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Showing papers on "Vortex published in 1997"


Journal ArticleDOI
TL;DR: In this paper, Jeong et al. used a conditional sampling scheme to extract the entire extent of dominant vortical structures near the wall in a numerically simulated turbulent channel flow.
Abstract: Coherent structures (CS) near the wall (i.e. y + ≤ 60) in a numerically simulated turbulent channel flow are educed using a conditional sampling scheme which extracts the entire extent of dominant vortical structures. Such structures are detected from the instantaneous flow field using our newly developed vortex definition (Jeong & Hussain 1995) - a region of negative λ 2, the second largest eigenvalue of the tensor SikSkj + ΩikΩkj - which accurately captures the structure details (unlike velocity-, vorticity- or pressure-based eduction). Extensive testing has shown that λ 2 correctly captures vortical structures, even in the presence of the strong shear occurring near the wall of a boundary layer. We have shown that the dominant near-wall educed (i.e. ensemble averaged after proper alignment) CS are highly elongated quasi-streamwise vortices; the CS are inclined 9° in the vertical (x, y)-plane and tilted ±4° in the horizontal (x, z)-plane. The vortices of alternating sign overlap in x as a staggered array; there is no indication near the wall of hairpin vortices, not only in the educed data but also in instantaneous fields. Our model of the CS array reproduces nearly all experimentally observed events reported in the literature, such as VITA, Reynolds stress distribution, wall pressure variation, elongated low-speed streaks, spanwise shear, etc. In particular, a phase difference (in space) between streamwise and normal velocity fluctuations created by CS advection causes Q4 ('sweep’) events to dominate Q2 ('ejection’) and also creates counter-gradient Reynolds stresses (such as Ql and Q3 events) above and below the CS. We also show that these effects are adequately modelled by half of a Batchelor's dipole embedded in (and decoupled from) a background shear U(y). The CS tilting (in the (x, z)-plane) is found to be responsible for sustaining CS through redistribution of streamwise turbulent kinetic energy to normal and spanwise components via coherent pressure-strain effects.

647 citations


Journal ArticleDOI
TL;DR: In this article, the physics of vortex axisymmetrization was examined further, with the goal of elucidating the dynamics of outward-propagating spiral bands in hurricanes.
Abstract: In this paper we examine further the physics of vortex axisymmetrization, with the goal of elucidating the dynamics of outward-propagating spiral bands in hurricanes. the basic shysics is illustrated most simply for stable vorticity monopoles on an f-plane. Unlike the dynamics of sheared disturbances in rectilinear shear flow, axisymmetrizing disturbances on a vortex are accompanied by outward-propagating vortex Rossby-waves whose restoring mechanism is associated with the radial gradient of storm vorticity. Expressions for both phase and group velocities are derived and verified; they confirm earlier speculations on the existence of vortex Rossbywaves in hurricanes. Effects of radially propagating vortex Rossby-waves on the mean vortex are also analysed. In conjunction with sustained injection of vorticity near the radius of maximum winds, these results reveal a new mechanism of vortex intensification. the basic theory is then applied to a hurricane-like vortex in a shallow-water asymmetric-balance model. the wave mechanics developed here shows promise in elucidating basic mechanisms of hurricane evolution and structure changes, such as the formation of secondary eye-walls. Radar observations possessing adequate temporal resolution are consistent with the predictions of this work, though more refined observations are needed to quantify further the impact of mesoscale banded disturbances on the evolution of the hurricane vortex.

569 citations


Journal ArticleDOI
TL;DR: In this article, the properties of light beams carrying phase singularities, or optical vortices, were studied both in theory and experiment, and the general rule for angular-momentum density distribution in a combined beam was established.
Abstract: We analyze the properties of light beams carrying phase singularities, or optical vortices. The transformations of topological charge during free-space propagation of a light wave, which is a combination of a Gaussian beam and a multiple charged optical vortex within a Gaussian envelope, are studied both in theory and experiment. We revise the existing knowledge about topological charge conservation, and demonstrate possible scenarios where additional vortices appear or annihilate during free propagation of such a combined beam. Coaxial interference of optical vortices is also analyzed, and the general rule for angular-momentum density distribution in a combined beam is established. We show that, in spite of any variation in the number of vortices in a combined beam, the total angular momentum is constant during the propagation.

