Showing papers on "Vortex published in 1996"

Leading-edge vortices in insect flight

Abstract: INSECTS cannot fly, according to the conventional laws of aerodynamics: during flapping flight, their wings produce more lift than during steady motion at the same velocities and angles of attack1–5. Measured instantaneous lift forces also show qualitative and quantitative disagreement with the forces predicted by conventional aerodynamic theories6–9. The importance of high-life aerodynamic mechanisms is now widely recognized but, except for the specialized fling mechanism used by some insect species1,10–13, the source of extra lift remains unknown. We have now visualized the airflow around the wings of the hawkmoth Manduca sexta and a 'hovering' large mechanical model—the flapper. An intense leading-edge vortex was found on the down-stroke, of sufficient strength to explain the high-lift forces. The vortex is created by dynamic stall, and not by the rotational lift mechanisms that have been postulated for insect flight14–16. The vortex spirals out towards the wingtip with a spanwise velocity comparable to the flapping velocity. The three-dimensional flow is similar to the conical leading-edge vortex found on delta wings, with the spanwise flow stabilizing the vortex.

Optical vortex trapping of particles

Abstract: Summary form only given. We demonstrate that a low-index dielectric particle can be stably trapped in three-dimensions using a stationary, focused Gaussian beam containing an optical vortex. This work is, to our knowledge, the first demonstration of three-dimensional trapping of a spherical low-index particle using a single, stationary beam. Vortex traps allow the trapping of low- and high-index particles with less risk of damage and better isolation.

An experimental study of round jets in cross-flow

Abstract: The structure of round jets in cross-flow was studied using flow visualization techniques and flying-hot-wire measurements. The study was restricted to jet to freestream velocity ratios ranging from 2.0 to 6.0 and Reynolds numbers based on the jet diameter and free-stream velocity in the range of 440 to 6200.Flow visualization studies, together with time-averaged flying-hot-wire measurements in both vertical and horizontal sectional planes, have allowed the mean topological features of the jet in cross-flow to be identified using critical point theory. These features include the horseshoe (or necklace) vortex system originating just upstream of the jet, a separation region inside the pipe upstream of the pipe exit, the roll-up of the jet shear layer which initiates the counter-rotating vortex pair and the separation of the flat-wall boundary layer leading to the formation of the wake vortex system beneath the downstream side of the jet.The topology of the vortex ring roll-up of the jet shear layer was studied in detail using phase-averaged flying-hot-wire measurements of the velocity field when the roll-up was forced. From these data it is possible to examine the evolution of the shear layer topology. These results are supported by the flow visualization studies which also aid in their interpretation.The study also shows that, for velocity ratios ranging from 4.0 to 6.0, the unsteady upright vortices in the wake may form by different mechanisms, depending on the Reynolds number. It is found that at high Reynolds numbers, the upright vortex orientation in the wake may change intermittently from one configuration of vortex street to another. Three mechanisms are proposed to explain these observations.

Direct Observation of Vortex Dynamics in Superconducting Films with Regular Arrays of Defects

Abstract: The microscopic mechanism of the matching effect in a superconductor, which manifested itself as the production of peaks or cusps in the critical current at specific values of the applied magnetic field, was investigated with Lorentz microscopy to allow direct observation of the behavior of vortices in a niobium thin film having a regular array of artificial defects. Vortices were observed to form regular and consequently rigid lattices at the matching magnetic field, at its multiples, and at its fractions. The dynamic observation furthermore revealed that vortices were most difficult to move at the matching field, whereas excess vortices moved easily.

An objective determination of the polar vortex using Ertel's potential vorticity

Abstract: We have developed objective criteria for choosing the location of the northern hemisphere polar vortex boundary region and the onset and breakup dates of the vortex. By determining the distribution of Ertel's potential vorticity (Epv) on equivalent latitudes, we define the vortex edge as the location of maximum gradient of Epv constrained by the location of the maximum wind jet calculated along Epv isolines. We define the vortex boundary region to be at the local maximum convex and concave curvature in the Epv distribution surrounding the edge. We have determined that the onset and breakup dates of the vortex on the 450 K isentropic surface occur when the maximum wind speed calculated along Epv isolines rises above and falls below approximately 15.2 m s -1 . We use 1992-1993 as a test case to study the onset and breakup periods, and we find that the increase of polar vortex Epv values is associated with the dominance of the term in the potential vorticity equation involving the movement of air through the surface due to the diabatic circulation. We also find that the decrease is associated with the dominance of the term involving radiatively induced changes in the stability of the atmosphere.

