computation of the optimal control for control of vortex shedding behind a thin airfoil through unsteady blowing on the airfoil surface. Accuracy of the reduced-order model is quantified by comparing flow fields obtained from the reduced-order models with those from the full-order simulations under the same free-stream conditions. A control of vortex shedding is demonstrated for Reynolds number 100. It is found that downstream directed blowing on the upper surface of the airfoil near the leading edge is more efficient in mitigating flow separation and suppressing the vortex shedding. Copyright © 2005 John Wiley & Sons, Ltd. [source] An eddy viscosity model with near-wall modificationsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 9 2005M. M. Rahman Abstract An extended version of the isotropic k,, model is proposed that accounts for the distinct effects of low-Reynolds number (LRN) and wall proximity. It incorporates a near-wall correction term to amplify the level of dissipation in nonequilibrium flow regions, thus reducing the kinetic energy and length scale magnitudes to improve prediction of adverse pressure gradient flows, involving flow separation and reattachment. The eddy viscosity formulation maintains the positivity of normal Reynolds stresses and the Schwarz' inequality for turbulent shear stresses. The model coefficients/functions preserve the anisotropic characteristics of turbulence. The model is validated against a few flow cases, yielding predictions in good agreement with the direct numerical simulation (DNS) and experimental data. Comparisons indicate that the present model is a significant improvement over the standard eddy viscosity formulation. Copyright © 2005 John Wiley & Sons, Ltd. [source] Low Reynolds number k,, model for near-wall flowINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 4 2005M. M. Rahman Abstract A wall-distance free k,, turbulence model is developed that accounts for the near-wall and low Reynolds number effects emanating from the physical requirements. The model coefficients/functions depend non-linearly on both the strain rate and vorticity invariants. Included diffusion terms and modified C,(1,2) coefficients amplify the level of dissipation in non-equilibrium flow regions, thus reducing the kinetic energy and length scale magnitudes to improve prediction of adverse pressure gradient flows, involving flow separation and reattachment. The model is validated against a few flow cases, yielding predictions in good agreement with the direct numerical simulation (DNS) and experimental data. Copyright © 2005 John Wiley & Sons, Ltd. [source] Design of a high-efficiency hydrofoil through the use of computational fluid dynamics and multiobjective optimizationAICHE JOURNAL, Issue 7 2009N. Spogis Abstract A computational fluid dynamics (CFD) model is proposed, based on ANSYS-CFX tools coupled to optimization models inside the commercial optimization software modeFRONTIER in order to obtain an optimal design of a high-efficiency impeller for solids suspension. The analysis of impeller shape performance was carried out using the shear-stress transport (SST) turbulence model with streamline curvature correction. This turbulence model combined the advantages of the ,,, and ,,, models, ensuring a proper relation between turbulent stress and turbulent kinetic energy, allowing an accurate and robust prediction of the impeller blade flow separation. The multiple frames of reference and the frozen rotor frame change models were used for the rotor/stator interaction inside the mixing vessel. The optimization procedure used seven design variables, two nonlinear constraints and two objective functions. The objective functions chosen (among many other possible options) to evaluate the impeller performance were the maximum solid distribution throughout the vessel (homogeneous suspension) reflected by a low variance between local solid concentration and average solid concentration inside the vessel and the higher pumping effectiveness, which was defined as the quotient of the flow and power numbers. The first objective function searches for impeller configurations able to provide good solid suspension, since it aims to achieve homogeneous suspension. The second objective function aims to reduce power consumption for a high-pumping capacity of the impeller. These criteria were considered enough to characterize the optimized impeller. Results indicated that the optimized impeller presented an increase of the pumping impeller capacity and homogeneous solid suspension with low-power consumption, especially when compared with the PBT 45° impeller. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source] The heart of the South American rattlesnake, Crotalus durissusJOURNAL OF MORPHOLOGY, Issue 9 2010Bjarke Jensen Abstract Most anatomical and physiological studies of the sauropsid heart have focused on species with extraordinary physiologies, and detailed anatomical descriptions of hearts from sauropsids with more common physiologies are therefore warranted. Here, we present a comprehensive study of the cardiac anatomy of the South American rattlesnake (Crotalus durissus). The cardiovascular physiology of this species has been investigated in a number of studies, whereas only a few cursory studies exist on the cardiac anatomy of viperid snakes. The heart of C. durissus is typically squamate in many regards. Both atria are thin-walled sacs, and the right atrium is the most voluminous. The single ventricle contains three major septa; the vertical septum, the muscular ridge (MR), and the bulbuslamelle. These partially divide the ventricle into three chambers; the systemic and left-sided cavum arteriosum (CA), the pulmonary and right-sided cavum pulmonale, and the medial cavum venosum (CV). The MR is the most developed septum, and several additional and minor septa are found within the CA and CV. An extraordinary thin cortical layer encloses the ventricle, and it is irrigated by a remarkably rich arborization of coronary arteries. Previous studies show high degrees of blood flow separation in the Crotalus heart, and this can only be explained by the coordinated actions of the septa and the prominent atrioventricular valves. J. Morphol. 271:1066-1077, 2010. © 2010 Wiley-Liss, Inc. [source] Numerical Investigation of Turbulent Flow around a Rotating Stepped Cylinder for Corrosion StudyTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 1 2003Kyung-Soo Yang Abstract Direct numerical simulation has been carried out for turbulent flow set up by a rotating cylinder with two backward-facing steps axisymmetrically mounted in the circumferential direction. This flow geometry creates a qualitatively similar flow pattern as observed near a sudden pipe expansion or a plane backward-facing step, characterized by flow separation and reattachment. A region of intense turbulence intensity and high wall-shear-stress fluctuations is formed in the recirculating region downstream of the step, where high mass-transfer capacity was also experimentally observed. Since corrosion is frequently mass-transfer controlled, our findings put forward this apparatus as a useful tool for future corrosion research. On a effectué une simulation numérique directe de l'écoulement turbulent créé par un cylindre rotatif ayant deux contractions axisymétriques dans la direction circonférentielle. Cette géométrie crée un profil d'écoulement qualitativement similaire à celui qu'on observe près d'une expansion de conduite soudaine ou d'une contraction planaire, caractérisés par la séparation et le ré-attachement de l'écoulement. Une région d'intense turbulence et de fortes fluctuations de contraintes de cisaillement pariétal se forment dans la région en recirculation en aval de la contraction, où une grande capacité de transfert de matière a également été observée expérimentalement. Étant donné que la corrosion dépend souvent du transfert de matière, nos résultats font la promotion de cet appareillage en tant qu'outil utile pour la recherche future sur la corrosion. [source] Influence of lee waves on the near-surface flow downwind of the PenninesTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 627 2007P. F. Sheridan Abstract The results of a recent field experiment focusing on the near-surface pressure and flow fields downstream of the Pennines in northern England are presented. The main aim of the experiment is the improvement of wind forecasts downstream of orography. Trapped lee waves commonly occur in westerly flow in this region, and during the experiment there were numerous instances of apparent flow separation, indicating the formation of lee-wave rotors. The spatial variability of the near-surface flow in these circumstances is closely linked to the positions of lee-wave crests and troughs aloft, and appears to be a response to pressure gradients induced by the lee waves. For large-amplitude waves, it has been possible to demonstrate a correlation between the fractional change of the flow speed across the measurement array (which if large enough may lead to flow separation) and a normalized pressure-perturbation amplitude. For a group of lee-wave cases during which the cross-mountain flow is strong, a rapid decrease in the Scorer parameter within the lower portion of the troposphere appears to be a prerequisite for rotors to form. However, this does not guarantee their occurrence. For a fixed Scorer-parameter profile, idealized two-dimensional simulations indicate that the lee-wave-induced pressure-perturbation amplitude, and hence the occurrence of rotors, is controlled largely by the strength of the wind upstream close to the mountain-top level. It seems that the combination of a favourable Scorer-parameter profile and sufficiently strong low-level winds is required for rotors to develop. © Crown Copyright 2007. Reproduced with the permission of Her Majesty's Stationery Office. Published by John Wiley & Sons, Ltd. [source] Observations of downslope