491 citations


Journal ArticleDOI
TL;DR: In this article, the authors studied a class of non-Hermitian random quantum-mechanical problems and showed that the delocalization of flux lines from extended defects in type-II superconductors subject to a tilted external magnetic field occurs in response to a sufficiently large constant imaginary vector potential.
Abstract: A delocalization phenomenon is studied in a class of non-Hermitian random quantum-mechanical problems. Delocalization arises in response to a sufficiently large constant imaginary vector potential. The transition is related to depinning of flux lines from extended defects in type-II superconductors subject to a tilted external magnetic field. The physical meaning of the complex eigenvalues and currents of the non-Hermitian system is elucidated in terms of properties of tilted vortex lines. The singular behavior of the penetration length describing stretched exponential screening of a perpendicular magnetic field (transverse Meissner effect), the surface transverse magnetization, and the trapping length is determined near the flux-line depinning point.

443 citations


Journal ArticleDOI
TL;DR: The sound generated by vortex pairing in a two-dimensional compressible mixing layer is investigated in this article, where direct numerical simulations of the Navier-Stokes equations are used to compute both the near-field region and a portion of the acoustic field.
Abstract: The sound generated by vortex pairing in a two-dimensional compressible mixing layer is investigated Direct numerical simulations (DNS) of the Navier-Stokes equations are used to compute both the near-field region and a portion of the acoustic field The acoustic analogy due to Lilley (1974) is also solved with acoustic sources determined from the near-field data of the DNS It is shown that several commonly made simplifications to the acoustic sources can lead to erroneous predictions for the acoustic field Predictions based on the quadrupole form of the source terms derived by Goldstein (1976a, 1984) are in excellent agreement with the acoustic field from the DNS However, despite the low Mach number of the flow, the acoustic far field generated by the vortex pairings cannot be described by considering compact quadrupole sources The acoustic sources have the form of modulated wave packets and the acoustic far field is described by a superdirective model (Crighton & Huerre 1990) The presence of flow-acoustic interactions in the computed source terms causes the acoustic field predicted by the acoustic analogy to be very sensitive to small changes in the description of the source

368 citations


Journal ArticleDOI
TL;DR: In this paper, optical vortices that have quasi-point core functions, such as optical vortex solitons, may orbit one another at rates that are orders of magnitude larger than those with nonlocalized cores.
Abstract: Optical vortices in linear and nonlinear media may exhibit propagation dynamics similar to hydrodynamic vortex phenomena. Analytical and numerical methods are used to describe and investigate the interaction between vortices and the background field. We demonstrate that optical vortices that have quasi-point core functions, such as optical vortex solitons, may orbit one another at rates that are orders of magnitude larger than those with nonlocalized cores.

349 citations


Journal ArticleDOI
TL;DR: In this article, a global shallow-water model based on the flux-form semi-lagrangian scheme is described, which is a multidimensional semi-Lagrangian extension of the higher order Godunov-type finite-volume schemes (e.g., the piece-wise parabolic method).
Abstract: A global shallow-water model based on the flux-form semi-lagrangian scheme is described. the mass-conserving flux-form semi-Lagrangian scheme is a multidimensional semi-Lagrangian extension of the higher order Godunov-type finite-volume schemes (e.g., the piece-wise parabolic method). Unlike the piece-wise parabolic methodology, neither directional splitting nor a Riemann solver is involved. A reverse engineering procedure is introduced to achieve the goal of consistent transport of the absolute vorticity and the mass, and hence, the potential vorticity. Gravity waves are treated explicitly, in a manner that is consistent with the forward-in-time flux-form semi-Lagrangian transport scheme. Due to the finite-volume nature of the flux-form semi-lagrangian scheme and the application of the monotonicity constraint, which can be regarded as a subgrid-scale flux parametrization, essentially noise-free solutions are obtained without additional diffusion. Two selected shallow-water test cases proposed by Williamson et al. (1992) and a stratospheric vortex erosion simulation are presented. Discussions on the accuracy and computational efficiency are given based on the comparisons with a Eulerian spectral model and two advective-form semi-implicit semi-Lagrangian models.

