Showing papers on "Vortex published in 1999"

Mechanisms for generating coherent packets of hairpin vortices in channel flow

Abstract: The evolution of a single hairpin vortex-like structure in the mean turbulent field of a low-Reynolds-number channel flow is studied by direct numerical simulation. The structure of the initial three-dimensional vortex is extracted from the two-point spatial correlation of the velocity field by linear stochastic estimation given a second-quadrant ejection event vector. Initial vortices having vorticity that is weak relative to the mean vorticity evolve gradually into omega-shaped vortices that persist for long times and decay slowly. As reported in Zhou, Adrian & Balachandar (1996), initial vortices that exceed a threshold strength relative to the mean flow generate new hairpin vortices upstream of the primary vortex. The detailed mechanisms for this upstream process are determined, and they are generally similar to the mechanisms proposed by Smith et al. (1991), with some notable differences in the details. It has also been found that new hairpins generate downstream of the primary hairpin, thereby forming, together with the upstream hairpins, a coherent packet of hairpins that propagate coherently. This is consistent with the experimental observations of Meinhart & Adrian (1995). The possibility of autogeneration above a critical threshold implies that hairpin vortices in fully turbulent fields may occur singly, but they more often occur in packets. The hairpins also generate quasi-streamwise vortices to the side of the primary hairpin legs. This mechanism bears many similarities to the mechanisms found by Brooke & Hanratty (1993) and Bernard, Thomas & Handler (1993). It provides a means by which new quasi-streamwise vortices, and, subsequently, new hairpin vortices can populate the near-wall layer.

Vortices in a Bose-Einstein Condensate

Abstract: We have created vortices in two-component Bose-Einstein condensates. The vortex state was created through a coherent process involving the spatial and temporal control of interconversion between the two components. Using an interference technique, we map the phase of the vortex state to confirm that it possesses angular momentum. We can create vortices in either of the two components and have observed differences in the dynamics and stability.

Drift waves and transport

Abstract: Drift waves occur universally in magnetized plasmas producing the dominant mechanism for the transport of particles, energy and momentum across magnetic field lines. A wealth of information obtained from quasistationary laboratory experiments for plasma confinement is reviewed for drift waves driven unstable by density gradients, temperature gradients and trapped particle effects. The modern understanding of Bohm transport and the role of sheared flows and magnetic shear in reducing the transport to the gyro-Bohm rate are explained and illustrated with large scale computer simulations. The types of mixed wave and vortex turbulence spontaneously generated in nonuniform plasmas are derived with reduced magnetized fluid descriptions. The types of theoretical descriptions reviewed include weak turbulence theory, Kolmogorov anisotropic spectral indices, and the mixing length. A number of standard turbulent diffusivity formulas are given for the various space-time scales of the drift-wave turbulent mixing.

The autonomous cycle of near-wall turbulence

Abstract: Numerical experiments on modified turbulent channels at moderate Reynolds numbers are used to differentiate between several possible regeneration cycles for the turbulent fluctuations in wall-bounded flows. It is shown that a cycle exists which is local to the near-wall region and does not depend on the outer flow. It involves the formation of velocity streaks from the advection of the mean profile by streamwise vortices, and the generation of the vortices from the instability of the streaks. Interrupting any of those processes leads to laminarization. The presence of the wall seems to be only necessary to maintain the mean shear. The generation of secondary vorticity at the wall is shown to be of little importance in turbulence generation under natural circumstances. Inhibiting its production increases turbulence intensity and drag.

