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Flow Domain (flow + domain)
Selected AbstractsFunnel-and-Gate Performance in a Moderately Heterogeneous Flow DomainGROUND WATER MONITORING & REMEDIATION, Issue 3 2001Lacrecia C. Bilbrey The funnel-and-gate ground water remediation technology (Starr and Cherry 1994) has received increased attention and application as an in situ alternative to the typical pump-and-treat system. Understanding the effects of heterogeneity on system performance can mean the difference between a successful remediation project and one that fails to meet its cleanup goals. In an attempt to characterize and quantify the effects of heterogeneity on funnel-and-gate system performance, a numerical modeling study of 15 simulated heterogeneous flow domains was conducted. Each realization was tested to determine if the predicted capture width met the capture width expected for a homogeneous flow domain with the same hulk properties. This study revealed that the capture width of the funnel-and-gate system varied significantly with the level of heterogeneity of the aquifer. Two possible remedies were investigated for bringing systems with less than acceptable capture widths to acceptable levels of performance. First, it was determined that enlarging the funnel and gate via a factor of safety applied to the design capture width could compensate for the capture width variation in the heterogeneous flow domains. In addition, it was shown that the use of a pumping well downstream of the funnel and gate could compensate for the effects of aquifer heterogeneity on the funnel-and-gate capture width. However, if a pumping well is placed downstream of the funnel and gate to control the hydraulic gradient through the gate, consideration should be given to the gate residence time in relation to the geochemistry of the contaminant removal or destruction process in the gate. [source] A two-grid fictitious domain method for direct simulation of flows involving non-interacting particles of a very small sizeINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 11 2010A. Dechaume Abstract The full resolution of flows involving particles whose scale is hundreds or thousands of times smaller than the size of the flow domain is a challenging problem. A naive approach would require a tremendous number of degrees of freedom in order to bridge the gap between the two spatial scales involved. The approach used in the present study employs two grids whose grid size fits the two different scales involved, one of them (the micro-scale grid) being embedded into the other (the macro-scale grid). Then resolving first the larger scale on the macro-scale grid, we transfer the so obtained data to the boundary of the micro-scale grid and solve the smaller size problem. Since the particle is moving throughout the macro-scale domain, the micro-scale grid is fixed at the centroid of the moving particle and therefore moves with it. In this study we combine such an approach with a fictitious domain formulation of the problem resulting in a very efficient algorithm that is also easy to implement in an existing CFD code. We validate the method against existing experimental data for a sedimenting sphere, as well as analytical results for motion of an inertia-less ellipsoid in a shear flow. Finally, we apply the method to the flow of a high aspect ratio ellipsoid in a model of a human lung airway bifurcation. Copyright © 2009 John Wiley & Sons, Ltd. [source] Volumetric methods for evaluating energy loss and heat transfer in cavity flows,INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 12 2007Stuart Norris Abstract Methods have been developed for calculating irreversible energy losses and rates of heat transfer from computational fluid dynamics solutions using volume integrations of energy dissipation or entropy production functions. These methods contrast with the more usual approach of performing first law energy balances over the boundaries of a flow domain. Advantages of the volumetric approach are that the estimates involve the whole flow domain and are hence based on more information than would otherwise be used, and that the energy dissipation or entropy production functions allow for detailed assessment of the mechanisms and regions of energy loss or entropy production. Volume integrations are applied to the calculation of viscous losses in a lid-driven cavity flow, and to the viscous losses and heat transfer due to natural convection in a side-heated cavity. In the convection problem comparison with the entropy increase across a stationary heat conducting layer leads to a novel volume integral expression for the Nusselt number. The predictions using this method compare well with traditional surface integrals and benchmark results. Copyright © 2007 John Wiley & Sons, Ltd. [source] Finite volume solution of the Navier,Stokes equations in velocity,vorticity formulationINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 6 2005Baoshan Zhu Abstract For the incompressible Navier,Stokes equations, vorticity-based formulations have many attractive features over primitive-variable velocity,pressure formulations. However, some features interfere with the use of the numerical methods based on the vorticity formulations, one of them being the lack of a boundary conditions on vorticity. In this paper, a novel approach is presented to solve the velocity,vorticity integro-differential formulations. The general numerical method is based on standard finite volume scheme. The velocities needed at the vertexes of each control volume are calculated by a so-called generalized Biot,Savart formula combined with a fast summation algorithm, which makes the velocity boundary