331 citations


Journal ArticleDOI
TL;DR: The leading-edge vortex had a strong axial flow veolocity, which stabilized it and reduced its diamater as discussed by the authors, and the vortex separated from the wing at approximately 75 per cent of the wing length and fed vorticity into a large, tangled tip vortex.
Abstract: Recent flow visualisation experiments with the hawkmoth, Manduca sexta, revealed small but clear leading-edge vortex and a pronounced three-dimensional flow. Details of this flow pattern were studied with a scaled-up, robotic insect ('the flapper') that accurately mimicked the wing movements of a hovering hawkmoth. Smoke released from the leading edge of the flapper wing confirmed the existence of a small, strong and stable leading-edge vortex, increasing in size from wingbase to wingtip. Between 25 and 75 per cent of the wing length, its diameter increased approximately from 10 to 50 per cent of the wing chord. The leading-edge vortex had a strong axial flow veolocity, which stabilized it and reduced its diamater. The vortex separated from the wing at approximately 75 per cent of the wing length and thus fed vorticity into a large, tangled tip vortex. If the circulation of the leading-edge vortex were fully used for lift generation, it could support up to two-thirds of the hawkmoth's weight during the downstroke. The growth of this circulation with time and spanwise position clearly identify dynamic stall as the unsteady aerodynamic mechanism responsible for high lift production by hovering hawkmoths and possibly also by many other insect species.

328 citations


Journal ArticleDOI
Abstract: Water tunnel experiments were conducted to examine the effect of hole exit geometry on the near-field characteristics of crossflow jets. Hole shapes investigated were round, elliptical, square, and rectangular, all having the same cross-sectional area. Laser-induced fluorescence (LIF) and particle image velocimetry (PIV) were used.The vorticity around the circumference of the jet was tracked to identify its relative contributions to the nascent streamwise vortices, which evolve eventually into kidney vortices downstream. The distinction between sidewall vorticity and that from the leading and trailing edges, though blurred for a round hole, became clear for a square or a rectangular hole. The choice of non-circular holes also made it possible to reveal the unexpected double-decked structures of streamwise vortices and link them to the vorticity generated along the wall of the hole.The lowermost vortex pair of the double-decked structures, located beneath the jet, is what we call a ‘steady’ vortex pair. This pair is always present and has the same sense of rotation as the kidney vortices. The origin of these lower-deck vortices is the hole sidewall boundary layer: as the jet emanates from the hole, the crossflow forces the sidewall boundary layer to roll up into nascent kidney vortices. Here, hole width sets the lateral separation of these steady sidewall vortices.The vortices comprising the upper deck ride intermittently over the top of the ‘steady’ lower pair. The sense of rotation of these upper-deck vortices depends on hole geometry and can be the same as, or opposite to, the lower pair. The origin of the upper deck is the hole leading-edge boundary layer. This vorticity, initially aligned transverse to the crossflow direction, is realigned by the entrainment of crossflow momentum and thus induces a streamwise component of vorticity. Depending on hole geometry, this induced streamwise vorticity can be opposite to the lower-deck vortex pair. The opposing pair, called the ‘anti-kidney’ pair, competes with the nascent kidney-vortices and affects the jet lift-off. The hole trailing-edge boundary layer can likewise be turned toward the streamwise direction. In this case, the turning is caused by the strong reverse flow just downstream of the jet.In the present range of parameters, all hole boundary layer vorticity, regardless of its origin along the hole circumference, is found to influence the kidney vortices downstream.

317 citations


Journal ArticleDOI
TL;DR: In this paper, high-speed video imaging was used to characterize liquid flow patterns and velocity distributions inside liquid slugs were determined by particle imaging velocimetry (PIV).

296 citations


01 Feb 1997
TL;DR: In this paper, the roll-up of a wingtip vortex, at Reynolds number based on chord of 4.6 million, was studied with an emphasis on suction side and near wake measurements.
Abstract: The roll-up of a wingtip vortex, at Reynolds number based on chord of 4.6 million was studied with an emphasis on suction side and near wake measurements. The research was conducted in a 32 in. x 48 in. low-speed wind tunnel. The half-wing model had a semi-span of 36 in. a chord of 48 in. and a rounded tip. Seven-hole pressure probe measurements of the velocity field surrounding the wingtip showed that a large axial velocity of up to 1.77 U(sub infinity) developed in the vortex core. This level of axial velocity has not been previously measured. Triple-wire probes have been used to measure all components of the Reynolds stress tensor. It was determined from correlation measurements that meandering of the vortex was small and did not appreciably contribute to the turbulence measurements. The flow was found to be turbulent in the near-field (as high as 24 percent RMS w - velocity on the edge of the core) and the turbulence decayed quickly with streamwise distance because of the nearly solid body rotation of the vortex core mean flow. A streamwise variation of the location of peak levels of turbulence, relative to the core centerline, was also found. Close to the trailing edge of the wing, the peak shear stress levels were found at the edge of the vortex core, whereas in the most downstream wake planes they occurred at a radius roughly equal to one-third of the vortex core radius. The Reynolds shear stresses were not aligned with the mean strain rate, indicating that an isotropic-eddy-viscosity based prediction method cannot accurately model the turbulence in the cortex. In cylindrical coordinates, with the origin at the vortex centerline, the radial normal stress was found to be larger than the circumferential.

