Showing papers on "Vortex published in 1993"

The structure of intense vorticity in isotropic turbulence

Abstract: The structure of the intense-vorticity regions is studied in numerically simulated homogeneous, isotropic, equilibrium turbulent flow fields at four different Reynolds numbers, in the range Re, = 35-170. In accordance with previous investigators this vorticity is found to be organized in coherent, cylindrical or ribbon-like, vortices (‘worms’). A statistical study suggests that they are simply especially intense features of the background, O(o’), vorticity. Their radii scale with the Kolmogorov microscale and their lengths with the integral scale of the flow. An interesting observation is that the Reynolds number y/v, based on the circulation of the intense vortices, increases monotonically with ReA, raising the question of the stability of the structures in the limit of Re, --z co. Conversely, the average rate of stretching of these vortices increases only slowly with their peak vorticity, suggesting that self-stretching is not important in their evolution. One- and two-dimensional statistics of vorticity and strain are presented; they are non-Gaussian and the behaviour of their tails depends strongly on the Reynolds number. There is no evidence of convergence to a limiting distribution in this range of Re,, even though the energy spectra and the energy dissipation rate show good asymptotic properties in the higher-Reynolds-number cases. Evidence is presented to show that worms are natural features of the flow and that they do not depend on the particular forcing scheme.

Boson localization and correlated pinning of superconducting vortex arrays.

Abstract: A theory of vortex pinning in high-temperature superconductors by correlated disorder in the form of twin boundaries, grain boundaries, and columnar defects is described. Mapping vortex trajectories onto boson world lines leads to a ``superfluid'' flux liquid at high temperatures, as well as low-temperature ``Bose-glass'' and ``Mott-insulator'' phases, in which the flux lines are localized. Currents perpendicular to the average vortex direction act like an electric field applied to charged bosons, while currents parallel to the field act like an imaginary magnetic field in this approach. We discuss the equilibrium and dynamic properties of these phases, and propose a scaling theory for the flux-liquid to Bose-glass transition, at which the linear resistivity vanishes. Although the Bose-glass predictions share some features with vortex-glass behavior predicted for point disorder, the response to tilting the magnetic field in the two cases differs dramatically, thus allowing the two theories to be distinguished experimentally.

Bénard cells and Taylor vortices

Abstract: Part I. Benard Convection and Rayleigh-Benard Convection: 1. Benard's experiments 2. Linear theory of Rayleigh-Benard convection 3. Theory of surface tension driven Benard convection 4. Surface tension driven Benard convection experiments 5. Linear Rayleigh-Benard convection experiments 6. Supercritical Rayleigh-Benard convection experiments 7. Nonlinear theory of Rayleigh-Benard convection 8. Miscellaneous topics Part II. Taylor Vortex Flow: 9. Circular Couette flow 10. Rayleigh's stability criterion 11. G. I. Taylor's work 12. Other early experiments 13. Supercritical Taylor vortex experiments 14. Experiments with two independently rotating cylinders 15. Nonlinear theory of Taylor vortices 16. Miscellaneous topics.

Optical Vortices and Their Propagation

Abstract: Free space propagation of an array of optical vortices nested in a smooth (Gaussian) beam is studied in the paraxial regime. It is found that when the vortices have all the same charge, their relative positions, as well as their positions within the host beam are invariant upon propagation. The array simply expands or contracts with the host beam and rotates rigidly. Vortices of opposite charges, in contrast, attract each other. Pairs can collide and annihilate. As an illustration, numerical simulation is used to compare the propagation of a pair of vortices of equal charges with that of a pair of opposite charges.

An Initialization Scheme of Hurricane Models by Vortex Specification

Abstract: A scheme is presented to improve the representation of a tropical cyclone in the initial condition of a high-resolution hurricane model. In the proposed method, a crudely resolved tropical cyclone in the large-scale analysis is replaced by a vortex that is properly specified for use in the prediction model. Appropriate filters are used to remove the vortex from the large-scale analysis so that a smooth environmental field remains. The new specified bogus vortex takes the form of a deviation from this environmental held so that it can be easily merged with the latter field at the correct position. The specified vortex consists of both axisymmetric and asymmetric components. The symmetric component is generated by the time integration of an axisymmetric version of the hurricane prediction model. This ensures dynamical and thermodynamical consistency in the vortex structure, including the moisture field, and also compatibility of the vortex with the resolution and physics of the hurricane model. In ...