Flow control with noncircular jets

Abstract: Noncircular jets have been the topic of extensive research in the last fifteen years. These jets were identified as an efficient technique of passive flow control that allows significant improvements of performance in various practical systems at a relatively low cost because noncircular jets rely solely on changes in the geometry of the nozzle. The applications of noncircular jets discussed in this review include improved large- and small-scale mixing in low- and high-speed flows, and enhanced combustor performance, by improving combustion efficiency, reducing combustion instabilities and undesired emissions. Additional applications include noise suppression, heat transfer, and thrust vector control (TVC). The flow patterns associated with noncircular jets involve mechanisms of vortex evolution and interaction, flow instabilities, and fine-scale turbulence augmentation. Stability theory identified the effects of initial momentum thickness distribution, aspect ratio, and radius of curvature on the initial flow evolution. Experiments revealed complex vortex evolution and interaction related to selfinduction and interaction between azimuthal and axial vortices, which lead to axis switching in the mean flow field. Numerical simulations described the details and clarified mechanisms of vorticity dynamics and effects of heat release and reaction on noncircular jet behavior.

Locomotor forces on a swimming fish: three-dimensional vortex wake dynamics quantified using digital particle image velocimetry.

Abstract: Quantifying the locomotor forces experienced by swimming fishes represents a significant challenge because direct measurements of force applied to the aquatic medium are not feasible. However, using the technique of digital particle image velocimetry (DPIV), it is possible to quantify the effect of fish fins on water movement and hence to estimate momentum transfer from the animal to the fluid. We used DPIV to visualize water flow in the wake of the pectoral fins of bluegill sunfish (Lepomis macrochirus) swimming at speeds of 0.5-1.5 L s(-)(1), where L is total body length. Velocity fields quantified in three perpendicular planes in the wake of the fins allowed three-dimensional reconstruction of downstream vortex structures. At low swimming speed (0.5 L s(-)(1)), vorticity is shed by each fin during the downstroke and stroke reversal to generate discrete, roughly symmetrical, vortex rings of near-uniform circulation with a central jet of high-velocity flow. At and above the maximum sustainable labriform swimming speed of 1.0 L s(-)(1), additional vorticity appears on the upstroke, indicating the production of linked pairs of rings by each fin. Fluid velocity measured in the vicinity of the fin indicates that substantial spanwise flow during the downstroke may occur as vortex rings are formed. The forces exerted by the fins on the water in three dimensions were calculated from vortex ring orientation and momentum. Mean wake-derived thrust (11.1 mN) and lift (3.2 mN) forces produced by both fins per stride at 0.5 L s(-)(1) were found to match closely empirically determined counter-forces of body drag and weight. Medially directed reaction forces were unexpectedly large, averaging 125 % of the thrust force for each fin. Such large inward forces and a deep body that isolates left- and right-side vortex rings are predicted to aid maneuverability. The observed force balance indicates that DPIV can be used to measure accurately large-scale vorticity in the wake of swimming fishes and is therefore a valuable means of studying unsteady flows produced by animals moving through fluids.

Polygonal Eyewalls, Asymmetric Eye Contraction, and Potential Vorticity Mixing in Hurricanes

Abstract: Hurricane eyewalls are often observed to be nearly circular structures, but they are occasionally observed to take on distinctly polygonal shapes. The shapes range from triangles to hexagons and, while they are often incomplete, straight line segments can be identified. Other observations implicate the existence of intense mesovortices within or near the eye region. Is there a relation between polygonal eyewalls and hurricane mesovortices? Are these phenomena just curiosities of the hurricane’s inner-core circulation, or are they snapshots of an intrinsic mixing process within or near the eye that serves to determine the circulation and thermal structure of the eye? As a first step toward understanding the asymmetric vorticity dynamics of the hurricane’s eye and eyewall region, these issues are examined within the framework of an unforced barotropic nondivergent model. Polygonal eyewalls are shown to form as a result of barotropic instability near the radius of maximum winds. After reviewing linear theory, simulations with a high-resolution pseudospectral numerical model are presented to follow the instabilities into their nonlinear regime. When the instabilities grow to finite amplitude, the vorticity of the eyewall region pools into discrete areas, creating the appearance of polygonal eyewalls. The circulations associated with these pools of vorticity suggest a connection to hurricane mesovortices. At later times the vorticity is ultimately rearranged into a nearly monopolar circular vortex. While the evolution of the finescale vorticity field is sensitive to the initial condition, the macroscopic end-states are found to be similar. In fact, the gross characteristics of the numerically simulated end-states are predicted analytically using a generalization of the minimum enstrophy hypothesis. In an effort to remove some of the weaknesses of the minimum enstrophy approach, a maximum entropy argument developed previously for rectilinear shear flows is extended to the vortex problem, and end-state solutions in the limiting case of tertiary mixing are obtained. Implications of these ideas for real hurricanes are discussed.