winds and rotors in the Falkland IslandsTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 605 2005S. D. Mobbs Abstract A field campaign aimed at observing the near-surface flow field across and downwind of a mountain range on the Falkland Islands, South Atlantic, is described. The objective was to understand and eventually predict orographically generated turbulence. The instrumentation was based primarily on an array of automatic weather stations (AWSs), which recorded 30 s mean surface pressure, wind speed and direction (at 2 m), temperature and relative humidity for approximately one year. These measurements were supported by twice-daily radiosonde releases. The densest part of the AWS array was located to the south of the Wickham mountain range, across Mount Pleasant Airfield (MPA). In northerly flow the array provides a detailed study of the flow downwind of the mountain range. The dataset contains several episodes in which the flow downwind of the mountains is accelerated relative to the upwind flow. During some of these episodes short-lived (typically ,1 hour) periods of unsteady flow separation are observed and these are associated with the formation of rotors aloft. Such events present a significant hazard to aviation at MPA. Examination of radiosonde profiles suggests that the presence of a strong temperature inversion at a height similar to the mountain height is a necessary condition for both downwind acceleration and the formation of rotors. The data are used to show that the downwind fractional speed-up is proportional to the non-dimensional mountain height (based on upstream near-surface winds and a depth-averaged Brunt,Väisälä frequency diagnosed from radiosonde data). Similarly, a relationship is established between a quantity that describes the spatial variability of the flow downwind of the mountains and the upstream wind and depth-averaged Brunt,Väisälä frequency. The dependence of the flow behaviour on the Froude number (defined in the usual way for two-layer shallow-water flow) and ratio of mountain height to inversion height is presented in terms of a flow regime diagram. © Royal Meteorological Society, 2005. S. B. Vosper's and P. F. Sheridan's contributions are Crown copyright [source] Inlet and Outlet Devices for Rotary Blood Pumps,ARTIFICIAL ORGANS, Issue 10 2004Xinwei Song Abstract: The purposes of inlet and outlet devices for rotary blood pumps, including inducers and diffusers for axial pumps, inlet and exit volutes for centrifugal pumps, and inlet and outlet cannulas, are to guide the blood into the impeller, where the blood is accelerated, and to convert the high kinetic energy into pressure after the impeller discharge, respectively. The designs of the inlet and outlet devices have an important bearing on the pump performance. Their designs are highly dependent on computational fluid dynamics (CFD) analysis, guided by intuition and experience. For inlet devices, the design objectives are to eliminate separated flow, to minimize recirculation, and to equalize the radial components of velocity. For outlet devices, the design goals are to reduce speed, to minimize energy loss, and to avoid flow separation and whirl. CFD analyses indicate the velocity field and pressure distribution. Geometrical optimization of these components has been implemented in order to improve the flow pattern. [source] Buffeting in transonic flow prediction using time-dependent turbulence modelINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 2 2005A. Kourta Abstract In transonic flow conditions, the shock wave/turbulent boundary layer interaction and flow separations on wing upper surface induce flow instabilities, ,buffet', and then the buffeting (structure vibrations). This phenomenon can greatly influence the aerodynamic performance. These flow excitations are self-sustained and lead to a surface effort due to pressure fluctuations. They can produce enough energy to excite the structure. The objective of the present work is to predict this unsteady phenomenon correctly by using unsteady Navier,Stokes-averaged equations with a time-dependent turbulence model based on the suitable (k,,) turbulent eddy viscosity model. The model used is based on the turbulent viscosity concept where the turbulent viscosity coefficient (C,) is related to local deformation and rotation rates. To validate this model, flow over a flat plate at Mach number of 0.6 is first computed, then the flow around a NACA0012 airfoil. The comparison with the analytical and experimental results shows a good agreement. The ONERA OAT15A transonic airfoil was chosen to describe buffeting phenomena. Numerical simulations are done by using a Navier,Stokes SUPG (streamline upwind Petrov,Galerkin) finite-element solver. Computational results show the ability of the present model to predict physical phenomena of the flow oscillations. The unsteady shock wave/boundary layer interaction is described. 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