Showing papers on "Vortex published in 1992"

A new modelling of cavitating flows : a numerical study of unsteady cavitation on a hydrofoil section

Abstract: A new cavity model that can explain the interaction between viscous effects including vortices and cavitation bubbles is presented in this study. This model, which is named a bubble two-phase flow (BTF) model, treats the inside and outside of a cavity as one continuum by regarding the cavity as a compressible viscous fluid whose density changes greatly. Navier–Stokes equations including cavitation bubble clusters are solved in finite-difference form by a time-marching scheme, where the growth and collapse of a bubble cluster is given by a modified Rayleigh's equation. Computation was made on a two-dimensional flow field around a hydrofoil NACA0015 at angles of attack of 8° and 20°. The Reynolds number was 3 × 105. The experiments were also performed at the same Reynolds number for comparison. The computed results by the BTF cavity model can express the feature of cloud-type cavitation shed from the trailing edge of the attached cavities when the angle of attack is 8°. It shows the mechanism of cavitation cloud generation and large-scale vortices. The boundary layer separates at the cavity leading edge. Then it rolls up and produces the cavitation cloud. In other words, the instability of the shear layer may produce the cavitation cloud. When the angle of attack is 20°, the flow was fully separated from the leading edge of the hydrofoil and vortex cavitation occurs in the separated region. The BTF cavity model can also express the generation of such vortex cavitation and the effect of cavitation nuclei in the uniform flow.

The role of streamwise vorticity in the near-field entrainment of round jets

Abstract: The role of streamwise vortex structures in the near-field (x/d < 10) evolution of a round jet is examined. In free shear layers the streamwise vorticity develops into Bernal-Roshko structures which are streamwise vortex pairs. Similar structures are shown to exist in round jets. These structures, which evolve and amplify in the braid region between primary vortical structures, are shown to drastically alter the entrainment process in the near field and to increase the rate at which fluid is entrained into the jet. As the flow evolves downstream, the efficiency of the streamwise vorticity in entraining fluid increases relative to that of the azimuthal vorticity. Beyond the end of the potential core regime, the entrainment process is mainly controlled by streamwise vorticity. These processes are identified via flow visualization and confirmed by detailed global entrainment measurements.

Optical vortex solitons observed in Kerr nonlinear media

Abstract: Optical vortex-soliton filaments are observed in a bulk self-defocusing Kerr nonlinear refractive medium. The dark cylindrical core, located at the axis of a 2\ensuremath{\pi} helical phase ramp, is stationary and stable, with a size that depends inversely on the field strength. Wave guiding of a weak probe beam within the core is reported. A single optical vortex soliton was experimentally created using a quasihelical phase mask. Pairs having opposite topological charge were experimentally and numerically investigated using a convective Kelvin-Helmholtz instability of dark soliton stripes.

The structure of the polar vortex

Abstract: The paper develops a comparative picture of the 1987 Southern Hemisphere and 1989 Northern Hemisphere lower stratospheric, polar vortex circulation and constituent distributions as observed by the Airborne Antarctic Ozone Experiment, August 17-September 22, 1987, and Airborne Arctic Stratospheric Expedition, January 3-February 19, 1989 aircraft campaigns. Overall, both polar vortices define a region of highly isolated air, where the exchange of trace gases occurs principally at the vortex edge through erosional wave activity. Aircraft measurement showed that between 50 and 100 mbar, horizontally stratified long-lived tracers such as N2O are displaced downward 2-3 km on the cyclonic (poleward) side of the jet with the meridional tracer gradient sharpest at the jet core. Eddy mixing rates, computed using parcel ensemble statistics, are an order of magnitude or more lower on the cyclonic side of the jet compared to those on the anticyclonic side. Poleward zonal mean meridional flow on the anticyclonic side of the jet terminates in a descent zone at the jet core.