Direct simulations of low-Reynolds-number turbulent flow in a rotating channel

Abstract: Direct numerical simulations of fully developed pressure-driven turbulent flow in a rotating channel have been performed The unsteady Navier–Stokes equations were written for flow in a constantly rotating frame of reference and solved numerically by means of a finite-difference technique on a 128 × 128 × 128 computational mesh The Reynolds number, based on the bulk mean velocity Um and the channel half-width h, was about 2900, while the rotation number Ro = 2|Ω|h/Um varied from 0 to 05 Without system rotation, results of the simulation were in good agreement with the accurate reference simulation of Kim, Moin & Moser (1987) and available experimental data The simulated flow fields subject to rotation revealed fascinating effects exerted by the Coriolis force on channel flow turbulence With weak rotation (Ro = 001) the turbulence statistics across the channel varied only slightly compared with the nonrotating case, and opposite effects were observed near the pressure and suction sides of the channel With increasing rotation the augmentation and damping of the turbulence along the pressure and suction sides, respectively, became more significant, resulting in highly asymmetric profiles of mean velocity and turbulent Reynolds stresses In accordance with the experimental observations of Johnston, Halleen & Lezius (1972), the mean velocity profile exhibited an appreciable region with slope 2Ω At Ro = 050 the Reynolds stresses vanished in the vicinity of the stabilized side, and the nearly complete suppression of the turbulent agitation was confirmed by marker particle trackings and two-point velocity correlations Rotational-induced Taylor-Gortler-like counter-rotating streamwise vortices have been identified, and the simulations suggest that the vortices are shifted slightly towards the pressure side with increasing rotation rates, and the number of vortex pairs therefore tend to increase with Ro

Vortex-induced vibrations of a long flexible circular cylinder

Abstract: In an experimental study of the vortex-induced oscillations of a long flexible circular cylinder, the observed stationary amplitudes describe an hysteresis loop partially different from earlier studies. Each branch of the loop is associated with a vortex shedding mode and, as a jump from one branch to the other occurs, the phase difference between the cylinder displacement and the vortex shedding undergoes an abrupt change. The critical flow velocities at which the jump occurs concur with the flow visualization observations of Williamson & Roshko (1988) on the vortex shedding modes near the fundamental synchronization region. Impulsive regimes, obtained at a given flow velocity with the cylinder initially at rest or pre-excited, and progressive regimes resulting from a variation of the flow velocity, are examined. The occurrence of bifurcations is detected for a flow velocity range in the case of the impulsive regimes. The coordinates of the bifurcations define a boundary between two vortex shedding modes, a boundary that verifies the critical curve obtained by Williamson & Roshko (1988). The experimental set-up of this study simulates half the wavelength of a vibrating cable, eliminates the end effects present in oscillating rigid cylinder set-up and has one of the lowest damping ratios reported for the study of this phenomenon.

Effect of tabs on the flow and noise field of an axisymmetric jet

Abstract: The effect of vortex generators, in the form of small tabs projecting normally into the flow at the nozzle exit, on the characteristics of an axisymmetric jet is investigated experimentally over the jet Mach number range of 0.3-1.81. The tabs eliminate screech noise from supersonic jets and alter the shock structure drastically. They distort the jet cross section and increase the jet spread rate significantly. The distortion produced is essentially the same at subsonic and underexpanded supersonic conditions. Thus, the underlying mechanism must be independent of compressibility effects. A tab with a height as small as 2 percent of the jet diameter, but larger than the efflux boundary-layer thickness, is found to produce a significant effect. Flow visualization reveals that each tab introduces an 'indentation' into the high speed side of the shear layer via the action of streamwise vortices. These vortices are inferred to be of the 'trailing vortex' type rather than of the 'necklace vortex' type. It is apparent that a substantial pressure differential must exist between the upstream and the downstream sides of the tab to effectively produce these trailing vortices. This explains why the tabs are ineffective in the overexpanded flow, as in that case an adverse pressure gradient exists near the nozzle exit which reduces the pressure differential produced by the tab.