Large-eddy simulations of a round jet in crossflow

Abstract: This paper reports on a series of large-eddy simulations of a round jet issuing normally into a crossflow. Simulations were performed at two jet-to-crossflow velocity ratios, 2.0 and 3.3, and two Reynolds numbers, 1050 and 2100, based on crossflow velocity and jet diameter. Mean and turbulent statistics computed from the simulations match experimental measurements reasonably well. Large-scale coherent structures observed in experimental flow visualizations are reproduced by the simulations, and the mechanisms by which these structures form are described. The effects of coherent structures upon the evolution of mean velocities, resolved Reynolds stresses, and turbulent kinetic energy along the centreplane are discussed. In this paper, the ubiquitous far-field counter-rotating vortex pair is shown to originate from a pair of quasi-steady ‘hanging’ vortices. These vortices form in the skewed mixing layer that develops between jet and crossflow fluid on the lateral edges of the jet. Axial flow through the hanging vortex transports vortical fluid from the near-wall boundary layer of the incoming pipe flow to the back side of the jet. There, the hanging vortex encounters an adverse pressure gradient and breaks down. As this breakdown occurs, the vortex diameter expands dramatically, and a weak counter-rotating vortex pair is formed that is aligned with the jet trajectory.

Transfer of energy to two-dimensional large scales in forced, rotating three-dimensional turbulence

Abstract: Forced turbulence in a rotating frame is studied using numerical simulations in a triply periodic box. The random forcing is three dimensional and localized about an intermediate wavenumber kf. The results show that energy is transferred to scales larger than the forcing scale when the rotation rate is large enough. The scaling of the energy spectrum approaches E(k)∝k−3 for k

Jet characteristics of a plunging airfoil

Abstract: Water-tunnel tests of a NACA 0012 airfoil that was oscillated sinusoidally in plunge are described. The flowered downstream of the airfoil was explored by dye flow visualization and single-component laser Doppler velocimetry (LDV) measurements for a range of freestream speeds, frequencies, and amplitudes of oscillation. The dye visualizations show that the vortex patterns generated by the plunging airfoil change from drag-producing wake flows to thrust-producing jet flows as soon as the ratio of maximum plunge velocity to freestream speed, i.e., the nondimensional plunge velocity, exceeds approximately 0.4. The LDV measurements show that the nondimensional plunge velocity is the appropriate parameter to collapse the maximum streamwise velocity data covering a nondimensional plunge velocity range from 0.18 to 9.3

Transport by coherent barotropic vortices

Abstract: ▪ Abstract This article reviews the transport properties of coherent vortices in rotating barotropic flows. It is shown that vortices induce regular Lagrangian motion inside their cores and are highly impermeable to inward and outward particle fluxes. Passive tracers can be trapped inside vortex cores for long times and are transported by the vortex motion over large distances. Absolute dispersion in vortex-dominated flows is discussed by studying particle dynamics in 2D turbulence, point-vortex systems, and subsurface float trajectories in the ocean. Finally, it is shown that anticyclonic coherent vortices in cyclonically rotating reference frames can concentrate heavy impurities (e.g. dust grains) in their cores. This process may play an important role in the formation of planetesimals in the early solar nebula.

