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Vorticity
Kinds of Vorticity Terms modified by Vorticity Selected AbstractsVorticity and the Nernst effect in cuprate superconductorsANNALEN DER PHYSIK, Issue 1-2 2004N.P. Ong Abstract We present a review of the vortex-Nernst effect in the 3 cuprate families La2-xSrxCuO4, Bi2Sr2CaCu2O8, and YBa2Cu3Oy, and discuss the scenario that the superconducting transition in the hole-doped cuprates corresponds to the destruction of long-range phase coherence rather than the vanishing of the order-parameter amplitude. [source] Dissipation of vortices in CFD-simulationsPROCEEDINGS IN APPLIED MATHEMATICS & MECHANICS, Issue 1 2003Michael Krieger Dipl.-Ing. The aim of this investigation is to find out the crucial factors for the meshing of a wing and especially the area behind the wingtip with the intention to keep numerical vortex-dissipation low to be able to analyse the trailing edge vortices of the wing in inviscid CFD-simulations. In a series of basic test-simulations spatial vortex propagation is observed for different cell types, mesh sizes and vorticities. A numerical viscosity is determined by comparing numerical results with an analytical solution for vortex-decay. The information gained is applied to a numerical analysis of a trailing edge vortex of a rectangular wing. The results of the numerical calculations are compared to PIV-measurements of trailing edge vortices and analytic results of linear wing theory. [source] Three-dimensional study of the pressure field and advantages of hemispherical crucible in silicon Czochralski crystal growthCRYSTAL RESEARCH AND TECHNOLOGY, Issue 6 2010F. Mokhtari Abstract The effects of several growth parameters in cylindrical and spherical Czochralski crystal process are studied numerically and particularly, we focus on the influence of the pressure field. We present a set of three-dimensional computational simulations using the finite volume package Fluent in two different geometries, a new geometry as cylindro-spherical and the traditional configuration as cylindro-cylindrical. We found that the evolution of pressure which is has not been studied before; this important function is strongly related to the vorticity in the bulk flow, the free surface and the growth interface. It seems that the pressure is more sensitive to the breaking of symmetry than the other properties that characterize the crystal growth as temperature or velocity fields. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Models of Earth's main magnetic field incorporating flux and radial vorticity constraintsGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 1 2007A. Jackson SUMMARY We describe a new technique for implementing the constraints on magnetic fields arising from two hypotheses about the fluid core of the Earth, namely the frozen-flux hypothesis and the hypothesis that the core is in magnetostrophic force balance with negligible leakage of current into the mantle. These hypotheses lead to time-independence of the integrated flux through certain ,null-flux patches' on the core surface, and to time-independence of their radial vorticity. Although the frozen-flux hypothesis has received attention before, constraining the radial vorticity has not previously been attempted. We describe a parametrization and an algorithm for preserving topology of radial magnetic fields at the core surface while allowing morphological changes. The parametrization is a spherical triangle tesselation of the core surface. Topology with respect to a reference model (based on data from the Oersted satellite) is preserved as models at different epochs are perturbed to optimize the fit to the data; the topology preservation is achieved by the imposition of inequality constraints on the model, and the optimization at each iteration is cast as a bounded value least-squares problem. For epochs 2000, 1980, 1945, 1915 and 1882 we are able to produce models of the core field which are consistent with flux and radial vorticity conservation, thus providing no observational evidence for the failure of the underlying assumptions. These models are a step towards the production of models which are optimal for the retrieval of frozen-flux velocity fields at the core surface. [source] Experimental study on flow characteristics of a sleeved jet into a main crossflowHEAT TRANSFER - ASIAN RESEARCH (FORMERLY HEAT TRANSFER-JAPANESE RESEARCH), Issue 1 2004H.L. Wu Abstract Experiments were carried out on the hydraulic mechanism of the thermal shock caused by cold jet injection at a T-junction with thermal sleeve in the reactor cooling system using digital particle imaging velocimetry (DPIV) technique to measure the flow in the main duct and in the annular space of the sleeve tube. The flow and vorticity characteristics were investigated at jet-to-crossflow velocity ratios of 0.5 to 4. There was a stream of discharge from the annular space at the rear part of the sleeve near the jet exit, which resulted in decreasing the influence of the jet on the downstream wall. The intensive vorticity in the near wake mainly originated from the shear layer vorticity of the jet and the annular discharge stream. The intensive vorticity soon broke down and dissipated, and further developed into the counterrotating vortex pair in the far wake. The flow in the annulus was closely dependent on R, and thermal protection of the sleeve would become evident at higher R. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(1): 24,31, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.10131 [source] Numerical stability of unsteady stream-function vorticity calculationsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN BIOMEDICAL ENGINEERING, Issue 6 2003E. Sousa Abstract The stability of a numerical solution of the Navier,Stokes equations is usually approached by con- sidering the numerical stability of a discretized advection,diffusion equation for either a velocity component, or in the case of two-dimensional flow, the vorticity. Stability restrictions for discretized advection,diffusion equations are a very serious constraint, particularly when a mesh is refined in an explicit scheme, so an accurate understanding of the numerical stability of a discretization procedure is often of equal or greater practical importance than concerns with accuracy. The stream-function vorticity formulation provides two equations, one an advection,diffusion equation for vorticity and the other a Poisson equation between the vorticity and the stream-function. These two equations are usually not coupled when considering numerical stability. The relation between the stream-function and the vorticity is linear and so has, in principle, an exact inverse. This allows an algebraic method to link the interior and the boundary vorticity into a single iteration scheme. In this work, we derive a global time-iteration matrix for the combined system. When applied to a model problem, this matrix formulation shows differences between the numerical stability of the full system equations and that of the discretized advection,diffusion equation alone. It also gives an indication of how the wall vorticity discretization affects stability. Despite the added algebraic complexity, it is straightforward to use MATLAB to carry out all the matrix operations. Copyright © 2003 John Wiley & Sons, Ltd. [source] Nonreflecting boundary conditions based on nonlinear multidimensional characteristicsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 1 2010Qianlong Liu Abstract Nonlinear characteristic boundary conditions based on nonlinear multidimensional characteristics are proposed for 2- and 3-D compressible Navier,Stokes equations with/without scalar transport equations. This approach is consistent with the flow physics and transport properties. Based on the theory of characteristics, which is a rigorous mathematical technique, multidimensional flows can be decomposed into acoustic, entropy, and vorticity waves. Nonreflecting boundary conditions are derived by setting corresponding characteristic variables of incoming waves to zero and by partially damping the source terms of the incoming acoustic waves. In order to obtain the resulting optimal damping coefficient, analysis is performed for problems of pure acoustic plane wave propagation and arbitrary flows. The proposed boundary conditions are tested on two benchmark problems: cylindrical acoustic wave propagation and the wake flow behind a cylinder with strong periodic vortex convected out of the computational domain. This new approach substantially minimizes the spurious wave reflections of pressure, density, temperature, and velocity as well as vorticity from the artificial boundaries, where strong multidimensional flow effects exist. The numerical simulations yield accurate results, confirm the optimal damping coefficient obtained from analysis, and verify that the method substantially improves the 1-D characteristics-based nonreflecting boundary conditions for complex multidimensional flows. Copyright © 2009 John Wiley & Sons, Ltd. [source] Numerical study of an inviscid incompressible flow through a channel of finite lengthINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 12 2009Vasily N. Govorukhin Abstract A two-dimensional inviscid incompressible flow in a rectilinear channel of finite length is studied numerically. Both the normal velocity and the vorticity are given at the inlet, and only the normal velocity is specified at the outlet. The flow is described in terms of the stream function and vorticity. To solve the unsteady problem numerically, we propose a version of the vortex particle method. The vorticity field is approximated using its values at a set of fluid particles. A pseudo-symplectic integrator is employed to solve the system of ordinary differential equations governing the motion of fluid particles. The stream function is computed using the Galerkin method. Unsteady flows developing from an initial perturbation in the form of an elliptical patch of vorticity are calculated for various values of the volume flux of fluid through the channel. It is shown that if the flux of fluid is large, the initial vortex patch is washed out of the channel, and when the flux is reduced, the initial perturbation evolves to a steady flow with stagnation regions. Copyright © 2008 John Wiley & Sons, Ltd. [source] A viscous vortex particle method for deforming bodies with application to biolocomotionINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 