The natural and forced formation of spot-like vortex dislocations in the transition of a wake

Abstract: The three-dimensional transition of the flow behind a bluff body is studied, with an emphasis placed on the evolution of large-scale structures in the wake. It has previously been found that there are two fundamental modes of three-dimensional vortex shedding in the wake of a circular cylinder (each mode being dependent on the range of Reynolds number), with a spanwise lengthscale of the same order as the primary streamwise wavelength of the vortex street. However. it is shown in the present study that the wake transition also involves the appearance of large-scale spot-like ‘vortex dislocations’, that grow downstream to a size of the order of 10–20 primary wavelengths. Vortex dislocations are generated between spanwise vortex-shedding cells of different frequency. The presence of these dislocations explains the large intermittent velocity irregularities that were originally found by Roshko (1954) and later by Bloor (1964) to characterize transition. The presence of these vortex dislocations in wake transition is largely responsible for the break-up to turbulence of the wake as it travels downstream.In order to study their evolution in detail, dislocations have been (passively) forced to occur at a local spanwise position with the use of a small ring disturbance. It is found that ‘two-sided’ dislocations are stable in a symmetric in-phase configuration, and that they induce quasi-periodic velocity spectra and (beat) dislocation-frequency oscillations in the near wake. Intrinsic to these dislocations is a mechanism by which they spread rapidly in the spanwise direction, involving helical twisting of the vortices and axial core flows. This is felt to be a fundamental mechanism by which vortices develop large-scale distortions in natural transition. As the wake travels downstream, the energy at the low dislocation frequency decays slowly (in contrast to the rapid decay of other frequencies), leaving the downstream wake dominated by the large dislocation structures. Distinct similarities are found between the periodic forced dislocations and the intermittent dislocations that occur in natural transition. Further similarities of dislocations in different types of flow suggest that vortex or phase dislocations could conceivably be a generic feature of transition in all shear flows.

The three-dimensional evolution of a plane mixing layer - The Kelvin-Helmholtz rollup

Abstract: The Kelvin Helmholtz roll up of three dimensional, temporally evolving, plane mixing layers were simulated numerically. All simulations were begun from a few low wavenumber disturbances, usually derived from linear stability theory, in addition to the mean velocity profile. The spanwise disturbance wavelength was taken to be less than or equal to the streamwise wavelength associated with the Kelvin Helmholtz roll up. A standard set of clean structures develop in most of the simulations. The spanwise vorticity rolls up into a corrugated spanwise roller, with vortex stretching creating strong spanwise vorticity in a cup shaped region at the vends of the roller. Predominantly streamwise rib vortices develop in the braid region between the rollers. For sufficiently strong initial three dimensional disturbances, these ribs collapse into compact axisymmetric vortices. The rib vortex lines connect to neighboring ribs and are kinked in the opposite direction of the roller vortex lines. Because of this, these two sets of vortex lines remain distinct. For certain initial conditions, persistent ribs do not develop. In such cases the development of significant three dimensionality is delayed. When the initial three dimensional disturbance energy is about equal to, or less than, the two dimensional fundamental disturbance energy, the evolution of the three dimensional disturbance is nearly linear (with respect to the mean and the two dimensional disturbances), at least until the first Kelvin Helmholtz roll up is completed.

Transition to dissipation in a model of superflow.

Abstract: A direct numerical study of a model of superflow, the nonlinear Schr\"odinger equation, and simple analytical arguments shows a very striking phenomenon: The flow around a disk creates a drag force beyond a well-defined threshold velocity, linked to the emission of vortices from the perimeter of the disk.

Three-dimensional dynamics and transition to turbulence in the wake of bluff objects

Abstract: The wakes of bluff objects and in particular of circular cylinders are known to undergo a ‘fast’ transition, from a laminar two-dimensional state at Reynolds number 200 to a turbulent state at Reynolds number 400. The process has been documented in several experimental investigations, but the underlying physical mechanisms have remained largely unknown so far. In this paper, the transition process is investigated numerically, through direct simulation of the Navier—Stokes equations at representative Reynolds numbers, up to 500. A high-order time-accurate, mixed spectral/spectral element technique is used. It is shown that the wake first becomes three-dimensional, as a result of a secondary instability of the two-dimensional vortex street. This secondary instability appears at a Reynolds number close to 200. For slightly supercritical Reynolds numbers, a harmonic state develops, in which the flow oscillates at its fundamental frequency (Strouhal number) around a spanwise modulated time-average flow. In the near wake the modulation wavelength of the time-average flow is half of the spanwise wavelength of the perturbation flow, consistently with linear instability theory. The vortex filaments have a spanwise wavy shape in the near wake, and form rib-like structures further downstream. At higher Reynolds numbers the three-dimensional flow oscillation undergoes a period-doubling bifurcation, in which the flow alternates between two different states. Phase-space analysis of the flow shows that the basic limit cycle has branched into two connected limit cycles. In physical space the period doubling appears as the shedding of two distinct types of vortex filaments.Further increases of the Reynolds number result in a cascade of period-doubling bifurcations, which create a chaotic state in the flow at a Reynolds number of about 500. The flow is characterized by broadband power spectra, and the appearance of intermittent phenomena. It is concluded that the wake undergoes transition to turbulence following the period-doubling route.