The structure and development of a wing-tip vortex

Abstract: Experiments have been performed on the tip vortex trailing from a rectangular NACA 0012 half-wing. Preliminary studies showed the vortex to be insensitive to the introduction of a probe and subject only to small wandering motions. Meaningful velocity measurements could therefore be made using hot-wire probes.Detailed analysis of the effects of wandering was performed to properly reveal the flow structure in the core region and to give confidence in measurements made outside the core. A theory has been developed to correct mean-velocity profiles for the effects of wandering and to provide complete quantitative estimates of its amplitude and contributions to Reynolds stress fields. Spectral decomposition was found to be the most effective method of separating these contributions from velocity fluctuations due to turbulence.Outside the core the flow structure is dominated by the remainder of the wing wake which winds into an ever-increasing spiral. There is no large region of axisymmetric turbulence surrounding the core and little sign of turbulence generated by the rotational motion of the vortex. Turbulence stress levels vary along the wake spiral in response to the varying rates of strain imposed by the vortex. Despite this complexity, the shape of the wake spiral and its turbulent structure reach an approximately self-similar form.On moving from the spiral wake to the core the overall level of velocity fluctuations greatly increases, but none of this increase is directly produced by turbulence. Velocity spectra measured at the vortex centre scale in a manner that implies that the core is laminar and that velocity fluctuations here are a consequence of inactive motion produced as the core is buffeted by turbulence in the surrounding spiral wake. Mean-velocity profiles through the core show evidence of a two-layered structure that dies away with distance downstream.

The Effect of Vertical Shear on Tropical Cyclone Intensity Change

Abstract: The effect of vertical shear on tropical cyclone intensity change is usually explained in terms of “ventilation” where heat and moisture at upper levels are advected away from the low-level circulation, which inhibits development. A simple two-layer diagnostic balance model is used to provide an alternate explanation of the effect of shear. When the upper-layer wind in the vortex environment differs from that in the lower layer, the potential vorticity (PV) pattern associated with the vortex circulation becomes tilted in the vertical. The balanced mass field associated with the tilted PV pattern requires an increased midlevel temperature perturbation near the vortex center. It is hypothesized that this midlevel warming reduces the convective activity and inhibits the storm development. Previous studies have shown that diabatic heating near the storm center acts to reduce the vertical tilt of the vortex circulation. These studies have also shown that there is an adiabatic process that acts to redu...

Bosons in anisotropic traps: Ground state and vortices

Abstract: We solve the Gross-Pitaevskii equations for a dilute atomic gas in a magnetic trap, modeled by an anisotropic harmonic potential. We evaluate the wave function and the energy of the Bose-Einstein condensate as a function of the particle number, both for positive and negative scattering length. The results for the transverse and the vertical size of the cloud of atoms, as well as for the kinetic and potential energy per particle, are compared with the predictions of approximated models. We also compare the aspect ratio of the velocity distribution with experimental estimates available for $^{87}\mathrm{Rb}$. Vortex states are considered and the critical angular velocity for production of vortices is calculated. We show that the presence of vortices significantly increases the stability of the condensate in the case of attractive interactions. \textcopyright{} 1996 The American Physical Society.

Control of laminar vortex shedding behind a circular cylinder using splitter plates

Abstract: Laminar vortex shedding behind a circular cylinder and its control using splitter plates attached to the cylinder are simulated. The vortex shedding behind a circular cylinder completely disappears when the length of the splitter plate is larger than a critical length, and this critical length is found to be proportional to the Reynolds number. The Strouhal number of the vortex shedding is rapidly decreasing with the increased plate length until the plate length (l) is nearly the same as the cylinder diameter (d). On the other hand, at 1

Axis switching and spreading of an asymmetric jet: the role of coherent structure dynamics

