Flow Features (flow + feature)

Distribution by Scientific Domains

Selected Abstracts

Numerical modeling of highly swirling flows in a through-flow cylindrical hydrocyclone

AICHE JOURNAL, Issue 10 2006
Jordan 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]

An investigation of pulsatile flow in a model cavo-pulmonary vascular system

K. Chitra
Abstract The complexities in the flow pattern in a cavo-pulmonary vascular system,after application of the Fontan procedure in the vicinity of the superior vena cava, inferior vena cava, and the confluence at the T-junction,are analysed. A characteristic-based split (CBS) finite element scheme involving the artificial compressibility approach is employed to compute the resulting flow. Benchmarking of the CBS scheme is carried out using standard problems and with the flow features observed in an experimental model with the help of a dye visualization technique in model scale. The transient flow variations in a total cavo-pulmonary connection (TCPC) under pulsatile conditions are investigated and compared with flow visualization studies. In addition to such qualitative flow investigations, quantitative analysis of energy loss and haemodynamic stresses have also been performed. The comparisons show good agreement between the numerical and experimental flow patterns. The numerically predicted shear stress values indicate that the pulsatile flow condition is likely to be more severe than steady flow, with regard to the long-term health of the surgically corrected TCPC. Copyright © 2008 John Wiley & Sons, Ltd. [source]

1-D numerical modelling of shallow flows with variable horizontal density

Feifei Zhang Leighton
Abstract A1-D numerical model is presented for vertically homogeneous shallow flows with variable horizontal density. The governing equations represent depth-averaged mass and momentum conservation of a liquid,species mixture, and mass conservation of the species in the horizontal direction. Here, the term ,species' refers to material transported with the liquid flow. For example, when the species is taken to be suspended sediment, the model provides an idealized simulation of hyper-concentrated sediment-laden flows. The volumetric species concentration acts as an active scalar, allowing the species dynamics to modify the flow structure. A Godunov-type finite volume scheme is implemented to solve the conservation laws written in a deviatoric, hyperbolic form. The model is verified for variable-density flows, where analytical steady-state solutions are derived. The agreement between the numerical predictions and benchmark test solutions illustrates the ability of the model to capture rapidly varying flow features over uniform and non-uniform bed topography. A parameter study examines the effects of varying the initial density and depth in different regions. Copyright © 2009 John Wiley & Sons, Ltd. [source]

A visual incompressible magneto-hydrodynamics solver with radiation, mass, and heat transfer

Necdet AslanArticle first published online: 8 JAN 200
Abstract A visual two-dimensional (2D) nonlinear magneto-hydrodynamics (MHD) code that is able to solve steady state or transient charged or neutral convection problems under the radiation, mass, and heat transfer effects is presented. The flows considered are incompressible and the divergence conditions on the velocity and magnetic fields are handled by similar relaxation schemes in the form of pseudo-iterations between the real time levels. The numerical method utilizes a matrix distribution scheme that runs on structured or unstructured triangular meshes. The time-dependent algorithm developed here utilizes a semi-implicit dual time stepping technique with multistage Runge-Kutta (RK) algorithm. It is possible for the user to choose different normalizations (natural, forced, Boussinesq, Prandtl, double-diffusive and radiation convection) automatically. The code is visual and runs interactively with the user. The graphics algorithms work multithreaded and allow the user to follow certain flow features (color graphs, vector graphs, one-dimensional profiles) during runs, see (Comput. Fluids 2007; 36:961,973) for details. With the code presented here nonlinear steady or time-dependent evolution of heated and stratified neutral and charged liquids, convection of mixture of neutral and charged gases, double-diffusive and salinity natural convection flows with internal heat generation/absorption and radiative heat transfer flows can be investigated. In addition, the numerical method (combining concentration, radiation, heat transfer, and MHD effects) takes the advantage of local time stepping and employs simplified residual jacobian matrix to increase pseudo-convergence rate. This code is currently being improved to simulate three-dimensional problems with parallel processing. Copyright © 2009 John Wiley & Sons, Ltd. [source]