Non-Gaussianity and coherent vortex simulation for two-dimensional turbulence using an adaptive orthogonal wavelet basis

Abstract: We decompose turbulent flows into two orthogonal parts: a coherent, inhomogeneous, non-Gaussian component and an incoherent, homogeneous, Gaussian component. The two components have different probability distributions and different correlations, hence different scaling laws. This separation into coherent vortices and incoherent background flow is done for each flow realization before averaging the results and calculating the next time step. To perform this decomposition we have developed a nonlinear scheme based on an objective threshold defined in terms of the wavelet coefficients of the vorticity. Results illustrate the efficiency of this coherent vortex extraction algorithm. As an example we show that in a 256 2 computation 0.7% of the modes correspond to the coherent vortices responsible for 99.2% of the energy and 94% of the enstrophy. We also present a detailed analysis of the nonlinear term, split into coherent and incoherent components, and compare it with the classical separation, e.g., used for large eddy simulation, into large scale and small scale components. We then propose a new method, called coherent vortex simulation ~CVS!, designed to compute and model two-dimensional turbulent flows using the previous wavelet decomposition at each time step. This method combines both deterministic and statistical approaches: ~i! Since the coherent vortices are out of statistical equilibrium, they are computed deterministically in a wavelet basis which is remapped at each time step in order to follow their nonlinear motions. ~ii! Since the incoherent background flow is homogeneous and in statistical equilibrium, the classical theory of homogeneous turbulence is valid there and we model statistically the effect of the incoherent background on the coherent vortices. To illustrate the CVS method we apply it to compute a two-dimensional turbulent mixing layer. © 1999 American Institute of Physics. @S1070-6631~99!04608-5# I. INTRODUCTION In this article we introduce a new approach for computing turbulence which is based on the observation that turbulent flows contain both an organized part ~the coherent vortices! and a random part ~the incoherent background flow !. The direct computation of fully developed turbulent flows involves such a large number of degrees of freedom that it is out of reach for the present and near future. Therefore some statistical modeling is needed to drastically reduce the computational cost. The problem is difficult because the statistical structure of turbulence is not Gaussian, although most statistical models assume simple Gaussian statistics. The approach we propose is to split the problem in two: ~i! the determinist computation of the non-Gaussian components of the flow and ~ii! the statistical modeling of the Gaussian components ~which can be done easily since they are completely characterized by their mean and variance! .W e

Topics in bifurcation theory and applications

Abstract: Centre manifolds normal forms, and bifurcations of vector fields near critical points - unperturbed vector fields perturbed vector fields Couette-Taylor problem - formulation of the problem Couette flow bifurcations from Couette flow bifurcations from Taylor vortex flow centre manifolds, normal forms, and bifurcations of vector fields near closed orbits - preliminaries adaptation of Floquet thoery unperturbed case perturbed case.

Laminar flow past a rotating circular cylinder

Abstract: The present study numerically investigates two-dimensional laminar flow past a circular cylinder rotating with a constant angular velocity, for the purpose of controlling vortex shedding and understanding the underlying flow mechanism. Numerical simulations are performed for flows with Re=60, 100, and 160 in the range of 0⩽α⩽2.5, where α is the circumferential speed at the cylinder surface normalized by the free-stream velocity. Results show that the rotation of a cylinder can suppress vortex shedding effectively. Vortex shedding exists at low rotational speeds and completely disappears at α>αL, where αL is the critical rotational speed which shows a logarithmic dependence on Re. The Strouhal number remains nearly constant regardless of α while vortex shedding exists. With increasing α, the mean lift increases linearly and the mean drag decreases, which differ significantly from those predicted by the potential flow theory. On the other hand, the amplitude of lift fluctuation stays nearly constant with in...

Predicted signatures of rotating Bose–Einstein condensates

Abstract: Superfluids are distinguished from normal fluids by their peculiar response1 to rotation: circulating flow in superfluid helium2,3, a strongly coupled Bose liquid, can appear only as quantized vortices4,5,6. The newly created Bose–Einstein condensates7,9—clouds of millions of ultracold, weakly interacting alkali-metal atoms that occupy a single quantum state—offer the possibility of investigating superfluidity in the weak-coupling regime. An outstanding question is whether Bose–Einstein condensates exhibit a mesoscopic quantum analogue of the macroscopic vortices in superfluids, and what its experimental signature would be. Here we report calculations of the low-energy states of a rotating, weakly interacting Bose gas. We find a succession of transitions between stable vortex patterns of differing symmetries that are in general qualitative agreement with observations5 of rotating superfluid helium, a strong-coupling superfluid. Counterintuitively, the angular momentum per particle is not quantized. Some angular momenta are forbidden, corresponding to asymmetrical unstable states that provide a physical mechanism for the entry of vorticity into the condensate.