Journal ArticleDOI
TL;DR: In this paper, a class of subgrid stress models for large-eddy simulation (LES) is presented based on the idea of structure-based Reynolds-stress closure, where the subgrid structure of the turbulence is assumed to consist of stretched vortices whose orientations are determined by the resolved velocity field.
Abstract: A class of subgrid stress (SGS) models for large-eddy simulation (LES) is presented based on the idea of structure-based Reynolds-stress closure. The subgrid structure of the turbulence is assumed to consist of stretched vortices whose orientations are determined by the resolved velocity field. An equation which relates the subgrid stress to the structure orientation and the subgrid kinetic energy, together with an assumed Kolmogorov energy spectrum for the subgrid vortices, gives a closed coupling of the SGS model dynamics to the filtered Navier-Stokes equations for the resolved flow quantities. The subgrid energy is calculated directly by use of a local balance between the total dissipation and the sum of the resolved-scale dissipation and production by the resolved scales. Simple one- and two-vortex models are proposed and tested in which the subgrid vortex orientations are either fixed by the local resolved velocity gradients, or rotate in response to the evolution of the gradient field. These models are not of the eddy viscosity type. LES calculations with the present models are described for 32^(3) decaying turbulence and also for forced 32^(3) box turbulence at Taylor Reynolds numbers R-lambda in the range R(lambda)similar or equal to 30 (fully resolved) to R-lambda=infinity. The models give good agreement with experiment for decaying turbulence and produce negligible SGS dissipation for forced turbulence in the limit of fully resolved flow.

Journal ArticleDOI
TL;DR: In this article, simulation results of flow-induced vibrations of an infinitely long flexible cable at Reynolds numbers Re = 100 and Re = 200, corresponding to laminar and early transitional flow states, respectively, are presented.
Abstract: We present simulation results of flow-induced vibrations of an infinitely long flexible cable at Reynolds numbers Re = 100 and Re = 200, corresponding to laminar and early transitional flow states, respectively. The question as to what cable motions and flow patterns prevail is investigated in detail. Both standing wave and travelling wave responses are realized but in general the travelling wave is the preferred response. A standing wave cable response produces an interwoven pattern of vorticity, while a travelling wave cable response produces oblique vortex shedding. A sheared inflow produces a mixed standing wave/travelling wave cable response and chevron-like patterns with vortex dislocations in the wake. The lift force on the cable as well as its motion amplitudes are larger for the standing wave response. At Re = 200, the cable and wake response are no longer periodic, and the maximum amplitude of the cable is about one cylinder diameter, in agreement with experimental results.

Journal ArticleDOI
TL;DR: In this paper, the point vortex model is used for a variety of problems in hydrodynamics as well as in plasma physics, and the results from several calculations are shown and the sources of numerical errors are explained.

Journal ArticleDOI
TL;DR: In this paper, the development of Typhoon Irving is investigated using a variety of data, including special research aircraft data from the Tropical Cyclone Motion (TCM-92) experiment, objective analyses, satellite data, and traditional surface and sounding data.
Abstract: The development of Typhoon Irving is investigated using a variety of data, including special research aircraft data from the Tropical Cyclone Motion (TCM-92) experiment, objective analyses, satellite data, and traditional surface and sounding data. The development process is treated as a dry-adiabatic vortex dynamics problem, and it is found that environmental and mesoscale dynamics mutually enhance each other in a cooperative interaction during cyclone formation. Synoptic-scale interactions result in the evolution of the hostile environment toward more favorable conditions for storm development. Mesoscale interactions with the low-level, large-scale circulations and with other midlevel, mesoscale features result in development of vorticity in the midlevels and enhancement of the low-level vorticity associated with the developing surface cyclone. Multiple developments of mesoscale convective systems after the storm reaches tropical depression strength suggests both an increase in low-level confluence and a tendency toward recurrent development of associated mesoscale convective vortices. This is observed in both aircraft data and satellite imagery where subsequent interactions, including mergers with the low-level, tropical depression vortex, are observed. A contour dynamics experiment suggests that the movement of mesoscale convective systems in satellite imagery corresponds well to the movement of their associated midlevel vortices. Results from a simple baroclinic experiment show that the midlevel vortices affect the large-scale, low-level circulation in two ways: 1) initially, interactions between midlevel vortices produce a combined vortex of greater depth; 2) interaction between midlevel vortices and the low-level circulation produces a development downward of the midlevel vorticity. This strengthens the surface vortex and develops a more cohesive vortex that extends from the surface through the midtroposphere.