Real-time observation of vortex lattices in a superconductor by electron microscopy

Abstract: THE dynamic behaviour of the quantized vortices of magnetic flux that penetrate type II superconductors, and specifically their interaction with pinning sites, is a topic of both scientific and technological interest, particularly since the discovery of high-transition-temperature (high- Tc) superconductors1. Until now, however, it has not been possible to study this behaviour in 'real time'. The Bitter technique2, scanning tunnelling microscopy3 and scanning electron microscopy4 have so far provided only static images, whereas a magneto-optical technique that provides time-resolved information5 does not resolve individual vortices. Here we report the real-time observation of vortices in a thin film of niobium, using the technique of Lorentz microscopy6. The coherent and penetrating beam of a recently developed 300-kV field-emission electron microscope6 allows us to observe, at 30 frames per second, the motion of thermally activated vortices, and their response to an applied magnetic field.

Vortex Shedding From a Circular Cylinder of Finite Length Placed on a Ground Plane

Abstract: This paper describes a study of changes in the vortex formation and the turbulent wake from a circular cylinder with a finite aspect ratio, placed on a ground plane. The experiment was carried out in an N.P.L. blow down type wind-tunnel, with a working section of 500 mm × 500 mm × 2,000 mm, and between the Reynolds number 2.5 × 104 and 4.7 × 104 . The surface-pressure distributions on the circular cylinder were measured and the drag coefficient was determined from these measurements. Vortices of two kinds generated in the flow-field around the cylinder were observed. The power spectrum, auto-correlation, space-correlation, velocity defects, and turbulent intensities in the turbulent wake behind a circular cylinder were also measured. It was found that the flow pattern changed rapidly above aspect ratio H/D = 4, with vortex shedding changing from symmetric “arch” type to antisymmetric “Karman” type.

Quantification of the inelastic interaction of unequal vortices in two‐dimensional vortex dynamics

Abstract: The interaction of two isolated vortices having uniform vorticity is examined in detailed contour dynamics calculations, and quantified using a diagnostic that measures the coherence of the final state. The two vortices have identical vorticity, leaving two basic parameters that determine the evolution: the radius ratio and separation distance. It is found that the term ‘‘vortex merger’’ inadequately describes the general interaction that takes place. Five regimes are found: (1) elastic interaction, (2) partial straining‐out, (3) complete straining‐out, (4) partial merger, and (5) complete merger. Regime 5 is what used to be called ‘‘merger,’’ but occurs in less than one‐quarter of the parameter space. Contrary to popular belief, inelastic vortex interactions (IVI’s) do not always lead to vortex growth. In fact, in over half of the parameter space, smaller vortices are produced. These results bring into question commonly accepted ideas about nearly inviscid two‐dimensional turbulence.

Vortex motion and the Hall effect in type-II superconductors: A time-dependent Ginzburg-Landau theory approach