On CFL evolution strategies for implicit upwind methods in linearized Euler equations

H. M. Bücker
Abstract In implicit upwind methods for the solution of linearized Euler equations, one of the key issues is to balance large time steps, leading to a fast convergence behavior, and small time steps, needed to sufficiently resolve relevant flow features. A time step is determined by choosing a Courant,Friedrichs,Levy (CFL) number in every iteration. A novel CFL evolution strategy is introduced and compared with two existing strategies. Numerical experiments using the adaptive multiscale finite volume solver QUADFLOW demonstrate that all three CFL evolution strategies have their advantages and disadvantages. A fourth strategy aiming at reducing the residual as much as possible in every time step is also examined. Using automatic differentiation, a sensitivity analysis investigating the influence of the CFL number on the residual is carried out confirming that, today, CFL control is still a difficult and open problem. Copyright © 2008 John Wiley & Sons, Ltd. [source]

A mesh patching method for finite volume modelling of shallow water flow

Keming Hu
Abstract A new mesh-patching model is presented for shallow water flow described by the 2D non-linear shallow water (NLSW) equations. The mesh-patching model is based on AMAZON, a high-resolution NLSW engine with an improved HLLC approximate Riemann solver. A new patching algorithm has been developed, which not only provides improved spatial resolution of flow features in particular parts of the mesh, but also simplifies and speeds up the (structured) grid generation process for an area with complicated geometry. The new patching technique is also compatible with increasingly popular parallel computing and adaptive grid techniques. The patching algorithm has been tested with moving bores, and results of test problems are presented and compared to previous work. Copyright © 2005 John Wiley & Sons, Ltd. [source]

On MILES based on flux-limiting algorithms,

F. F. Grinstein
Abstract Non-classical large eddy simulation (LES) approaches based on using the unfiltered flow equations instead of the filtered ones have been the subject of considerable interest during the last decade. In the monotonically integrated LES (MILES) approach, flux-limiting schemes are used to emulate the characteristic turbulent flow features in the high-wave number end of the inertial subrange region. Mathematical and physical aspects of implicit sub grid scale modelling using nonlinear flux-limiters are conveniently addressed using the modified LES-equation formalism. In this study, the performance of MILES is demonstrated as a function of the flux-limiting scheme in selected representative case studies. Published in 2005 by John Wiley & Sons, Ltd. [source]

Numerical methods for large-eddy simulation in general co-ordinates

Gefeng Tang
Abstract Large scale unsteady motions in many practical engineering flows play a very important role and it is very unlikely that these unsteady flow features can be captured within the framework of Reynolds averaged Navier,Stokes approach. Large-eddy simulation (LES) has become, arguably, the only practical numerical tool for predicting those flows more accurately since it is still not realistic to apply DNS to practical engineering flows with the current and near future available computing power. Numerical methods for the LES of turbulent flows in complex geometry have been developed and applied to predict practical engineering flows successfully. The method is based on body-fitted curvilinear co-ordinates with the contravariant velocity components of the general Navier,Stokes equations discretized on a staggered orthogonal mesh. For incompressible flow simulations the main source of computational expense is due to the solution of a Poisson equation for pressure. This is especially true for flows in complex geometry. A multigrid 3D pressure solver is developed to speed up the solution. In addition, the Poisson equation for pressure takes a simpler form with no cross-derivatives when orthogonal mesh is used and hence resulting in increased convergence rate and producing more accurate solutions. Copyright © 2004 John Wiley & Sons, Ltd. [source]