12 2009Li Jeany Zhang Abstract Bio-inspired mechanics of locomotion generally consist of the interaction of flexible structures with the surrounding fluid to generate propulsive forces. In this work, we extend, for the first time, the viscous vortex particle method (VVPM) to continuously deforming two-dimensional bodies. The VVPM is a high-fidelity Navier,Stokes computational method that captures the fluid motion through evolution of vorticity-bearing computational particles. The kinematics of the deforming body surface are accounted for via a surface integral in the Biot,Savart velocity. The spurious slip velocity in each time step is removed by computing an equivalent vortex sheet and allowing it to flux to adjacent particles; hence, no-slip boundary conditions are enforced. Particles of both uniform and variable size are utilized, and their relative merits are considered. The placement of this method in the larger class of immersed boundary methods is explored. Validation of the method is carried out on the problem of a periodically deforming circular cylinder immersed in a stagnant fluid, for which an analytical solution exists when the deformations are small. We show that the computed vorticity and velocity of this motion are both in excellent agreement with the analytical solution. Finally, we explore the fluid dynamics of a simple fish-like shape undergoing undulatory motion when immersed in a uniform free stream, to demonstrate the application of the method to investigations of biomorphic locomotion. Copyright © 2008 John Wiley & Sons, Ltd. [source] A spectral projection method for the analysis of autocorrelation functions and projection errors in discrete particle simulationINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 7 2008André Kaufmann Abstract Discrete particle simulation is a well-established tool for the simulation of particles and droplets suspended in turbulent flows of academic and industrial applications. The study of some properties such as the preferential concentration of inertial particles in regions of high shear and low vorticity requires the computation of autocorrelation functions. This can be a tedious task as the discrete point particles need to be projected in some manner to obtain the continuous autocorrelation functions. Projection of particle properties on to a computational grid, for instance, the grid of the carrier phase, is furthermore an issue when quantities such as particle concentrations are to be computed or source terms between the carrier phase and the particles are exchanged. The errors committed by commonly used projection methods are often unknown and are difficult to analyse. Grid and sampling size limit the possibilities in terms of precision per computational cost. Here, we present a spectral projection method that is not affected by sampling issues and addresses all of the above issues. The technique is only limited by computational resources and is easy to parallelize. The only visible drawback is the limitation to simple geometries and therefore limited to academic applications. The spectral projection method consists of a discrete Fourier-transform of the particle locations. The Fourier-transformed particle number density and momentum fields can then be used to compute the autocorrelation functions and the continuous physical space fields for the evaluation of the projection methods error. The number of Fourier components used to discretize the projector kernel can be chosen such that the corresponding characteristic length scale is as small as needed. This allows to study the phenomena of particle motion, for example, in a region of preferential concentration that may be smaller than the cell size of the carrier phase grid. The precision of the spectral projection method depends, therefore, only on the number of Fourier modes considered. Copyright © 2008 John Wiley & Sons, Ltd. [source] Using vorticity to define conditions at multiple open boundaries for simulating flow in a simplified vortex settling basinINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 1 2007A. N. Ziaei Abstract In this paper a method is developed to define multiple open boundary (OB) conditions in a simplified vortex settling basin (VSB). In this method, the normal component of the momentum equation is solved at the OBs, and tangential components of vorticity are calculated by solving vorticity transport equations only at the OBs. Then the tangential vorticity components are used to construct Neumann boundary conditions for tangential velocity components. Pressure is set to its ambient value, and the divergence-free condition is satisfied at these boundaries by employing the divergence as the Neumann condition for the normal-direction momentum equation. The 3-D incompressible Navier,Stokes equations in a primitive-variable form are solved using the SIMPLE algorithm. Grid-function convergence tests are utilized to verify the numerical results. The complicated laminar flow structure in the VSB is investigated, and preliminary assessment of two popular turbulence models, k,, and k,,, is conducted. Copyright © 2006 John Wiley & Sons, Ltd. [source] A combined vortex and panel method for numerical simulations of viscous flows: a comparative