conditions implicitly satisfied by maintaining the kinematic compatibility of the velocity and vorticity fields. The well-known fractional step approaches are used to solve the vorticity transport equation. The paper describes in detail how we accurately impose no normal-flow and no tangential-flow boundary conditions. We impose a no-flux boundary condition on solid objects by the introduction of a proper amount of vorticity at wall. The diffusion term in the transport equation is treated implicitly using a conservative finite update. The diffusive fluxes of vorticity into flow domain from solid boundaries are determined by an iterative process in order to satisfy the no tangential-flow boundary condition. As application examples, the impulsively started flows through a flat plate and a circular cylinder are computed using the method. The present results are compared with the analytical solution and other numerical results and show good agreement. Copyright © 2005 John Wiley & Sons, Ltd. [source] Computation of unsteady viscous incompressible flows in generalized non-inertial co-ordinate system using Godunov-projection method and overlapping meshesINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 11 2002H. Pan Abstract Time-dependent incompressible Navier,Stokes equations are formulated in generalized non-inertial co-ordinate system and numerically solved by using a modified second-order Godunov-projection method on a system of overlapped body-fitted structured grids. The projection method uses a second-order fractional step scheme in which the momentum equation is solved to obtain the intermediate velocity field which is then projected on to the space of divergence-free vector fields. The second-order Godunov method is applied for numerically approximating the non-linear convection terms in order to provide a robust discretization for simulating flows at high Reynolds number. In order to obtain the pressure field, the pressure Poisson equation is solved. Overlapping grids are used to discretize the flow domain so that the moving-boundary problem can be solved economically. Numerical results are then presented to demonstrate the performance of this projection method for a variety of unsteady two- and three-dimensional flow problems formulated in the non-inertial co-ordinate systems. Copyright © 2002 John Wiley & Sons, Ltd. [source] GENSMAC3D: a numerical method for solving unsteady three-dimensional free surface flowsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 7 2001M.F. Tomé Abstract A numerical method for solving three-dimensional free surface flows is presented. The technique is an extension of the GENSMAC code for calculating free surface flows in two dimensions. As in GENSMAC, the full Navier,Stokes equations are solved by a finite difference method; the fluid surface is represented by a piecewise linear surface composed of quadrilaterals and triangles containing marker particles on their vertices; the stress conditions on the free surface are accurately imposed; the conjugate gradient method is employed for solving the discrete Poisson equation arising from a velocity update; and an automatic time step routine is used for calculating the time step at every cycle. A program implementing these features has been interfaced with a solid modelling routine defining the flow domain. A user-friendly input data file is employed to allow almost any arbitrary three-dimensional shape to be described. The visualization of the results is performed using computer graphic structures such as phong shade, flat and parallel surfaces. Results demonstrating the applicability of this new technique for solving complex free surface flows, such as cavity filling and jet buckling, are presented. Copyright © 2001 John Wiley & Sons, Ltd. [source] Finite volume multigrid method of the planar contraction flow of a viscoelastic fluidINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 8 2001H. Al Moatssime Abstract This paper reports on a numerical algorithm for the steady flow of viscoelastic fluid. The conservative and constitutive equations are solved using the finite volume method (FVM) with a hybrid scheme for the velocities and first-order upwind approximation for the viscoelastic stress. A non-uniform staggered grid system is used. The iterative SIMPLE algorithm is employed to relax the coupled momentum and continuity equations. The non-linear algebraic equations over the flow domain are solved iteratively by the symmetrical coupled Gauss,Seidel (SCGS) method. In both, the full approximation storage (FAS) multigrid algorithm is used. An Oldroyd-B fluid model was selected for the calculation. Results are reported for planar 4:1 abrupt contraction at various Weissenberg numbers. The solutions are found to be stable and smooth. The solutions show that at high Weissenberg number the domain must be long enough. The convergence of the method has been verified with grid refinement. All the calculations have been performed on a PC equipped with a Pentium III processor at 550 MHz. Copyright © 2001 John Wiley & Sons, Ltd. [source] Numerical modeling of highly swirling flows in a through-flow cylindrical hydrocycloneAICHE JOURNAL, Issue 10 2006Jordan Ko Abstract This article aims to identify the most appropriate numerical methodology for simulating hydrocyclone flows with high swirl numbers. The numerical results are validated against the tangential velocity measurements from a cylindrical hydrocyclone with a swirl number of 8.1, which is twice the typical swirl magnitude of industrial hydrocyclones. The linear and quadratic formulations of the Reynolds stress transport (RST) model are used to simulate the anisotropic swirling turbulent flow three-dimensionally