Abstract: Vortex motion in type-II superconductors is studied starting from a variant of the time-dependent Ginzburg-Landau equations, in which the order-parameter relaxation time is taken to be complex. Using a method due to Gor'kov and Kopnin, we derive an equation of motion for a single vortex (B\ensuremath{\ll}${\mathit{H}}_{\mathit{c}2}$) in the presence of an applied transport current. The imaginary part of the relaxation time and the normal-state Hall effect both break ``particle-hole symmetry,'' and produce a component of the vortex velocity parallel to the transport current, and consequently a Hall field due to the vortex motion. Various models for the relaxation time are considered, allowing for a comparison to some phenomenological models of vortex motion in superconductors, such as the Bardeen-Stephen and Nozi\`eres-Vinen models, as well as to models of vortex motion in neutral superfluids. In addition, the transport energy, Nernst effect, and thermopower are calculated for a single vortex. Vortex bending and fluctuations can also be included within this description, resulting in a Langevin-equation description of the vortex motion. The Langevin equation is used to discuss the propagation of helicon waves and the diffusional motion of a vortex line. The results are discussed in light of the rather puzzling sign change of the Hall effect which has been observed in the mixed state of the high-temperature superconductors.

Free-surface cusps associated with flow at low Reynolds number

Abstract: When two cylinders are counter-rotated at low Reynolds number about parallel horizontal axes below the free surface of a viscous fluid, the rotation being such as to induce convergence of the flow on the free surface, then above a certain critical angular velocity Ωc, the free surface dips downwards and a cusp forms. This paper provides an analysis of the flow in the neighbourhood of the cusp, via an idealized problem which is solved completely: the cylinders are represented by a vortex dipole and the solution is obtained by complex variable techniques. Surface tension effects are included, but gravity is neglected. The solution is analytic for finite capillary number [Cscr ], but the radius of curvature on the line of symmetry on the free surface is proportional to exp (−32π[Cscr ]) and is extremely small for [Cscr ] [gsim ] 0.25, implying (in a real fluid) the formation of a cusp. The equation of the free surface is cubic in (x, y) with coefficients depending on [Cscr ], and with a cusp singularity when [Cscr ] = ∞.The influence of gravity is considered through a stability analysis of the free surface subjected to converging uniform strain, and a necessary condition for the development of a finite-amplitude disturbance of the free surface is obtained.An experiment was carried out using the counter-rotating cylinders as described above, over a range of capillary numbers from zero to 60; the resulting photographs of a cross-section of the free surface are shown in figure 13. For Ω Ωc, the downward-pointing cusp forms, and its structure shows good agreement with the foregoing theory.

Vortex structure and dynamics in the near field of a coaxial jet

Abstract: We present results from an experimental and numerical investigation into the structure of vortex patterns and the dynamics of their interactions for the incompressible flow in the near field of a round coaxial jet issuing into a quiescent ambient fluid. A two-colour planar laser-induced-fluorescence technique is used to document the flow field via still photographs and cine sequences over a limited range of parameters. We examine the effects of varying the velocity ratio as well as the absolute velocities of the two coaxial streams for equal densities and for a single area ratio. Results show that a variety of widely differing near-field vortex patterns can arise, with very different interaction dynamics, which can depend both on the velocity ratio and on the absolute velocities of the two streams. The observed vortex structures and their dynamics are interpreted in terms of the instability of the initially cylindrical and concentric vorticity layers separating each of the fluid streams, and their subsequent rollup to form wake-like or shear-layer-like vortices. Our results show that in addition to the velocity jump across each of these vorticity layers, an accounting of the layer thicknesses and the wake defect within each layer can be essential to understanding the resulting near-field structure that occurs. Ensuing dynamical interactions between the vortices formed from each layer can produce a strong coupling between the development of the two layers. These resulting vortex structures and interaction dynamics are also seen to produce widely differing mixing patterns in the jet near field.

Viscous and inviscid instabilities of a trailing vortex

Abstract: The linear stability of the trailing line vortex model of Batchelor (1964) is studied using a spectral collocation and matrix eigenvalue method. The entire unstable region in the swirl/axial wavenumber parameter space is mapped out for various azimuthal wavenumbers for both the inviscid and viscous stability problem. The results of the study provide a direct numerical validation of the large-azimuthal-wavenumber asymptotic analysis of Leibovich and Stewartson (1983). It is shown that accurate results are obtained up to azimuthal wavenumbers of 10,000 and greater, and the agreement with the asymptotic theory is excellent.