Evidence for a Singularity of the Three Dimensional, Incompressible Euler Equations

Abstract: Three-dimensional, incompressible Euler calculations of the interaction of perturbed anti-parallel vortex tubes using a variety of smooth initial profiles in bounded domains with bounded initial vorticity is discussed. It will be shown that trends towards either exponential, non-singular growth of the peak vorticity or power law, singular behavior can be strongly dependent on details of the initial conditions. A numerical method that uses symmetries and additional resolution in the direction and location of maximum coin-pression is used to simulate periodic boundary conditions in all directions. For the initial condition that yields singular type behavior the growth of the peak vorticity roughly obeys (t c − t)−1 and the growth of the strain along the vorticity at this point obeys (t c − t)−γwhere γ ≈ 1.

Vortex-Induced Forces on Oscillating Bluff Cylinders

Abstract: : Vortex-induced forces and consequent vibration of long cylindrical structures are important for a large number of engineering applications, while the complexity of the underlying physical mechanisms is such that this is one of the canonical problems of fluid mechanics Vortex shedding force varies in frequency and magnitude along the length of the structure, causing the response at any point to be amplitude-modulated in space and time The focus is on the measurement, via forced-oscillation experiments, of the vortex-induced lift and drag forces acting on circular cylinders undergoing sinusoidal and amplitude- modulated oscillations Basic concepts on vortex formation and vortex-induced vibrations, a review of the existing literature, and details of the experimental apparatus and data processing methods are presented Stationary and sinusoidal oscillation tests are presented Several novel properties are described, among them the role of the lift force phase angle in causing the amplitude-limited nature of VIV, and use of the lift force excitation region in contrast with the often-quoted but quite different lift force lock-in region Next, a comprehensive data error analysis, and a simple VIV prediction scheme are described New data on amplitude-modulated oscillations are presented The concept of control of the mean wake velocity profile via the control of the major vortical feature is explored, with possible applications being the reduction of the in-line wake velocity and alteration of the wake signature

Global regularity for vortex patches

Abstract: We present a proof of Chemin's [4] result which states that the boundary of a vortex patch remains smooth for all time if it is initially smooth.

Applications of Shock-Induced Mixing to Supersonic Combustion

Abstract: Families of two-dimensional, unsteady shock-induced vortical flows are simulated numerically The flows consist of one or more regions of light gas, surrounded by heavy gas, being overtaken by a normal shock wave The interaction of the density gradient at each light/heavy interface with the pressure gradient from the shock wave generates vorticity This causes the light gas regions to roll up into one or more counter-rotating vortex pairs, which stir and mix the light and heavy gases The mixing is characterized by an asymptotic stretching rate The effects of shock strength, light/heavy gas density ratio, and geometry on the mixing are investigated These two-dimensional, unsteady flows are analogous to three-dimensional, steady flows that may be used in SCRAMJET combustors demanding rapid and efficient mixing of fuel and oxidizer For such applications, 1) the fuel injectors should be elongated in the direction of the shock; 2) multiple smaller injectors are preferable to a single larger injector; 3) injectors should be arranged in groups of closely spaced pairs, rather than uniformly; and 4) multiple shock waves should be utilized, if possible

Vortex reconnection in superfluid helium.

Abstract: A useful physical model for superfluid turbulence considers the flow to consist of a dense tangle of vortex lines which evolve and interact. It has been suggested that these vortex lines can dynamically reconnect upon close approach. Here, we consider the nonlinear Schr\"odinger equation model of superfluid quantum mechanics, and use numerical simulation to study this topology changing core-scale process. Our results support the idea that vortex reconnection will occur whenever filaments come within a few core lengths of one another.

Vortices in Rotating Fluids

Abstract: The emergence of coherent vortex structures is a characteristic feature of quasi-geostrophic or two-dimensional turbulence and because of their relevance to large-scale geophysical flows, the dynamics of these structures has been studied increasingly over the past decade. In the oceans and in the atmospheres vortices or eddies are abundant. For this reason, the study of isolated vortices in rotating fluids-including their dynamics, instability properties, mutual interaction behavior, and effects due to bottom topog­ raphy-is of fundamental interest for refined models of geostrophic turbulence and of the general oceanic and atmospheric circulations. Pro­ cesses of heat transfer, dispersion of biochemical components, and the transport of other physical properties are closely connected with coherent structures. The dynamics of two-dimensional vortices in homogeneous fluids is also a key problem in free shear flows. With or without background rotation, the main question concerns the stability of these vortices to three­ dimensional disturbances. In addition, two-dimensional vortices also play an important role in tokamak-confined plasmas as well as in astrophysical situations such as accretion discs of neutron stars. During the past decade, considerable insight has been gained in the