study of a vortex particle method and a finite volume methodINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 10 2005Kwang-Soo Kim Abstract This paper describes and compares two vorticity-based integral approaches for the solution of the incompressible Navier,Stokes equations. Either a Lagrangian vortex particle method or an Eulerian finite volume scheme is implemented to solve the vorticity transport equation with a vorticity boundary condition. The Biot,Savart integral is used to compute the velocity field from a vorticity distribution over a fluid domain. The vorticity boundary condition is improved by the use of an iteration scheme connected with the well-established panel method. In the early stages of development of flows around an impulsively started circular cylinder, and past an impulsively started foil with varying angles of attack, the computational results obtained by the Lagrangian vortex method are compared with those obtained by the Eulerian finite volume method. The comparison is performed separately for the pressure fields as well. The results obtained by the two methods are in good agreement, and give a better understanding of the vorticity-based methods. Copyright © 2005 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] Liquid vorticity computation in non-spherical bubble dynamicsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 2 2005A. A. Aganin Abstract The purpose of this work is to compare efficiency of a number of numerical techniques of computation of liquid vorticity from non-spherical bubble oscillations. The techniques based on the finite-difference method (FDM), the collocation method (one with differentiating (CMd) the integral boundary condition and another without it (CM)) and the Galerkin method (GM) have been considered. The central-difference approximations are used in FDM. Sinus functions are chosen as the basis in GM. Problems of decaying a small distortion of the spherical shape of a bubble and dynamics of a bubble under harmonic liquid pressure variation with various parameters are used for comparison. The FDM technique has been found to be most efficient in all the cases. Copyright © 2004 John Wiley & Sons, Ltd. [source] Anisotropic, isothermal, turbulent swirling flow in a complex combustor geometryINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 10-11 2005L. N. Jones Abstract The performance of popular second moment closure (LRR, SSG) turbulence models is assessed and compared against experimental data for anisotropic swirling flow in a cylindrical combustion chamber. In contrast to previous studies, where the dissipation anisotropy is correlated with the stress anisotropy, the benefit of approximating the former for swirling flows in terms of the mean strain and vorticity is investigated. Second moment closure models are found to predict mean and turbulent flow quantities reasonably well everywhere except near the wall. The anisotropic dissipation model is found to improve prediction of mean flow quantities near the chamber axis and acts to preserve turbulence further downstream. Copyright © 2005 John Wiley & Sons, Ltd. [source] A semi-implicit method conserving mass and potential vorticity for the shallow water equations on the sphereINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 8-9 2005Luca Bonaventura Abstract A semi-implicit discretization for the shallow water equations is discussed, which uses triangular Delaunay cells on the sphere as control volumes and conserves mass and potential vorticity. The geopotential gradient, the Coriolis force terms and the divergence of the velocity field are discretized implicitly, while an explicit time discretization is used for the non-linear advection terms. The results obtained with a preliminary implementation on some idealized test cases are presented, showing that the main features of large scale atmospheric flows are well represented by the proposed method. Copyright © 2004 John Wiley & Sons, Ltd. [source] Godunov-type adaptive grid model of wave,current interaction at cuspate beachesINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 6 2004Benedict D. Rogers Abstract This paper presents a second-order accurate Godunov-type numerical scheme for depth- and period-averaged wave,current interaction. A flux Jacobian is derived for the wave conservation equations and its eigensystem determined, enabling Roe's approximate Riemann solver to be used to evaluate convective fluxes. Dynamically adaptive quadtree grids are used to focus on local hydrodynamic features, where sharp gradients occur in the flow variables. Adaptation criteria based on depth-averaged vorticity, wave-height gradient, wave steepness and the magnitude of velocity gradients are found to produce accurate solutions for nearshore circulation at a half-sinusoidal beach. However, the simultaneous combination of two or more separate criteria produces numerical instability and interference unless all criteria are satisfied for mesh depletion. Simulations of wave,current interaction at a multi-cusped beach match laboratory data from the United Kingdom Coastal Research Facility (UKCRF). A parameter study demonstrates the sensitivity of nearshore flow patterns to changes in relative cusp height, angle of wave