in the commercial software package FluentÔ. The tangential velocity profiles predicted by the quadratic RST model are in good agreement with experimental data. They also show Rankine vortex patterns over the entire flow domain. In contrast, the linear RST model fails to predict this important swirl flow feature. In addition, both models predicted a complex axial flow reversal pattern not previously reported in hydrocyclones. This study clearly shows that the quadratic RST model is preferable for future hydrocyclone simulations, especially when the swirl number is large. All necessary physical and numerical parameters used to obtain converged results are given in this article. © American Institute of Chemical Engineers AIChE J, 2006 [source] Computational Simulation of the Blood Separation ProcessARTIFICIAL ORGANS, Issue 8 2005Sandro De Gruttola Abstract:, The aim of this work is to construct a computational fluid dynamics model capable of simulating the quasitransient process of apheresis. To this end a Lagrangian,Eulerian model has been developed which tracks the blood particles within a delineated two-dimensional flow domain. Within the Eulerian method, the fluid flow conservation equations within the separator are solved. Taking the calculated values of the flow field and using a Lagrangian method, the displacement of the blood particles is calculated. Thus, the local blood density within the separator at a given time step is known. Subsequently, the flow field in the separator is recalculated. This process continues until a quasisteady behavior is reached. The simulations show good agreement with experimental results. They shows a complete separation of plasma and red blood cells, as well as nearly complete separation of red blood cells and platelets. The white blood cells build clusters in the low concentrate cell bed. [source] Computational Fluid Dynamics and Vascular AccessARTIFICIAL ORGANS, Issue 7 2002Ulf Krueger Abstract: Anastomotic intimal hyperplasia caused by unphysiological hemodynamics is generally accepted as a reason for dialysis access graft occlusion. Optimizing the venous anastomosis can improve the patency rate of arteriovenous grafts. The purpose of this study was to examine, evaluate, and characterize the local hemodynamics and, in particular, the wall shear stresses in conventional venous end-to-side anastomosis and in patch form anastomosis (Venaflo) by three-dimensional computational fluid dynamics (CFD). We investigated the conventional form of end-to-side anastomosis and a new patch form by numerical simulation of blood flow. The numerical simulation was done with a finite volume-based algorithm. The anastomotic forms were constructed with usual size and fixed walls. Subdividing the flow domain into multiple control volumes solved the fundamental equations. The boundary conditions were identical for both forms. The velocity profile of the patch form is better than that for the conventional form. The region of high static pressure caused by flow stagnation is reduced on the vein floor. The anastomotic wall shear stress is decreased. The results of this study strongly support patch form use to reduce the incidence of intimal hyperplasia and venous anastomotic stenoses. [source] CFD Investigation of the Mixing of Yield-Pseudoplastic Fluids with Anchor ImpellersCHEMICAL ENGINEERING & TECHNOLOGY (CET), Issue 8 2009P. Prajapati Abstract The study was carried out to simulate the 3D flow domain in the mixing of pseudoplastic fluids possessing yield stress with anchor impellers, using a computational fluid dynamics (CFD) package. The multiple reference frames (MRF) technique was employed to model the rotation of the impellers. The rheology of the fluid was approximated using the Herschel,Bulkley model. To validate the model, the CFD results for the power consumption were compared to the experimental data. After the flow fields were calculated, the simulations for tracer homogenization were performed to simulate the mixing time. The effects of impeller speed, fluid rheology, and impeller geometry on power consumption, mixing time, and flow pattern were explored. The optimum values of c/D (impeller clearance to tank diameter) and w/D (impeller blade width to tank diameter) ratios were determined on the basis of minimum mixing time. [source] Funnel-and-Gate Performance in a Moderately Heterogeneous Flow DomainGROUND WATER MONITORING & REMEDIATION, Issue 3 2001Lacrecia C. Bilbrey The funnel-and-gate ground water remediation technology (Starr and Cherry 1994) has received increased attention and application as an in situ alternative to the typical pump-and-treat system. Understanding the effects of heterogeneity on system performance can mean the difference between a successful remediation project and one that fails to meet its cleanup goals. In an attempt to characterize and quantify the effects of heterogeneity on funnel-and-gate system performance, a numerical modeling study of 15 simulated heterogeneous flow domains was conducted. Each realization was tested to determine if the predicted capture width met the capture width expected for a homogeneous flow domain with the same hulk properties. This study revealed that the capture width of the funnel-and-gate system varied significantly with the level of heterogeneity of the aquifer. Two possible remedies were investigated for bringing systems with less than acceptable capture widths to acceptable levels of performance. First, it was determined that enlarging the funnel and gate via a factor of safety applied to the design capture width could compensate for the capture width variation in the heterogeneous flow domains. In addition, it was shown that the use of a pumping well