Journal ArticleDOI
TL;DR: The rollup of a wingtip vortex, at a Reynoldsnumber based on chord of 4.6 £ 10 6, was studied with an emphasis on SUctionside and very near-wakemeasurements (upto x/c = 0.678 downstreamofthetrailingedge) as mentioned in this paper.
Abstract: The rollup of a wingtip vortex, at a Reynoldsnumber based on chord of 4 .6 £ 10 6 , was studied with an emphasis onsuctionsideandverynear-wakemeasurements(upto x/c = 0.678 downstreamofthetrailingedge).Theresearch was conducted in a 32 £ 48 in. (0.81 £ 1.22 m), low-speed wind tunnel. The rectangular half-wing model had a semispan of 36 in. (0.91 m), a chord of 48 in. (1.22 m), and a rounded tip. Seven-hole pressure probe measurements of the velocity ® eld surrounding the wingtip showed that a large axial velocity up to 1.77 U 1 developed in the vortex core. This high a level of core axial velocity has not been measured previously. Triple-wire probes were used to measure all components of the Reynolds stress tensor. It was determined from correlation measurements that

Journal ArticleDOI
TL;DR: Visualization experiments with Manduca sexta have revealed the presence of a leading-edge vortex and a highly three-dimensional flow pattern, confirming that the downstroke is the main provider of lift force.
Abstract: Visualization experiments with Manduca sexta have revealed the presence of a leading-edge vortex and a highly three-dimensional flow pattern. To further investigate this important discovery, a scaled-up robotic insect was built (the 'flapper') which could mimic the complex movements of the wings of a hovering hawkmoth. Smoke released from the leading edge of the flapper wing revealed a small but strong leading-edge vortex on the downstroke. This vortex had a high axial flow velocity and was stable, separating from the wing at approximately 75 per cent of the wing length. It connected to a large, tangled tip vortex, extending back to a combining stopping and starting vortex from pronation. At the end of the downstroke, the wake could be approximated as one vortex ring per wing. Based on the size and velocity of the vortex rings, the mean lift force during the downstroke was estimated to be about 1.5 times the body weight of a hawkmoth, confirming that the downstroke is the main provider of lift force.

Journal ArticleDOI
TL;DR: The aerodynamic mechanisms employed durng the flight of the hawkmoth, Manduca sexta, have been investigated through smoke visualization studies with tethered moths and stereophotographs suggest that the bound circulation may not be reversed between half strokes at the fastest flight speeds.
Abstract: The aerodynamic mechanisms employed durng the flight of the hawkmoth, Manduca sexta , have been investigated through smoke visualization studies with tethered moths. Details of the flow around the wings and of the overall wake structure were recorded as stereophotographs and high–speed video sequences. The changes in flow which accompanied increases in flight speed from 0.4 to 5.7 m s−1 were analysed. The wake consists of an alternating series of horizontal and vertical vortex rings which are generated by successive down– and upstrokes, respectively. The downstroke produces significantly more lift than the upstroke due to a leading–edge vortex which is stabilized by a radia flow moving out towards the wingtip. The leading–edge vortex grew in size with increasing forward flight velocity. Such a phenomenon is proposed as a likely mechanism for lift enhancement in many insect groups. During supination, vorticity is shed from the leading edge as postulated in the ‘flex’ mechanism. This vorticity would enhance upstroke lift if it was recaptured diring subsequent translation, but it is not. Instead, the vorticity is left behind and the upstroke circulation builds up slowly. A small jet provides additional thrust as the trailing edges approach at the end of the upstroke. The stereophotographs also suggest that the bound circulation may not be reversed between half strokes at the fastest flight speeds.