incidence, bed roughness, offshore wave height and assumed turbulent eddy viscosity. Only a small deviation from normal wave incidence is required to initiate a meandering longshore current. Nearshore circulation patterns are highly dependent on the offshore wave height. Reduction of the assumed eddy viscosity parameter causes the primary circulation cells for normally incident waves to increase in strength whilst producing rip-like currents cutting diagonally across the surf zone. Copyright © 2004 John Wiley & Sons, Ltd. [source] A numerical-variational procedure for laminar flow in curved square ductsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 12 2004P. M. Hatzikonstantinou Abstract A new numerical method is presented for the solution of the Navier,Stokes and continuity equations governing the internal incompressible flows. The method denoted as the CVP method consists in the numerical solution of these equations in conjunction with three additional variational equations for the continuity, the vorticity and the pressure field, using a non-staggered grid. The method is used for the study of the characteristics of the laminar fully developed flows in curved square ducts. Numerical results are presented for the effects of the flow parameters like the curvature, the Dean number and the stream pressure gradient on the velocity distributions, the friction factor and the appearance of a pair of vortices in addition to those of the familiar secondary flow. The accuracy of the method is discussed and the results are compared with those obtained by us, using a variation of the velocity,pressure linked equation methods denoted as the PLEM method and the results obtained by other methods. Copyright © 2004 John Wiley & Sons, Ltd. [source] Numerical simulation of turbulent flow through series stenosesINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 7 2003T. S. Lee Abstract The flow fields in the neighbourhoods of series vascular stenoses are studied numerically for the Reynolds numbers from 100 to 4000, diameter constriction ratios of 0.2,0.6 and spacing ratios of 1, 2, 3, 4 and ,. In this study, it has been further verified that in the laminar flow region, the numerical predictions by k,, turbulence model matched those by the laminar-flow modelling very well. This suggests that the k,, turbulence model is capable of the prediction of the laminar flow as well as the prediction of the turbulent stenotic flow with good accuracy. The extent of the spreading of the recirculation region from the first stenosis and its effects on the flow field downstream of the second stenosis depend on the stenosis spacing ratio, constriction ratio and the Reynolds number. For c1 = 0.5 with c2 , c1, the peak value of wall vorticity generated by the second stenosis is always less than that generated by the first stenosis. However, the maximum centreline velocity and turbulence intensity at the second stenosis are higher than those at the first stenosis. In contrast, for c1 = 0.5 with c2 = 0.6, the maximum values at the second stenosis are much higher than those at the first stenosis whether for centreline velocity and turbulence intensity or for wall vorticity. The peak values of the wall vorticity and the centreline disturbance intensity both grow up with the Reynolds number increasing. The present study shows that the more stenoses can result in a lower critical Reynolds number that means an earlier occurrence of turbulence for the stenotic flows. Copyright © 2003 John Wiley & Sons, Ltd. [source] Numerical simulation of vortical ideal fluid flow through curved channelINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 11 2003N. P. Moshkin Abstract A numerical algorithm to study the boundary-value problem in which the governing equations are the steady Euler equations and the vorticity is given on the inflow parts of the domain boundary is developed. The Euler equations are implemented in terms of the stream function and vorticity. An irregular physical domain is transformed into a rectangle in the computational domain and the Euler equations are rewritten with respect to a curvilinear co-ordinate system. The convergence of the finite-difference equations to the exact solution is shown experimentally for the test problems by comparing the computational results with the exact solutions on the sequence of grids. To find the pressure from the known vorticity and stream function, the Euler equations are utilized in the Gromeka,Lamb form. The numerical algorithm is illustrated with several examples of steady flow through a two-dimensional channel with curved walls. The analysis of calculations shows strong dependence of the pressure field on the vorticity given at the inflow parts of the boundary. Plots of the flow structure and isobars, for different geometries of channel and for different values of vorticity on entrance, are also presented. Copyright © 2003 John Wiley & Sons, Ltd. [source] Highly accurate solutions of the bifurcation structure of mixed-convection heat transfer using spectral methodINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 5 2002M. Selmi Abstract This paper is concerned with producing highly accurate solution and bifurcation structure using the pseudo-spectral method