Topological aspects of the dynamics of fluids and plasmas

Abstract: I Introductory Lectures- Relaxation under Topological Constraints- Knot Theory, Jones' Polynomials, Invariants of 3-Manifolds, and the Topological Theory of Fluid Dynamics- Topology of Knots- Stretching and Alignment in General Flow Fields: Classical Trajectories from Reynolds Number Zero to Infinity- Fast Dynamo Theory- II Relaxation and Minimum Energy States- Relaxation and Topology in Plasma Experiments- Taylor's Relaxation in an Unbounded Domain of Space- Force-Free Magnetic Fields with Constant Alpha- Minimum Energy Magnetic Fields with Toroidal Topology- Research Announcement on the "Energy" of Knots- III Helicity, Linkage, and Flow Topology- The Helicity of a Knotted Vortex Filament- A Heirarchy of Linking Integrals- Borromeanism and Bordism- Topology of Steady Fluid Flows- IV The Euler Equations: Extremal Properties and Finite Time Singularities- Extremal Properties and Hamiltonian Structure of the Euler Equations- Blow Up in Axisymmetric Euler Flows- Is There a Finite-Time Singularity in Axisymmetric Euler Flows?- Evidence for a Singularity of the Three-Dimensional Incompressible Euler Equations- Singularity Formation on Vortex Sheets: The Raleigh-Taylor Problem- V Vortex Interactions and the Structure of Turbulence- 2D Turbulence: New Results for Re ? ?- On Vortex Reconnection and Turbulence- New Aspects of Vortex Dynamics: Helical Waves, Core Dynamics, Viscous Helicity Generation, and Interaction with Turbulence- Intermittency Growth in 3D Turbulence- Dynamical Mechanisms for Intermittency Effects in Fully Developed Turbulence- The Multispiral Model of Turbulence and Intermittency- Measurements of Local Scaling of Turbulent Velocity Fields at High Reynolds Numbers- On the Determination of Universal Multifractal Parameters in Turbulence- VI Chaos, Instability and Dynamo Theory- Anomolous Transport and Fractal Kinectics- Kinematical Instability and Line-Stretching in Relation to the Geodesics of Fluid Motion- The Behavior of Active and Passive Particles in a Chaotic Flow- Chaos Associated with Fluid Inertia- Instability Criteria in Fluid Dynamics- Localized Instabilities in Fluids- Kinematic Fast Dynamo Action in a Time-Periodic Chaotic Flow- An Exact Turbulent Closure for the Hydromagnetic Dynamo- Author Index

Fluctuations of vortices in layered high-Tc superconductors.

Abstract: The temperature ${\mathit{T}}_{\mathit{s}}$ of spontaneous creation of vortex lines in Josephson coupled layered superconductors is obtained by taking into account the entropy contribution to the free energy due to thermal distortions. For Bi- and Ti-based high-${\mathit{T}}_{\mathit{c}}$ superconductors and superlattices, the superconducting critical temperature ${\mathit{T}}_{\mathit{s}}$ lies noticeably below the mean-field transition temperature ${\mathit{T}}_{\mathit{c}0}$(${\mathit{T}}_{\mathit{c}0}$-${\mathit{T}}_{\mathit{s}}$\ensuremath{\approxeq}4 K or more). The contribution of thermal distortions to the free enery causes significant changes in the temperature and field dependences of magnetization below ${\mathit{T}}_{\mathit{s}}$ seen in Bi compounds.

Quantized Vortices in Helium II

Abstract: Preface 1. Background on classical vortices 2. Background on liquid helium II 3. Vortex dynamics and mutual friction 4. The structure of quantized vortices 5. Vortex arrays 6. Vortex waves 7. Quantum turbulence 8. Nucleation of quantized vortices Index.

A study of singularity formation in vortex-sheet motion by a spectrally accurate vortex method