Journal ArticleDOI
TL;DR: In this paper, low-temperature decaying superfluid turbulence is studied using the nonlinear Schrodinger equation in the geometry of the Taylor-Green (TG) vortex flow with resolutions up to ${512}^{3}$.
Abstract: Low-temperature decaying superfluid turbulence is studied using the nonlinear Schr\"odinger equation in the geometry of the Taylor-Green (TG) vortex flow with resolutions up to ${512}^{3}$. The rate of (irreversible) kinetic energy transfer in the superfluid TG vortex is found to be comparable to that of the viscous TG vortex. At the moment of maximum dissipation, the energy spectrum of the superflow has an inertial range compatible with Kolmogorov's scaling. Physical-space visualizations show that the vorticity dynamics of the superflow is similar to that of the viscous flow, including vortex reconnection. The implications to experiments in low-temperature helium are discussed.

Journal ArticleDOI
TL;DR: Schlieren et al. as mentioned in this paper presented a quantitative, experimental study of a single, sonic, underexpanded, transverse, round jet injected into a Mach 1.6 mainframe.
Abstract: This paper presents a quantitative, experimental study of a single, sonic, underexpanded, transverse, round jet injected into a Mach 1.6 crosse ow. This investigation is applicable to studies of supersonic combustors, thrust vector control of rocket nozzles, the cooling of nozzle walls, and jet reaction force prediction. Schlieren/shadowgraph photography and two-component, frequency preshifted laser Doppler velocimetry are used to visualize the e ow and to measure three mean velocity components, e ve of the six kinematic Reynolds stresses, and turbulent kinetic energy at over 4000 locations throughout the e owe eld. The study focuses on the transverse, midline plane and on two crosse ow planes. These measurements are used to study the size and orientation of the recirculation regions upstream and downstream of the jet; the structure and strength of the bow shock, barrel shock, and Mach disk; the structure, strength, and development of the kidney-shaped, counter-rotating vortex pair; the growth of the annular shear layer between the jet plume and the crosse ow; and the growth of the boundary layer beneath the jet. In addition, the present study provides validation data for analytical and numerical predictions of the transverse jet e owe eld.

Journal ArticleDOI
TL;DR: In this paper, the authors performed non-intrusive measurements of the streamwise velocity in turbulent round jets in air by recording short-time displacements and distorsions of very thin tagging lines written spanwise into the flow.
Abstract: Non-intrusive measurements of the streamwise velocity in turbulent round jets in air are performed by recording short-time displacements and distorsions of very thin tagging lines written spanwise into the flow. The lines are written by Raman-exciting oxygen molecules and are interrogated by laser-induced electronic fluorescence (relief). This gives access to the spatial structure of transverse velocity increments without recourse to the Taylor hypothesis. The resolution is around 25 μm, less than twice the Kolmogorov scale η for the experiments performed (with Rλ≈360–600).The technique is validated by comparison with results obtained from other techniques for longitudinal or transverse structure functions up to order 8. The agreement is consistent with the estimated errors – a few percent on exponents determined by extended-self-similarity – and indicates significant departures from Kolmogorov (1941) scaling.Probability distribution functions of transverse velocity increments Δu over separations down to 1:8η are reported for the first time. Violent events, with Δu comparable to the r.m.s. turbulent velocity fluctuation, are found to take place with statistically significant probabilities. The shapes of the corresponding lines suggest the effect of intense slender vortex filaments.

Journal ArticleDOI
TL;DR: In this article, the axisymmetric vortex breakdown phenomenon in high-Reynolds-number swirling flows in a constant-area pipe is investigated. And the results are established through a rigorous mathematical analysis and provide a solid theoretical understanding of the dynamics of a swirling flow.
Abstract: This paper provides a new study of the axisymmetric vortex breakdown phenomenon. Our approach is based on a thorough investigation of the axisymmetric unsteady Euler equations which describe the dynamics of a swirling flow in a finite-length constant-area pipe. We study the stability characteristics as well as the time-asymptotic behaviour of the flow as it relates to the steady-state solutions. The results are established through a rigorous mathematical analysis and provide a solid theoretical understanding of the dynamics of an axisymmetric swirling flow. The stability and steady-state analyses suggest a consistent explanation of the mechanism leading to the axisymmetric vortex breakdown phenomenon in high-Reynolds-number swirling flows in a pipe. It is an evolution from an initial columnar swirling flow to another relatively stable equilibrium state which represents a flow around a separation zone. This evolution is the result of the loss of stability of the base columnar state when the swirl ratio of the incoming flow is near or above the critical level.