for the two-dimensional pressure-driven flow through a horizontal duct of a square cross-section that is heated by a uniform flux in the axial direction with a uniform temperature on the periphery. Two approaches are presented. In one approach, the streamwise vorticity, streamwise momentum and energy equations are solved for the stream function, axial velocity, and temperature. In the second approach, the streamwise vorticity and a combination of the energy and momentum equations are solved for stream function and temperature only. While the second approach solves less number of equations than the first approach, a grid sensitivity analysis has shown no distinct advantage of one method over the other. The overall solution structure composed of two symmetric and four asymmetric branches in the range of Grashof number (Gr) of 0,2 × 106 for a Prandtl number (Pr) of 0.73 has been computed using the first approach. The computed structure is comparable to that found by Nandakumar and Weinitschke (1991) using a finite difference scheme for Grashof numbers in the range of 0,1×106. The stability properties of some solution branches; however, are different. In particular, the two-cell structure of the isolated symmetric branch that has been found to be unstable by the study of Nandakumar and Weinitschke is found to be stable by the current study. Copyright © 2002 John Wiley & Sons, Ltd. [source] The shallow flow equations solved on adaptive quadtree gridsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 6 2001A. 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] A three-dimensional vortex particle-in-cell method for vortex motions in the vicinity of a wallINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 5 2001Chung Ho Liu Abstract A new vortex particle-in-cell method for the simulation of three-dimensional unsteady incompressible viscous flow is presented. The projection of the vortex strengths onto the mesh is based on volume interpolation. The convection of vorticity is treated as a Lagrangian move operation but one where the velocity of each particle is interpolated from an Eulerian mesh solution of velocity,Poisson equations. The change in vorticity due to diffusion is also computed on the Eulerian mesh and projected back to the particles. Where diffusive fluxes cause vorticity to enter a cell not already containing any particles new particles are created. The surface vorticity and the cancellation of tangential velocity at the plate are related by the Neumann conditions. The basic framework for implementation of the procedure is also introduced where the solution update comprises a sequence of two fractional steps. The method is applied to a problem where an unsteady boundary layer develops under the impact of a vortex ring and comparison is made with the experimental and numerical literature. Copyright © 2001 John Wiley & Sons, Ltd. [source] Seasonal variability in the response of the airflow characteristics to the changes in the macro-scale westerly flow intensity over Europe, 1971,2000INTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 4 2009Michal Marosz Abstract The aim of the research was to identify the seasonal variability of correlation between the air motion characteristics and common zonal circulation index (Rossby's Index). Air flow characteristics comprised divergence and vorticity. The spatial coverage of the research was the so called Euro-Atlantic Region covering parts of North Atlantic and Europe (40W,40E, 35N,75N). Temporal extent was 1971,2000 thus matching the latest WMO normal period. The data used was the components of the wind vector (u,zonal component, v,meridional component) and geopotential heights (hgt) from NCEP/NCAR Reanalysis. The response of the vorticity field is apparent and the greatest variability is noted at 1000 hPa. The mid and upper levels in the troposphere reveal quite homogenous response of vorticity to the macro-scale forcing. The response of the divergence field is also apparent though more complex in the vertical profile of the troposphere. The analysis of correlation was followed by the description of annual as well as seasonal shape of vorticity and divergence fields in the extremes of Rossby Index. Copyright © 2008 Royal Meteorological Society [source] Long-term trends of synoptic-scale breaking Rossby waves in the Northern Hemisphere between 1958 and 2001INTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 12 2008F. Isotta Abstract Breaking synoptic-scale Rossby waves are frequent features of the upper troposphere and lower stratosphere (UTLS) which affect both global- and regional-scale dynamics. Furthermore, they directly influence ozone distribution through meridional transport of ozone-rich air towards the south and ozone-poor air towards the north. Here, trends in the frequency of these breaking waves are assessed by analysing a 44-year climatology (1958,2002) of potential vorticity (PV) streamers on isentropic surfaces from 310 to 350 K. These streamers are viewed as breaking Rossby waves, whereby stratospheric (tropospheric) streamers indicate southward (northward) breaking waves. Two complementary techniques are used to analyse the trends. First, linear trends are computed using the least-squares regression