downstream of the funnel and gate could compensate for the effects of aquifer heterogeneity on the funnel-and-gate capture width. However, if a pumping well is placed downstream of the funnel and gate to control the hydraulic gradient through the gate, consideration should be given to the gate residence time in relation to the geochemistry of the contaminant removal or destruction process in the gate. [source] A numerical model for the flooding and drying of irregular domainsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 3 2002P. Brufau Abstract A numerical technique for the modelling of shallow water flow in one and two dimensions is presented in this work along with the results obtained in different applications involving unsteady flows in complex geometries. A cell-centred finite volume method based on Roe's approximate Riemann solver across the edges of both structured and unstructured cells is presented. The discretization of the bed slope source terms is done following an upwind approach. In some applications a problem arises when the flow propagates over adverse dry bed slopes, so a special procedure has been introduced to model the advancing front. It is shown that this modification reproduces exactly steady state of still water in configurations with strong variations in bed slope and contour. The applications presented are mainly related with unsteady flow problems. The scheme is capable of handling complex flow domains as will be shown in the simulations corresponding to the test cases that are going to be presented. Comparisons of experimental and numerical results are shown for some of the tests. Copyright © 2002 John Wiley & Sons, Ltd. [source] Sensitivity analyses for four pesticide leaching modelsPEST MANAGEMENT SCIENCE (FORMERLY: PESTICIDE SCIENCE), Issue 9 2003Igor G Dubus Abstract Sensitivity analyses using a one-at-a-time approach were carried out for leaching models which have been widely used for pesticide registration in Europe (PELMO, PRZM, PESTLA and MACRO). Four scenarios were considered for simulation of the leaching of two theoretical pesticides in a sandy loam and a clay loam soil, each with a broad distribution across Europe. Input parameters were varied within bounds reflecting their uncertainty and the influence of these variations on model predictions was investigated for accumulated percolation at 1-m depth and pesticide loading in leachate. Predictions for the base-case scenarios differed between chromatographic models and the preferential flow model MACRO for which large but transient pesticide losses were predicted in the clay loam. Volumes of percolated water predicted by the four models were affected by a small number of input parameters and to a small extent only, suggesting that meteorological variables will be the main drivers of water balance predictions. In contrast to percolation, predictions for pesticide loss were found to be sensitive to a large number of input parameters and to a much greater extent. Parameters which had the largest influence on the prediction of pesticide loss were generally those related to chemical sorption (Freundlich exponent nf and distribution coefficient Kf) and degradation (either degradation rates or DT50, QTEN value). Nevertheless, a significant influence of soil properties (field capacity, bulk density or parameters defining the boundary between flow domains in MACRO) was also noted in at least one scenario for all models. Large sensitivities were reported for all models, especially PELMO and PRZM, and sensitivity was greater where only limited leaching was simulated. Uncertainty should be addressed in risk assessment procedures for crop-protection products. Copyright © 2003 Society of Chemical Industry [source] Investigation of unsaturated flow in woven, braided and stitched fiber mats during mold-filling in resin transfer moldingPOLYMER COMPOSITES, Issue 4 2001J. Slade In Resin Transfer Molding (RPM), which is a process to manufacture polymer composites, the impregnation of fibrous reinforcement In the form of mats by a thermosetting resin is modeled as the flow of a Newtonian liquid through a single length-scale porous medium. While this approach is sufficiently accurate for random fiber-mats, it can lead to appreciable errors when applied to woven, braided, or stitched fiber-mats that contain two length scales. This work investigates the primary factors governing the isothermal unsaturated flow through such dual-scale porous media. Two studies were conducted to better understand this phenomenon: the first experimenatally investigated the flow, while the second theoretically modeled the flow and identified important parameters affecting such a flow with the help of dimensionless analysis. In the first study, one-dimensional constant injection rate experiments were performed using various fiber mats. The unsaturated flow behavior of various mats was characterized using a constant "sink" term in the continuity equation. Results indicated that for a given fiber-mat, the magnitude of the sink effect was a function of the capillary number. In the second study, a numerical model was developed to describe flow through dual-scale preforms in which the two flow domains, the inter- and intra-tow regions, were coupled. We identified a dimensionless number called the sink effect index , that characterizes the magnitude of liquid absorption by the tows and is a function of the relative resistance to flow in the tow and inter-tow regions, and the packing density of the tows. The parametric study of this index with the help of numerical simulations reveals its influence on the flow and identifies the distinct transient and steady-state flow regimes. [source] | |||||||||||||