Journal ArticleDOI
TL;DR: In this paper, a van der Pol equation was used to predict the vortex-excited vibrations of flexible cylindrical structures, and the model provided an asymptotic, self-limiting structural response at zero structural damping.

Proceedings ArticleDOI
29 Jun 1997
TL;DR: In this article, a two-dimensional, incompressible simulation of synthetic jet actuators is performed and three different velocity distributions along the orifice are examined and all three distributions give qualitatively similar flow features.
Abstract: A promising design of fluid actuators for flow control is investigated. Two-dimensional, incompressible simulations of synthetic jet actuators are performed. A modeled boundary condition is developed for the actuator. Three different velocity distributions along the orifice are examined and all three distributions give qualitatively similar flow features. Both laminar and turbulent jets are investigated. Results show very good agreement with experimental measurements. A jet flow develops, even though no net mass flow is introduced. Pairs of counterrotating vortices are observed near the jet exit as are observed in the experiments. Comparison of the synthetic jet computations is made with both pulsed jets and steady jets. Little difference is seen in the turbulent flow regime, but the laminar flow shows markedly different results between a steady jet and a synthetic jet. It has been demonstrated that the modeled boundary condition captures the essential features of the jet without modeling the detailed breakdown of the vortices or the details of the cavity flow. Nomenclature

Journal ArticleDOI
TL;DR: In this paper, a simulation of the nonlinear Schrodinger equation (NLSE) was performed on the Taylor-Green (TG) vortex and the energy conservation relations were derived in hydrodynamic form.
Abstract: Superfluid turbulence is studied using numerical simulations of the nonlinear Schrodinger equation (NLSE), which is the correct equation of motion for superflows at low temperatures. This equation depends on two parameters: the sound velocity and the coherence length. It naturally contains nonsingular quantized vortex lines. The NLSE mass, momentum, and energy conservation relations are derived in hydrodynamic form. The total energy is decomposed into an incompressible kinetic part, and other parts that correspond to acoustic excitations. The corresponding energy spectra are defined and computed numerically in the case of the two-dimensional vortex solution. A preparation method, generating initial data reproducing the vorticity dynamics of any three-dimensional flow with Clebsch representation is given and is applied to the Taylor–Green (TG) vortex. The NLSE TG vortex is studied with resolutions up to 5123. The energetics of the flow is found to be remarkably similar to that of the viscous TG vortex. The...

Journal ArticleDOI
TL;DR: In this paper, fine-and microstructure profiles collected over Fieberling Seamount at 32°26′N in the eastern North Pacific reveal a variety of intensified baroclinic motions driven by astronomical diurnal tides.
Abstract: Fine- and microstructure profiles collected over Fieberling Seamount at 32°26′N in the eastern North Pacific reveal a variety of intensified baroclinic motions driven by astronomical diurnal tides. The forced response consists of three phenomena coexisting in a layer 200 m thick above the summit plain: (i) an anticyclonic vortex cap of core relative vorticity − 0.5f, (ii) diurnal fluctuations of ±15 cm s−1 amplitude and 200-m vertical wavelength, and (iii) turbulence levels corresponding to an eddy diffusivity κe ≅ 10 × 10−4 m2 s−1. The vortex cannot be explained by Taylor–Proudman dynamics because of its − 0.3fN2 negative potential vorticity anomaly. The ±0.3f fortnightly cycle in the vortex’s strength suggests that it is at least partially maintained against dissipative erosion by tidal rectification. The diurnal motions are slightly subinertial, turning clockwise in time and counterclockwise with depth over the summit plain. They also exhibit a fortnightly cycle in their amplitude, pointing to...