technique. Statistically significant linear trends are found to vary in location and magnitude between isentropic levels and the four seasons. In winter significant trends are detected in the eastern Pacific between 340 and 350 K. A positive trend of stratospheric streamers in the Tropics is related to an increase of total column ozone, whereas the positive trend of tropospheric streamers in the mid-latitudes is associated with a decrease of total ozone. Secondly, a nonlinear trend analysis is performed using the seasonal-trend decomposition procedure based on Loess (STL). With this technique, the low-frequency variability of the time series is analysed during the 44-year period. For instance, over the eastern Atlantic on 350 K, a phase of decreasing PV streamer frequencies in the 1950s and 1960s is followed by a positive streamer tendency after the 1970s. Additionally, trends of the zonal wind are investigated. One prominent outcome of this analysis is the observation that equatorial easterlies over the Atlantic are weakening. A dynamically meaningful link exists between the trends observed in both wind velocity and PV streamers. Copyright © 2008 Royal Meteorological Society [source] Environmental links to reduced tropical cyclogenesis over the south-east CaribbeanINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 8 2007Alexandros P. Georgiadis Abstract Tropical cyclone formation over the Caribbean is not evenly distributed across the basin. Previous work identified the south-western sector as the area that dominates the hurricane activity of the whole basin. The south-eastern sector, in contrast, exhibits a distinct suppression in cyclogenesis. This study seeks to identify the restricting factors that differentiate the cyclogenesis climatology in the south-east Caribbean. It is based on statistical analysis of eight environmental variables using principal component analysis. The first three components are examined in detail. The first component, accounting for 31.7% of the variance within the data, differentiates the Caribbean from the rest of the subtropical Atlantic, primarily in terms of an increased thermodynamic potential for cyclogenesis and enhanced atmospheric humidity in the boundary layer. The second component, accounting for 31% of the variance, marks the south-west Caribbean as the only sub-region within the area of analysis where the easterlies curve southwards and the relative vorticity is cyclonic. The third component, accounting for 20.3% of the variance, differentiates the South Caribbean from the rest of the Atlantic, indicating it to be a region of increased spatial variation in the intensity of the easterlies and in absolute vorticity. The variance of cyclogenesis within the Caribbean is greatest along the second and third components. Thus, the genesis pattern in the basin is likely associated with the low-level wind-field and absolute vorticity. The divergence of the flow, when combined with the vorticity of the wind-field over the Caribbean, renders the background conditions of the eastern sector less prone to developing disturbances than is the case in the western Caribbean. Copyright © 2006 Royal Meteorological Society [source] Atmospheric large-scale dynamics during the 2004/2005 winter drought in portugalINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 5 2007J. Santos Abstract The unusually dry conditions during the 2004/2005 winter in Portugal led to the development of an extreme/severe drought episode throughout the country with major socioeconomic impacts. In fact, at some locations, this winter was the driest in at least the last 60 years. A K-means classification of days into a set of five weather regimes (WRs), relevant for winter precipitation in Portugal, reveals a large prevalence of the two driest weather regimes during the 2004/2005 winter. These two regimes are basically linked to either anticyclonic circulation or easterly winds over Portugal and their prevalence explains the significant precipitation deficit. Winter precipitation variability in Portugal is indeed skillfully represented by linear models where the predictors are the frequencies of occurrence (FO) of these weather regimes. The dominance of the ,dry phases' of the main coupled modes between winter precipitation in Portugal and the large-scale atmospheric circulation also supports the prevalence of the dry regimes and the corresponding lack of precipitation. The predominance of the dry regimes can be explained by a remarkably strong enhancement of the climate-mean North Atlantic ridge, manifested by dynamically coherent anomalies in the geopotential heights, vorticity and temperature fields over the North Atlantic. The persistence of a warm-core asymmetrical eddy over the North Atlantic, with a nearly barotropic equivalent structure, is a manifestation of this large-scale anomaly. The blocking of the westerlies and the consequent northward shift in the axis of maximum moisture transports over the North Atlantic was one of the most striking changes in the large-scale atmospheric flow. Consequently, the main track of the developing baroclinic disturbances was sufficiently distant from Portugal to hamper the development of rain-generating conditions. As these dynamical conditions