Three‐dimensional fluid simulations of the nonlinear drift‐resistive ballooning modes in tokamak edge plasmas

Abstract: A three‐dimensional study of the turbulence and sheared flow generated by the drift‐resistive ballooning modes in tokamak edge plasmas has been completed. The fluid simulations show that 10%–15% percent density fluctuations can develop in the nonlinear state when the self‐consistently generated shear flow is suppressed. These modes are also found to give rise to poloidally asymmetric particle transport. Characteristic scale lengths of these fluctuations are isotropic in the plane transverse to B and smaller than the connection length along the field line. Sheared poloidal flow is self‐consistently driven by both the Reynolds stress and the Stringer mechanisms. In the presence of self‐consistent shear flow, the transverse spectrum is no longer isotropic transverse to B. The vortices become elongated in the poloidal direction. Also, there is a substantial reduction in both the level of fluctuations of the density and potential and the associated particle transport. These features are in qualitative agreement with L–H transitions observed in tokamaks.

A mechanism for bypass transition from localized disturbances in wall-bounded shear flows

Abstract: The linear, nonlinear and breakdown stages in the transition of localized disturbances in plane Poiseuille flow is studied by direct numerical simulations and analysis of the linearized Navier–Stokes equations. Three-dimensionality plays a key role and allows for algebraic growth of the normal vorticity through the linear lift-up mechanism. This growth primarily generates elongated structures in the streamwise direction since it is largest at low streamwise wavenumbers. For finite-amplitude disturbances such structures will be generated essentially independent of the details of the initial disturbance, since the preferred nonlinear interactions transfer energy to low streamwise wavenumbers. The nonlinear interactions also give a decrease in the spanwise scales. For the stronger initial disturbances the streamwise vorticity associated with the slightly inclined streaks was found to roll up into distinct streamwise vortices in the vicinity of which breakdown occurred. The breakdown starts with a local rapid growth of the normal velocity bringing low-speed fluid out from the wall. This phenomenon is similar to the low-velocity spikes previously observed in transition experiments. Soon thereafter a small turbulent spot is formed. This scenario represents a bypass of the regular Tollmien–Schlichting, secondary instability process. The simulations have been carried out with a sufficient spatial resolution to ensure an accurate description of all stages of the breakdown and spot formation processes. The generality of the observed processes is substantiated by use of different types of initial disturbances and by Blasius boundary-layer simulations. The present results point in the direction of universality of the observed transition mechanisms for localized disturbances in wall-bounded shear flows.

Jupiter's Great Red Spot and Other Vortices

Abstract: A theoretical explanation of Jupiter's Great Red Spot (GRS) as the self-organization of vorticity in turbulence is presented. A number of properties of the GRS and other Jovian vortices that are unambiguous from the data are listed. The simplest possible model that explains these properties one at a time rather than in a difficult all-encompassing planetary global circulation model is presented. It is shown that Jovian vortices reflect the behavior of quasi-geostrophic (QG) vortices embedded in an east-west wind with bands of uniform potential vorticity. It is argued that most of the properties of the Jovian vortices can be easily explained and understood with QG theory. Many of the signatures of QG vortices are apparent on Voyager images. In numerical and laboratory experiments, QG vortices relax to approximately steady states like the Jovian vortices, rather than oscillating or rotating Kida ellipses.

Large-scale isentropic mixing properties of the Antarctic polar vortex from analyzed winds

Abstract: Winds derived from analyzed geopotential height fields are used to study quasi-horizontal mixing by the large-scale flow in the lower stratosphere during austral spring. This is the period when the Antarctic ozone hole appears and disappears. Trajectories are computed for large ensembles of particles initially inside and outside the main polar vortex. Mixing and transport are diagnosed through estimates of finite time Lyapunov exponents and Lagrangian dispersion statistics of the tracer trajectories. At 450 K and above prior to the vortex breakdown: Lyapunov exponents are a factor of 2 smaller inside the vortex than outside; diffusion coefficients are an order of magnitude smaller inside than outside the vortex; and the trajectories reveal little exchange of air across the vortex boundary. At lower levels (425 and 400 K) mixing is greater, and there is substantial exchange of air across the vortex boundary. In some years there are large wave events that expel small amounts of vortex air into the mid-latitudes. At the end of the spring season during the vortex breakdown there is rapid mixing of air across the vortex boundary, which is evident in the mixing diagnostics and the tracer trajectories.