Book
31 Aug 1997
TL;DR: This chapter discusses the development and dynamics of Coherent Structures in Near-Wall Turbulence: A New Look, and how Streamwise Rolls and Streaks Self-Sustain in a Shear Flow.
Abstract: Chapter 1 A Brief History of Boundary Layer Structure Research Chapter 2 Sustaining Mechanisms of Turbulent Boundary Layers: The Role of Vortex Development and Interactions Chapter 3 An Experimental Model for Near-Wall Structure Chapter 4 Vortex Kinematics, Dynamics, and Turbulent Momentum Transport in Wall Bounded Flows Chapter 5 The Role of Wall Vortices in Producing Turbulence Chapter 6 Formation of Coherent Hairpin Packets in Wall Turbulence Chapter 7 Self-sustaining Traits of Near-Wall Motions Underlying Boundary Layer Stress Transport Chapter 8 A View of the Structure of Turbulent Boundary Layers Chapter 9 Reynolds Stress Producing Motions in Smooth and Rough Wall Boundary Layers Chapter 10 Wall Layer Microturbulence Phenomenological Model and a Semi-Markov Probability Predictive Model for Active Control of Turbulent Boundary Layers Chapter 11 The Persistence of Viscous Effects in the Overlap Region, and the Mean Velocity in Turbulent Pipe and Channel Flows Chapter 12 Hierarchical, Self-Sustained Energy Cascade to Small Scales in Wall-bounded Shear Turbulence Chapter 13 How Streamwise Rolls and Streaks Self-Sustain in a Shear Flow Chapter 14 Dynamics of Coherent Structures in Wall Bounded Turbulence Chapter 15 On the Dynamics of Turbulent Boundary Layers Chapter 16 Genesis and Dynamics of Coherent Structures in Near-Wall Turbulence: A New Look

Journal ArticleDOI
01 Jul 1997-Icarus
TL;DR: In this article, the authors present a new mechanism for number density enhancement and size segregation of particles in a nonlaminar accretion disk, e.g., the solar nebula.

Journal ArticleDOI
L Civale1
TL;DR: In this article, the influence of the angular dispersion (splay) of the tracks on vortex pinning and creep is discussed, and measurements of the temperature, field and defect density dependence of the persistent currents and their time relaxation in YBCO are presented.
Abstract: Columnar defects introduced by irradiation with very energetic heavy ions are the most effective pinning centres for flux lines in high-temperature superconductors. This correlated disorder generates large increases in the critical current densities and expansion of the irreversible regime in and the various Bi- and Tl-based compounds. In single crystals and thin films, the pinning enhancement is strongly angular-dependent, and maximizes when the applied magnetic field is parallel to the amorphous latent tracks. In contrast, in the much more anisotropic Bi- and Tl- based materials this unidirectional anisotropy is very small due to the quasi two-dimensional character of the vortices. Some of the extensive experimental studies on this topic are reviewed. Measurements of the temperature, field and defect density dependence of the persistent currents and their time relaxation in YBCO are presented. The analysis of these results based on recent theoretical models permits the identification of various pinning and creep regimes. Studies in other compounds, including technologically relevant Bi-based tapes, are summarized. The influence of the angular dispersion (splay) of the tracks on vortex pinning and creep is discussed.

Journal ArticleDOI
TL;DR: In this article, the authors study numerically the simplest model of two incompressible, immiscible fluids shearing past one another, where the fluids are two-dimensional, inviscid, irrotational, density matched and separated by a sharp interface under a surface tension.
Abstract: We study numerically the simplest model of two incompressible, immiscible fluids shearing past one another. The fluids are two-dimensional, inviscid, irrotational, density matched, and separated by a sharp interface under a surface tension. The nonlinear growth and evolution of this interface is governed by only the competing effects of the Kelvin–Helmholtz instability and the dispersion due to surface tension. We have developed new and highly accurate numerical methods designed to treat the difficulties associated with the presence of surface tension. This allows us to accurately simulate the evolution of the interface over much longer times than has been done previously. A surprisingly rich variety of behavior is found. For small Weber numbers, where there are no unstable length-scales, the flow is dispersively dominated and oscillatory behavior is observed. For intermediate Weber numbers, where there are only a few unstable length-scales, the interface forms elongating and interpenetrating fingers of fluid. At larger Weber numbers, where there are many unstable scales, the interface rolls-up into a “Kelvin-Helmholtz” spiral with its late evolution terminated by the collision of the interface with itself, forming at that instant bubbles of fluid at the core of the spiral. Using locally refined grids, this singular event (a “topological” or “pinching” singularity) is studied carefully. Our computations suggest at least a partial conformance to a local self-similar scaling. For fixed initial data, the pinching singularity times decrease as the surface tension is reduced, apparently towards the singularity time associated with the zero surface tension problem, as studied by Moore and others. Simulations from more complicated, multi-modal initial data show the evolution as a combination of these fingers, spirals, and pinches.