are common to other reportedly dry winters, they effectively constitute a key factor for the occurrence of a precipitation deficit in Portugal. Copyright © 2006 Royal Meteorological Society [source] Near-surface-temperature lapse rates on the Prince of Wales Icefield, Ellesmere Island, Canada: implications for regional downscaling of temperatureINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 3 2007Shawn J. Marshall Abstract Screen temperatures were monitored from May 2001 to April 2003 in an array of 25 sites on the Prince of Wales Icefield, Ellesmere Island, Canada. The observational network covered an area of ca 15 650 km2 and spanned an altitude ranging from 130 to 2010 m above sea level. The spatial array provides a record of near-surface-temperature lapse rates and mesoscale temperature variability on the icefield. The mean daily lapse rate in the 2-year record is , 4.1° C km,1, with an average summer lapse rate of , 4.3° C km,1. Surface-temperature lapse rates in the region are therefore systematically less than the free-air lapse rates that are typically adopted for extrapolations of sea-level temperature to higher altitudes. Steep lapse rates, resembling moist adiabatic rates in the free air (,6 to , 7° C km,1), are more common in summer at our site and are associated with enhanced cyclonic activity (low-pressure and high relative vorticity) and southerly flow aloft. In contrast, northerly, anticyclonic flow prevails when summer lapse rates are weak (above , 2° C km,1). The low surface-temperature lapse rates and their systematic synoptic variability have important implications for applications that require downscaling or extrapolation of surface- or boundary-layer temperatures, such as modelling of glacier mass balance. We illustrate this in an analysis of observed versus modelled snowmelt on the icefield. Copyright © 2006 Royal Meteorological Society. [source] Episodes of alpine heavy precipitation with an overlying elongated stratospheric intrusion: a climatologyINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 9 2006Olivia Martius Abstract This study discusses the role of stratospheric intrusions (determined as potential vorticity (PV) streamers) as upper-level instigators of heavy precipitation along the Swiss Alpine south side (AS) on a climatological timescale. A climatology of streamers is used compiled on the basis of the ECMWF 40-year re-analysis data set (ERA-40). Days of extreme and heavy precipitation along the Swiss AS are determined from an existing observational Alpine precipitation climatology. For these days, the presence of streamers over western Europe as well as their location and orientation is recorded. On 73% of the extreme precipitation days, a streamer is situated over western Europe. The mean spatial frequency distribution of the streamers on the extreme precipitation days exhibits a structure that resembles in its form and location the ,archetypal heavy precipitation streamer' known from case studies. The frequency maximum is situated over southern England and the west coast of France. The same analysis is applied to three sub-domains (Valais, Grisons, Ticino) along the Swiss AS. Significant differences in the location and the orientation of the streamers for the sub-domains are found. The majority of streamers associated with heavy rain in the western-most sub-domain (Valais) are oriented in a cyclonically-sheared fashion, while for the Ticino the streamers are more anti-cyclonically orientated. Differences for events of increasing severity are analysed by comparing the form, location, amplitude (PV), and persistence of the streamers on extreme and heavy precipitation days. The precipitation distribution is shifted to higher intensities for more persistent streamers. There is no detectable difference found in the form parameters, but the southerly moisture flux into the domain is significantly larger during extreme precipitation days than during heavy precipitation days. Likewise, the seasonal variation in the percentage of streamer-related heavy precipitation, which is highest in autumn (85%), can be related to the seasonal variation of southerly moisture fluxes. Copyright © 2006 Royal Meteorological Society. [source] Laser diagnostic investigation of the bubble eruption patterns in the freeboard of fluidized beds: Simultaneous acetone PLIF and stereoscopic PIV measurementsAICHE JOURNAL, Issue 6 2009C. R. Müller Abstract For the first time PIV has been applied simultaneously with acetone-PLIF in the freeboard of a fluidized bed. Here, the eruption profile of single bubbles and a continuous stream of bubbles were studied. As stereoscopic PIV was applied the out-of-plane component of the velocity was also measured. The out-of-plane component is not negligible. The observed bubble eruption patterns were in general agreement with the bubble model of Levy and Lockwood,24 Yorquez-Ramirez and Duursma5 and Solimene et al.1 No qualitative difference between the eruption of a single bubble and a stream of bubbles was observed. Based on the calculated vorticity of the gas in the freeboard, it was found that the bubble induced turbulence decays fairly rapidly. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source] | ||||||||||||||||||||||