A model of solar wind fluctuations with two components: Alfvén waves and convective structures

Abstract: A two-component incompressible fluctuation model is presented to explain the radial evolution of the solar wind fluctuations. The basic idea is to consider the small-scale fluctuations in the solar wind as being composed of Alfven waves and convective structures. The major Alfven waves are believed to be created near the coronal base and to propagate outward along the magnetic field lines. The convective structures are defined as the small-scale variations perpendicular to the local magnetic field direction. They are either quasi-static or turbulent and slowly evolving in the plasma frame of reference. The small-scale perpendicular variations are connected, in the parallel direction, with large-scale magnetic field variations, which are convected by the solar wind as quasi-static structures during the wind expansion time. The decomposition of the original fluctuations can be done by using special space and time averages, which are defined by space averaging along the directions parallel and perpendicular to the local magnetic field vector and by time averaging in the plasma frame of reference. The equations of motion of the fluctuations and of the correlation functions for both Alfven waves and convective structures have been derived from the one-fluid MHD equations. A combination of the correlation functions of these two components is then used for a comparison with observational results. The influence of the angle between the sampling direction and the magnetic field vector on the final results has also been considered. As a first step to apply these equations, a simple model has been suggested that is based on the assumption that the fluctuations are only composed of outward propagating Alfven waves and static magnetic structures. For comparison with the observations, new statistical results from data obtained by Helios 1 during days 1–95, 1975, and Helios 2 during days 19–109, 1976, are presented. The numerical solutions are shown to describe well the basic evolution trend of the fluctuation energy, the normalized cross helicity, and the Alfven ratio. It is also shown that the basic physical process of the evolution of the convective structures is the convection of the fluctuating velocity vortex lines and the magnetic field lines by the expanding solar wind.

Optimal excitation of three‐dimensional perturbations in viscous constant shear flow

Abstract: The three‐dimensional perturbations to viscous constant shear flow that increase maximally in energy over a chosen time interval are obtained by optimizing over the complete set of analytic solutions. These optimal perturbations are intrinsically three dimensional, of restricted morphology, and exhibit large energy growth on the advective time scale, despite the absence of exponential normal modal instability in constant shear flow. The optimal structures can be interpreted as combinations of two fundamental types of motion associated with two distinguishable growth mechanisms: streamwise vortices growing by advection of mean streamwise velocity to form streamwise streaks, and upstream tilting waves growing by the down gradient Reynolds stress mechanism of two‐dimensional shear instability. The optimal excitation over a chosen interval of time comprises a combination of these two mechanisms, characteristically giving rise to tilted roll vortices with greatly amplified perturbation energy. It is suggested that these disturbances provide the initial growth leading to transition to turbulence, in addition to providing an explanation for coherent structures in a wide variety of turbulent shear flows.

A numerical investigation of the coherent vortices in turbulence behind a backward-facing step

Abstract: This paper presents a statistical and topological study of a complex turbulent flow over a backward-facing step by means of direct and large-eddy simulations. Direct simulations are first performed for an isothermal two-dimensional case. In this case, shedding of coherent vortices in the mixing layer is demonstrated. Both direct and large-eddy simulations are then carried out in three dimensions. The subgrid-scale model used is the structure-function model proposed by Metais & Lesieur (1992). Lowstep computations corresponding to the geometry of Eaton & Johnston's (1980) laboratory experiment give turbulence statistics in better agreement with the experimental data than both Smagorinsky's method and K-e modelling. Furthermore, calculations for a high step show that the eddy structure of the flow presents striking analogies with forced plane mixing layers: large billows are shed behind the step with intense longitudinal vortices strained between them.