Abstract: The effects of vortex generators and periodic excitation on vorticity dynamics and the phenomenon of axis switching in a free asymmetric jet are studied experimentally. Most of the data reported are for a 3:1 rectangular jet at a Reynolds number of 450 000 and a Mach number of 0.31. The vortex generators are in the form of ‘delta tabs’, triangular-shaped protrusions into the flow, placed at the nozzle exit. With suitable placement of the tabs, axis switching could be either stopped or augmented. Two mechanisms are identified governing the phenomenon. One, as described by previous researchers, is due to the difference in induced velocities for different segments of a rolled-up azimuthal vortical structure. The other is due to the induced velocities of streamwise vortex pairs in the flow. While the former mechanism, referred to here as the ωθ-dynamics, is responsible for a rapid axis switching in periodically forced jets, e.g. screeching supersonic jets, the effect of the tabs is governed mainly by the latter mechanism, referred to as the ωx-dynamics. Both dynamics can be active in a natural asymmetric jet; the tendency for axis switching caused by the ωθ-dynamics may be, depending on the streamwise vorticity distribution, either resisted or enhanced by the ωx-dynamics. While this simple framework qualitatively explains the various observations made on axis switching, mechanisms actually in play may be much more complex. The two dynamics are not independent as the flow field is replete with both azimuthal and streamwise vortical structures which continually interact. Phase-averaged measurements for a periodically forced case, over a volume of the flow field, are carried out in an effort to gain insight into the dynamics of these vortical structures. The results are used to examine such processes as the reorientation of the azimuthal vortices, the resultant evolution of streamwise vortex pairs, as well as the redistribution of streamwise vortices originating from secondary flow within the nozzle.

Control of turbulent separated flow over a backward-facing step by local forcing

Abstract: An experimental study was made of the flow over a backward-facing step. Excitations were given to separated flow by means of a sinusoidally oscillating jet issuing from a thin slit near the separation line. The Reynolds number based on the step height (H) varied 13000 ⩽ Re H ⩽ 33000. Effect of local forcing on the flow structure was scrutinized by altering the forcing amplitude (0 ⩽ A 0 ⩽ 0.07) and forcing frequency (0 ⩽ St H ⩽ 5.0). Small localized forcing near the separation edge enhanced the shear-layer growth rate and produced a large roll-up vortex at the separation edge. A large vortex in the shear layer gave rise to a higher rate of entrainment, which lead to a reduction in reattachment length as compared to the unforced flow. The normalized minimum reattachment length (x r )min/x x0 was obtained at St θ ≅ 0.01. The most effective forcing frequency was found to be comparable to the shedding frequency of the separated shear layer.

Magnetic structures in a dynamo simulation

Abstract: We use three-dimensional simulations to study compressible convection in a rotating frame with magnetic fields and overshoot into surrounding stable layers. The, initially weak, magnetic field is amplified and maintained by dynamo action and becomes organized into flux tubes that are wrapped around vortex tubes. We also observe vortex buoyancy which causes upward flows in the cores of extended down-draughts. An analysis of the angles between various vector fields shows that there is a tendency for the magnetic field to be parallel or antiparallel to the vorticity vector, especially when the magnetic field is strong. The magnetic energy spectrum has a short inertial range with a slope compatible with k +1/3 during the early growth phase of the dynamo. During the saturated state the slope is compatible with k -1 . A simple analysis based on various characteristic timescales and energy transfer rates highlights important qualitative ideas regarding the energy budget of hydromagnetic dynamos.

The emergence of jets and vortices in freely evolving, shallow-water turbulence on a sphere

Abstract: Results from a series of simulations of unforced turbulence evolving within a shallow layer of fluid on a rotating sphere are presented. Simulations show that the turbulent evolution in the spherical domain is strongly dependent on numerical and physical conditions. The independent effects of (1) (hyper)dissipation and initial spectrum, (2) rotation rate, and (3) Rossby deformation radius are carefully isolated and studied in detail. In the nondivergent and nonrotating case, an initially turbulent flow evolves into a vorticity quadrupole at long times, a direct consequence of angular momentum conservation. In the presence of sufficiently strong rotation, the nondivergent long‐time behavior yields a field dominated by polar vortices—as previously reported by Yoden and Yamada. In contrast, the case with a finite deformation radius (i.e., the full spherical shallow‐water system) spontaneously evolves toward a banded configuration, the number of bands increasing with the rotation rate. A direct application of...