Effects of blood models on flows through a stenosis

Panagiotis Neofytou
Abstract The paper presents a numerical investigation of non-Newtonian modelling effects on unsteady periodic flows in a two-dimensional (2D) channel with a stenosis. The geometry and boundary conditions were chosen so as to reproduce the flow features that are observed in real haemodynamic conditions. Three different non-Newtonian constitutive equations for modelling the shear characteristics of the blood namely the Casson, power-law and Quemada models, are utilized. Similarly with previous studies based on Newtonian modelling, the present simulations show the formation of several vortices downstream of the stenosis, as well as substantial variations of the wall shear stress throughout the unsteady cycle. Additionally, it is shown that: (i) there are substantial differences between the results obtained by Newtonian and non-Newtonian models, and (ii) the prediction of vortex formation, wall shear stress distribution and separation behind the stenosis is strongly dependent on the details of the non-Newtonian model employed in the simulations. Copyright © 2003 John Wiley & Sons, Ltd. [source]

The shallow flow equations solved on adaptive quadtree grids

A. G. L. Borthwick
Abstract This paper describes an adaptive quadtree grid-based solver of the depth-averaged shallow water equations. The model is designed to approximate flows in complicated large-scale shallow domains while focusing on important smaller-scale localized flow features. Quadtree grids are created automatically by recursive subdivision of a rectangle about discretized boundary, bathymetric or flow-related seeding points. It can be fitted in a fractal-like sense by local grid refinement to any boundary, however distorted, provided absolute convergence to the boundary is not required and a low level of stepped boundary can be tolerated. Grid information is stored as a tree data structure, with a novel indexing system used to link information on the quadtree to a finite volume discretization of the governing equations. As the flow field develops, the grids may be adapted using a parameter based on vorticity and grid cell size. The numerical model is validated using standard benchmark tests, including seiches, Coriolis-induced set-up, jet-forced flow in a circular reservoir, and wetting and drying. Wind-induced flow in the Nichupté Lagoon, México, provides an illustrative example of an application to flow in extremely complicated multi-connected regions. Copyright © 2001 John Wiley & Sons, Ltd. [source]

Diffusion of feed spray in fluid catalytic cracker riser

AICHE JOURNAL, Issue 4 2010
Yiping Fan
Abstract For fluid catalytic cracker (FCC) riser reactor, the diffusion pattern of feed spray and the flow features of catalysts in the feed injection zone were investigated in a cold-riser model that is made of 186-mm ID plexiglass pipe. In the feed injection zone, when a feed spray is introduced into the riser, a secondary flow of spray will occur. The secondary flow extends at first and then merges into the mainstream of spray. The occurrence of the secondary flow enhances the mixing of catalysts with feed. However, the extension of the secondary flow causes a violent catalyst backmixing; it is believed to be harmful to FCC reaction. The generation of the secondary flow of feed spray was theoretically analyzed by using the Kutta-Joukowski Lift Theorem. Furthermore, a FCC feed nozzle, which can control/utilize the secondary flow in riser, was proposed. The effects of the nozzle used in some commercial FCC units are quite desirable. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source]

Modeling Flow in a Compromised Pediatric Airway Breathing Air and Heliox

Mihai Mihaescu PhD
Abstract Objectives/Hypothesis: The aim of this study was to perform computer simulations of flow within an accurate model of a pediatric airway with subglottic stenosis. It is believed that the airflow characteristics in a stenotic airway are strongly related to the sensation of dyspnea. Methodology: Computed tomography images through the respiratory tract of an infant with subglottic stenosis, were used to construct the three-dimensional geometry of the airway. By using computational fluid dynamics (CFD) modeling to capture airway flow patterns during inspiration and expiration, we obtained information pertaining to flow velocity, static airway wall pressure, pressure drop across the stenosis, and wall shear stress. These simulations were performed with both air and heliox. Results: Unlike air, heliox maintained laminar flow through the stenosis. The calculated pressure drop over stenosis was lower for the heliox flow, in contrast to the airflow case. This lead to an approximately 40% decrease in airway resistance when using heliox, and presumably causes a decrease in the level of effort required for breathing. Conclusions: CFD simulations offer a quantitative method of evaluating airway flow dynamics in patients with airway abnormalities. CFD modeling illustrated the flow features and quantified flow parameters within a pediatric airway with subglottic stenosis. Simulations with air and heliox conditions mirrored the known clinical benefits of heliox as compared with air. We anticipate that computer simulation models will ultimately allow a better understanding of changes in flow caused by specific medical and surgical interventions in patients with conditions associated with dyspnea. [source]

Can mesoscale models reproduce meandering motions?