A Three-Dimensional Balance Theory for Rapidly Rotating Vortices

Abstract: A three-dimensional balance formulation for rapidly rotating vortices, such as hurricanes, is presented. The asymmetric balance (AB) theory represents a new mathematical framework for studying the slow evolution of rapidly rotating fluid systems. The AB theory is valid for large Rossby number; it makes no formal restriction on the magnitude of the divergence or vertical advection, which need not be small. The AB is an ordered expansion in the square of the ratio of orbital to inertial frequencies, the square of a local Rossby number. The approximation filters gravity and inertial waves from the system. Advantage is taken of the weak asymmetries near the vortex care as well as the tendency for low azimuthal wavenumber asymmetries to dominate. Linearization about a symmetric balanced vortex allows the three-dimensional asymmetric dynamics to be deduced properly. The AB formulation has a geopotential tendency equation with a three-dimensional elliptic operator. The AB system has a uniformly valid co...

Berry's phase and the Magnus force for a vortex line in a superconductor

Abstract: We show that the existence of the Magnus force is a general property of a vortex line in a superconductor by calculating the Berry phase for an adiabatic motion around a closed loop at zero temperature. We find that there is no influence of the disorder and the electromagnetic field on the existence of the Magnus force in the superconducting state, and its magnitude is proportional to the superfluid electron density.

Vortex dynamics and the production of Reynolds stress

Abstract: The physical mechanisms by which the Reynolds shear stress is produced from dynamically evolving vortical structures in the wall region of a direct numerical simulation of turbulent channel flow are explored. The complete set of quasistreamwise vortices are systematically located and tracked through the flow by the locus of the points of intersection of their centres of rotation with the (y, z) numerical grid planes. This approach assures positive identification of vortices of widely differing strengths, including those whose amplitude changes significantly in time. The process of vortex regeneration, and the means by which vortices grow, distort and interact over time are noted. Ensembles of particle paths arriving on fixed planes in the flow are used to represent the physical processes of displacement and acceleration transport (Bernard & Handler 1990a) from which the Reynolds stress is produced. By interweaving the most dynamically significant of the particle paths with the evolving vortical structures, the dynamical role of the vortices in producing Reynolds stress is exposed. This is found to include ejections of low-speed fluid particles by convecting structures and the acceleration and deceleration of fluid particles in the cores of vortices. Sweep dominated Reynolds stress close to the wall appears to be a manifestation of the regeneration process by which new vortices are created in the flow.

Calculation of Vortex Shedding Past a Square Cylinder with Various Turbulence Models

Abstract: The vortex-shedding flow past a square cylinder at Re = 22.000 was calculated with various turbulence models. The 2D periodic shedding motion was resolved in an unsteady calculation, and the superimposed stochastic turbulent fluctuations were simulated both with the k — e eddy-viscocity model and with a Reynolds-stress equation model. For both models, the viscosity-affected near-wall region was either bridged by wall functions or was resolved with a simpler one-equation model using a prescribed length-scale distribution. The k — e model with wall functions does not yield unsteady vortex motion while the other model variants do. The two-layer k —e model underpredicts severely the periodic fluctuations and also the Strouhal number and drag coefficient. The Reynoldsstress-equation models yield considerably better agreement with experiments, but tend to overpredict the periodic fluctuating motion and also miss some other details of the flow behaviour.

Vortical and turbulent structure of a lobed mixer free shear layer

Abstract: An experimental investigation of the vortical and turbulent structure in a free shear lager downstream of a lobed mixer has been conducted. Pulsed-laser sheet flow visualization with smoke and three-dimensional velocity measurements with hot-film anemometry were obtained for a lobed-mixer configuration and a baseline, planar configuration. Laminar and turbulent initial boundary-lager conditions were documented for both cases. The main result of this investigation is that a new vortex structure was confirmed to exist for the lobed mixer in addition to the well-known streamwise vortex array, consistent with the work of Manning

The Role of Tip Clearance in High-Speed Fan Stall

Abstract: A numerical experiment has been carried out to define the near-stall casing endwall flow field of a high-speed fan rotor. The experiment used a simulation code incorporating a simple clearance model, whose calibration is presented. The results of the simulation show that the interaction of the tip leakage vortex and the in-passage shock plays a major role in determining the fan flow range. More specifically, the computations imply that it is the area increase of this vortex as it passes through the in-passage shock that is the source of the blockage associated with stall. In addition, for fans of this type, it is the clearance over the forward portion of the fan blade that controls the flow processes leading to stall.