Abstract: Moore's asymptotic analysis of vortex-sheet motion predicts that the Kelvin–Helmholtz instability leads to the formation of a weak singularity in the sheet profile at a finite time. The numerical studies of Meiron. Baker & Orszag, and of Krasny, provide only a partial validation of his analysis. In this work, the motion of periodic vortex sheets is computed using a new, spectrally accurate approximation to the Birkhoff–Rott integral. As advocated by Krasny, the catastrophic effect of round-off error is suppressed by application of a Fourier filter, which itself operates near the level of the round-off. It is found that to capture the correct asymptotic behaviour of the spectrum, the calculations must be performed in very high precision, and second-order terms must be included in the Ansatz to the spectrum. The numerical calculations proceed from the initial conditions first considered by Meiron, Baker & Orszag. For the range of amplitudes considered here, the results indicate that Moore's analysis is valid only at times well before the singularity time. Near the singularity time the form of the singularity departs away from that predicted by Moore, with the real and imaginary parts of the solutions becoming differentiated in their behaviour; the real part behaves in accordance with Moore's prediction, while the singularity in the imaginary part weakens. In addition, the form of the singularity apparently depends upon the initial amplitude of the disturbance, with the results suggesting that either Moore's analysis gives the complete form of the singularity only in the zero amplitude limit, or that the initial data considered here is not yet sufficiently small for the behaviour to be properly described by the asymptotic analysis. Convergence of the numerical solution beyond the singularity time is not observed.

Numerical evidence of smooth self-similar dynamics and possibility of subsequent collapse for three-dimensional ideal flows

Abstract: Direct numerical simulations of the three‐dimensional Euler equations at resolutions up to 2563 for general periodic flows and 8643 for the symmetric Taylor–Green vortex are presented The spontaneous emergence of flat pancakelike structures that shrink exponentially in time is observed A simple self‐similar model that fits these observations is discussed Focusing instabilities similar to those leading to streamwise vortices in the context of free shear layers [J Fluid Mech 143, 253 (1984)], are expected to subsequently concentrate the vorticity and produce isolated vortex filaments A finite time singularity for the Euler equation is not excluded as the result of interactions among these filaments

A new numerical model of the middle atmosphere: 1. Dynamics and transport of tropospheric source gases

Abstract: Attention is given to a new model of the middle atmosphere which includes, in addition to the equations governing the zonal mean state, a potential vorticity equation for a single planetary-scale Rossby wave, and an IR radiative transfer code for the stratosphere and lower mesosphere, which replaces the Newtonian cooling parameterization used previously. It is shown that explicit computation of the planetary-scale wave field yields a more realistic representation of the zonal mean dynamics and the distribution of trace chemical species. Wave breaking produces a well-mixed 'surf zone' equatorward of the polar night vortex and drives a meridional circulation with downwelling on the poleward side of the vortex. This combination of mixing and downwelling produces shallow meridional gradients of trace gases in the subtropics and middle latitudes, and very steep gradients at the edge of the polar vortex. Computed distributions of methane and nitrous oxide are shown to agree well with observations.

Self-organizing particle dispersion mechanism in a plane wake

Abstract: Experimental and numerical results concerning solid particle motion in a plane wake are presented that demonstrate the importance of large‐scale vortex structures in self‐organizing dispersion processes. Previous studies have demonstrated that a time scale ratio involving the aerodynamic response time of the particles and a characteristic time of the vortex structures is an important parameter for indicating the qualitative and quantitative nature of the dispersion process. A stretching and folding mechanism associated with vortex development and merging interactions has been suggested as a description for characterizing particle dispersion in plane mixing layers at intermediate time scale ratios. For plane wakes where large‐scale vortex mergers rarely occur, a highly organized particle dispersion process focuses intermediate time scale ratio particles along the boundaries of the large‐scale vortices. The fractal correlation dimension associated with chaotic systems is found to be a useful parameter for quantifying the relative organization of the dispersion patterns as a function of the particle time scale ratio.

Stability of axial flow in an annulus with a rotating inner cylinder

Abstract: Flow between concentric cylinders with the inner cylinder rotating and an axial pressure gradient imposed in the annulus reveals a rich variety of flow regimes depending on the flow conditions. The occurrence of these flow regimes was studied experimentally by both visually and optically detecting the transition from one flow regime to another over a wide range of Taylor numbers for moderate axial Reynolds numbers. Seven flow regimes of toroidal vortices were identified, including Taylor vortices, wavy vortices, random wavy vortices, modulated wavy vortices, turbulent modulated wavy vortices, turbulent wavy vortices, and turbulent vortices. The toroidal vortices in these flow regimes look similar to the corresponding vortices when there is no axial flow, except that they translate with the axial flow at a speed slightly greater than the bulk axial velocity. Three flow regimes of helical vortices were observed at low Taylor numbers, including laminar helical vortices, stationary helical vortices, and wavy helical vortices. Depending on the flow parameters, the helical vortices had both positive and negative helix angles with respect to the bulk flow and appeared either stationary or moving downstream. Another flow regime consisting of the repeating sequential appearance of turbulent wavy vortices, turbulent helical vortices, and turbulent vortices was also observed.