Danijel Belu
Abstract The influence of meandering flow on dispersion of pollutants is frequently under-represented in dispersion models. In terms of measurements, meandering is primarily associated with time-scales between the turbulence and the applied averaging time, which is usually 1 h. The related spatial scales thus range roughly from 102 to 104 m (referred to here as submesoscales). As the state-of-the-art mesoscale models should be capable of reproducing flow features on scales larger than the turbulence, and as the meandering-generating mechanisms are not fully understood yet, it is useful to examine if the mesoscale models can reproduce meandering. For that purpose, the WRF/Chem model at 1/3 km horizontal resolution is used to simulate a weak-wind night during the CASES99 experiment. The measurements are used for detailed model verification. The model with its typical set-up fails to reproduce the variability at submesoscales and the locus of the under-representation is traced to too-strong horizontal diffusion. Reducing or removing the model diffusion allows the appearance of the submeso variability, whose spectral properties and the resulting plume behaviour agree well with the measurements. The linear correlation between the simulations with reproduced variability and the measurements is low, as is the case between two simulations with only slightly different set-up. The conclusion is that mesoscale models are able to reproduce the strength of variability and the effects of meandering, but only with reduced or removed horizontal diffusion. The question arises whether it is possible to obtain a linear correlation, i.e. to correctly reproduce individual modes at these scales at all. Copyright © 2010 Royal Meteorological Society [source]

On the onset of bora and the formation of rotors and jumps near a mountain gap

Alexander Gohm
Abstract This study investigates the onset phase of a strong Adriatic bora windstorm that occurred on 4 April 2002. The target area is a gap about 20 km wide embedded in the coastal mountain barrier of the Dinaric Alps that favours strong jet-like winds. Airborne-aerosol back-scatter lidar measurements on board the DLR Falcon research aircraft, together with surface and upper-air observations, are used to verify high-resolution numerical experiments conducted with the mesoscale atmospheric model RAMS and a single-layer shallow-water model (SWM). Especially during the breakthrough phase of the bora, the flow at the gap exit exhibits a complex spatial structure and temporal evolution. On a transect through the centre of the gap, a hydraulic jump forms; this is located close to the coast throughout the night, and starts to propagate downstream in the early morning. On a transect through the edge of the gap, a lee-wave-induced rotor becomes established, due to boundary-layer separation. It starts to propagate downstream about two hours after the jump. This flow evolution implies that the onset of strong winds at the coast occurs several hours earlier downstream of the centre of the gap than downwind of the edge of the gap. Consequently, the wind field in the vicinity of Rijeka airport, located downwind of the gap, is strongly inhomogeneous and transient, and represents a potential hazard to aviation. Measured bora winds at the surface exceed 20 ms,1, and the simulated wind speed in the gap wind layer exceeds 30 ms,1. The simulated turbulent kinetic energy exceeds 10 m2 s,2. RAMS indicates that wave-breaking near a critical level is the dominant mechanism for the generation of the windstorm. Gap jets can be identified downstream of several mountain passes. The simulated wave pattern above the Dinaric Alps, the wave decay with height due to directional wind shear and the strong flow descent on the leeward side of the barrier are supported by measured back-scatter intensities. Basic bora flow features, including gap jets and jumps, are remarkably well reproduced by SWM simulations. The RAMS reference run captures observed flow phenomena and the temporal flow evolution qualitatively well. A cold low-level bias, an overestimated bora inversion strength, and a slightly too-early bora onset are probably related to insufficient turbulent mixing in the boundary layer. The amplitude of trapped gravity waves, the time of the bora breakthrough and the inversion strength are found to be quite sensitive to the turbulence parametrization. Copyright © 2008 Royal Meteorological Society [source]