Calorimetric measurement of the latent heat of vortex-lattice melting in untwinned YBa 2 Cu 3 O 7-δ

Abstract: THE magnetic vortex lattice of copper oxide superconductors in the mixed (field-penetrated) state 'liquefies'1,2 on increasing the temperature T or the external magnetic field H, giving rise to an ohmic resistivity well below the fluctuation-dominated crossover to the normal state at the upper critical field Hc2(T). Theoretical work suggests that in clean materials this melting is a first-order phase transition3; features in the resistivity4–6 and magnetization7–10, as well as results from muon spin rotation11 and neutron-diffraction work12, have been cited to support this hypothesis. A calorimetric measurement of a latent heat provides the most definitive proof of the occurrence of a first-order transition, but such measurements require very high sensitivity. Here we report calorimetric measurements on an untwinned single crystal of YBa2Cu3O7–δ that have sufficient precision to clearly resolve the latent heat. The value obtained, ∼0.45kBT per vortex per superconducting layer (where kB is the Boltzmann constant), is consistent with that inferred from magnetization data using the Clapeyron equation. This result is compelling evidence for a first-order transition at a well defined phase boundary Hm(T).

High-frequency vortex dynamics in

Abstract: We present a phenomenological description of the high-frequency vortex dynamics in and discuss the main parameters related to vortex motion, namely the viscous drag coefficient , the pinning constant (Labusch parameter) and the depinning frequency . We demonstrate experimental results on the angular and temperature dependence of , and in and compare these results with existing models. We show how studies of the vortex viscosity may yield information on the superclean limit. This limit corresponds to the formation of the discrete excitation spectrum in the vortex core due to quantum confinement and small coherence length. From the low-temperature viscosity data we conclude that the superclean limit in is reached for magnetic field perpendicular to the c-axis.

Tropical Cyclone Motion and Evolution in Vertical Shear

Abstract: The motion and the evolution of tropical cyclone-like vortices in an environmental flow with vertical shear are investigated using a baroclinic primitive equation model. The study focuses on the fundamental dynamics of a baroclinic vortex in vertical shear, the influence of vortex structure, and the role of diabatic heating. The results show that the initial response of the vortex to the vertical shear is to tilt downshear. As soon as the tilt develops, the upper-level anticyclonic and lower-level cyclonic circulations begin to interact with each other. As a result of these interactions, the tilted axis of the vortex reaches a stable state after an initial adjustment, which varies with the structure of the vortex, its environmental flow shear, and the cumulus convective heating. The motion of an adiabatic vortex in vertical shear is controlled by both the steering of the environmental flow and vertical coupling mechanisms. Most of the vortices move with the environmental flow at about 650 hPa or ...

Efficient Foil Propulsion Through Vortex Control

Abstract: We investigate the problem of a heaving and pitching hydrofoil in an inflow that consists of a uniform velocity field and a staggered array of vortices. The foil can exploit the energy in such a Karman vortex street for efficient propulsion of animals or submarines. Through a two-dimensional inviscid analysis, we find that the phase between foil motion and the arrival of inflow vortices is a critical parameter. Everything else being equal, the highest efficiency is seen when this phase is such that the foil moves in close proximity to the oncoming vortices. Different modes of vortex interaction in the wake results from a variation in this phase, and we show that the flow downstream of the foil is related to the foil input power.

The evolution of a jet with vortex-generating tabs: real-time visualization and quantitative measurements

Abstract: An experimental study involving flow visualization and two-component LDV measurements has been undertaken to elucidate the deformation of an axisymmetric jet ( Re D ≈ 1950 and 4160) caused by tabs placed at the nozzle exit. Previous studies have shown the profound distortion of high-speed jets with tabs and have demonstrated that a pair of counter-rotating streamwise vortices generated by each tab are responsible for the deformation of the jet core. This work illustrates the distortion as well as some of the more subtle features of the tab effect. Extensive visualizations taken simultaneously from two perspectives reveal the real-time evolution of complex three-dimensional flow structures. Velocity data show the expected overall distortion, and the existence and strength of the streamwise vortices responsible for this deformation. Furthermore, a second set of weaker streamwise vortices was detected near each tab, the size and location of which was consistent with a horseshoe vortex system. The data showed a widespread increase in both Reynolds normal and shear stresses and generally indicated the accelerated development of the mixing layer when tabs were inserted. A brief analysis employing vortex dynamics - an alternative to previous work which utilized pressure gradient arguments-is used to explain the tab effect, resulting in similar conclusions.