The role of non-uniqueness in the development of vortex breakdown in tubes

Abstract: Numerical solutions of viscous, swirling flows through circular pipes of constant radius and circular pipes with throats have been obtained. Solutions were computed for several values of vortex circulation, Reynolds number and throat/inlet area ratio, under the assumptions of steady flow, rotational symmetry and frictionless flow at the pipe wall. When the Reynolds number is sufficiently large, vortex breakdown occurs abruptly with increased circulation as a result of the existence of non-unique solutions. Solution paths for Reynolds numbers exceeding approximately 1000 are characterized by an ensemble of three inviscid flow types: columnar (for pipes of constant radius), soliton and wavetrain. Flows that are quasi-cylindrical and which do not exhibit vortex breakdown exist below a critical circulation, dependent on the Reynolds number and the throat/inlet area ratio. Wavetrain solutions are observed over a small range of circulation below the critical circulation, while above the critical value, wave solutions with large regions of reversed flow are found that are primarily solitary in nature. The quasi-cylindrical (QC) equations first fail near the critical value, in support of Hall's theory of vortex breakdown (1967). However, the QC equations are not found to be effective in predicting the spatial position of the breakdown structure.

Vortex solutions in the Weinberg-Salam model

Abstract: We show that the Weinberg-Salam model has vortex solutions similar to semilocal strings for all values of the parameters. The stability of the solutions under small perturbations will depend on the parameters of the theory and, in particular, on the ratio of the Higgs boson mass to the Z boson mass.

Magnetic flux-line lattices and vortices in the copper oxide superconductors.

Abstract: A variety of recent experiments on both the static and the dynamic properties of vortices and flux-line lattices in the mixed state of the copper oxide superconductors are discussed. The experiments are of two basic types: (i) experiments that image the magnetic flux patterns either with magnetic decoration or neutrons and give information about static structures, and (ii) experiments that explore the dynamics of vortices either through the resistivity or other electrodynamic responses of the material. Results of these experiments argue in favor of the existence of a true phase transition in the high-field vortex state from a low-temperature superconducting vortex glass phase into a disordered high-temperature vortex fluid phase. The vortex glass phase transition model does a good job of explaining high-precision measurements of the dynamics at the transition. At low fields and temperatures, very long range hexatic order in the flux-line lattice is observed.

Smoke-Column Observations from Two Forest Fires Using Doppler Lidar and Doppler Radar

Abstract: To demonstrate the usefulness of active remote-sensing systems in observing forest fire plume behavior, we studied two fires, one using a 3.2-cm-wavelength Doppler radar, and one more extensively, using Doppler lidar. Both instruments observed the kinematics of the convection column, including the presence of two different types of rotation in the columns, and monitored the behavior of the smoke plume. The first fire, a forest fire that burned out of control, was observed by the Doppler radar during late-morning and afternoon hours. Strong horizontal ambient winds produced a bent-over convection column, which the radar observed to have strong horizontal flow at its edges and weaker flow along the centerline of the plume. This velocity pattern implies that the column consisted of a pair of counterrotating horizontal vortices (rolls), with rising motion along the centerline and sinking along the edges. The radar tracked the smoke plume for over 30 km. It also provided circular depolarization ratio ...

Kinematic alignment effects in turbulent flows

Abstract: It is shown that, in the neighborhood of a vortex whose maximum vorticity is large with respect to that in the surrounding flow, the two principal strains with the largest absolute values lie in the equatorial plane, so that the vorticity is automatically aligned to the intermediate eigenvector. This purely kinematic effect is offered as an explanation for the alignment properties recently reported for the velocity and pressure derivatives in turbulent flows. The model is compared with experimental evidence from numerical simulations. The observed ratio between the principal strains is also related to the properties of a two‐dimensional Burgers’ vortex.