Experimental and Computational Investigation of the Tip Clearance Flow in a Transonic Axial Compressor Rotor

Abstract: Experimental and computational techniques are used to investigate tip clearance flows in a transonic axial compressor rotor at design and part speed conditions. Laser anemometer data acquired in the endwall region are presented for operating conditions near peak efficiency and near stall at 100% design speed and at near peak efficiency at 60% design speed. The role of the passage shock/leakage vortex interaction in generating endwall blockage is discussed. As a result of the shock/vortex interaction at design speed, the radial influence of the tip clearance flow extends to 20 times the physical tip clearance height. At part speed, in the absence of the shock, the radial extent is only 5 times the tip clearance height. Both measurements and analysis indicate that under part-speed operating conditions a second vortex, which does not originate from the tip leakage flow, forms in the endwall region within the blade passage and exits the passage near midpitch. Mixing of the leakage vortex with primary flow downstream of the rotor at both design and part speed conditions is also discussed.

Dynamics of coherent structures and transition to turbulence in free square jets

Abstract: We report results of time‐dependent numerical simulation of spatially developing free square jets initialized with a thin square vortex‐sheet with slightly rounded corner‐regions The studies focus on the near field of jets with Mach number 03–06 and moderately high Reynolds numbers A monotonically‐integrated large‐eddy‐simulation approach is used, based on the solution of the unfiltered inviscid equations and appropriate inflow/outflow open boundary conditions The simulations show that the initial development of the square jet is characterized by the dynamics of vortex rings and braid vortices Farther downstream, strong vortex interactions lead to the breakdown of the vortices, and to a more disorganized flow regime characterized by smaller scale elongated vortices and spectral content consistent with that of the Kolmogorov (K41) inertial subrange Entrainment rates significantly larger than those for round jets are directly related to the enhanced fluid and momentum transport between jet and surroundings determined by the vortex dynamics underlying the axis‐rotation of the jet cross‐section The first axis‐rotation of the jet cross‐section can be directly correlated with self‐induced vortex‐ring deformation However, subsequent jet axis‐rotations are the result of strong interactions between ring and braid vortices, rather than being correlated with successive self‐induced vortex‐ring deformations, as previously conjectured based on laboratory observations The interaction between braid and ring vortices has the effect of inhibiting the periodic self‐induced axis‐rotations observed in the case of isolated square vortex rings

Autogeneration of near-wall vortical structures in channel flow

Abstract: The evolution of a symmetric pair of quasistreamwise vortical structures extracted from the two‐point correlation tensor of turbulent channel flow data by a linear stochastic estimation procedure is studied through direct numerical simulation. It is observed that an Ω‐shaped hairpin vortex is formed quickly from this initial structure. Sufficiently strong hairpin vortices are observed to generate a hierarchy of secondary hairpin vortices, and the mechanism of their creation closely resembles the formation of the primary hairpin vortex. New streamwise vortices are also generated, and this process provides a means for a few vortical disturbances of adequate strength to reproduce themselves and eventually populate the near‐wall layer.

Irreversibility, mechanical entanglement and thermal melting in superconducting vortex crystals with point impurities

Abstract: We discuss the onset of irreversibility and entanglement of vortex lines in high T c superconductors due to point disorder and thermal fluctuations using a simplified cage model. A combination of Flory arguments, known results from directed polymers in random media, and a Lindemann criterion are used to estimate the field and temperature dependence of irreversibility, mechanical entanglement and thermal melting. The qualitative features of this dependence, including its nonmonotonicity when disorder is sufficiently strong, are in good agreement with recent experiments.

Moving glass phase of driven lattices.

Abstract: We study periodic lattices, such as vortex lattices, driven by an external force in a random pinning potential. We show that effects of static disorder persist even at large velocity. It results in a novel moving glass state which has analogies with the static Bragg glass. The lattice flows through well-defined, elastically coupled, static channels. We predict barriers to transverse motion resulting in finite transverse critical current. Experimental tests of the theory are proposed.

Three-Dimensional Organized Vortices above Flexible Water Plants

Abstract: Open channel flow over flexible bottom vegetation is studied experimentally. Two-component laser-Doppler velocimetry is used to measure the longitudinal and vertical velocity components of the flow field. It is observed that the temporally averaged velocity profile has an inflection below the top of the vegetation layer. The turbulence intensity and the Reynolds stress are largest near the top of the layer. An advanced technique using particle image velocimetry is developed to measure the instantaneous two-dimensional and three-dimensional flow field. The measurements reveal that organized vortices are generated intermittently above the vegetation. A side view of the vortex shows an elliptical shape, and the part of the three-dimensional structure above the vegetation layer consists of a pair of vortices inclined downward toward the front. The period of the vortex generation is explained in terms of linear stability analysis of flows with an inflectional velocity profile. The wavy motion of flexible vegetation is induced by the movement of such organized vortices.