Secondary instability of crossflow vortices and swept-wing boundary-layer transition

Abstract: Crossflow instability of a three-dimensional boundary layer is a common cause of transition in swept-wing flows. The boundary-layer flow modified by the presence of finite-amplitude crossflow modes is susceptible to high-frequency secondary instabilities, which are believed to harbinger the onset of transition. The role of secondary instability in transition prediction is theoretically examined for the recent swept-wing experimental data by Reibert et al. (1996). Exploiting the experimental observation that the underlying three-dimensional boundary layer is convectively unstable, non-linear parabolized stability equations are used to compute a new basic state for the secondary instability analysis based on a two-dimensional eigenvalue approach. The predicted evolution of stationary crossflow vortices is in close agreement with the experimental data. The suppression of naturally dominant crossflow modes by artificial roughness distribution at a subcritical spacing is also confirmed. The analysis reveals a number of secondary instability modes belonging to two basic families which, in some sense, are akin to the ‘horseshoe’ and ‘sinuous’ modes of the Gortler vortex problem. The frequency range of the secondary instability is consistent with that measured in earlier experiments by Kohama et al. (1991), as is the overall growth of the secondary instability mode prior to the onset of transition (e.g. Kohama et al. 1996). Results indicate that the N-factor correlation based on secondary instability growth rates may yield a more robust criterion for transition onset prediction in comparison with an absolute amplitude criterion that is based on primary instability alone.

Evolution of a wave packet into vortex loops in a laminar separation bubble

Abstract: A laminar boundary layer develops in a favourable pressure gradient where the velocity profiles asymptote to the Falkner & Skan similarity solution. Flying-hot-wire measurements show that the layer separates just downstream of a subsequent region of adverse pressure gradient, leading to the formation of a thin separation bubble. In an effort to gain insight into the nature of the instability mechanisms, a small-magnitude impulsive disturbance is introduced through a hole in the test surface at the pressure minimum. The facility and all operating procedures are totally automated and phase-averaged data are acquired on unprecedently large and spatially dense measurement grids. The evolution of the disturbance is tracked all the way into the reattachment region and beyond into the fully turbulent boundary layer. The spatial resolution of the data provides a level of detail that is usually associated with computations.Initially, a wave packet develops which maintains the same bounded shape and form, while the amplitude decays exponentially with streamwise distance. Following separation, the rate of decay diminishes and a point of minimum amplitude is reached, where the wave packet begins to exhibit dispersive characteristics. The amplitude then grows exponentially and there is an increase in the number of waves within the packet. The region leading up to and including the reattachment has been measured with a cross-wire probe and contours of spanwise vorticity in the centreline plane clearly show that the wave packet is associated with the cat's eye pattern that is a characteristic of Kelvin–Helmholtz instability. Further streamwise development leads to the formation of roll-ups and contour surfaces of vorticity magnitude show that they are three-dimensional. Beyond this point, the behaviour is nonlinear and the roll-ups evolve into a group of large-scale vortex loops in the vicinity of the reattachment. Closely spaced cross-wire measurements are continued in the downstream turbulent boundary layer and Taylor's hypothesis is applied to data on spanwise planes to generate three-dimensional velocity fields. The derived vorticity magnitude distribution demonstrates that the second vortex loop, which emerges in the reattachment region, retains its identity in the turbulent boundary layer and it persists until the end of the test section.

Climatology of Arctic and Antarctic Polar Vortices Using Elliptical Diagnostics

Abstract: The climatological structure, and interannual variability, of the Arctic and Antarctic stratospheric polar vortices are examined by analysis of elliptical diagnostics applied to over 19 yr of potential vorticity data. The elliptical diagnostics define the area, center, elongation, and orientation of each vortex and are used to quantify their structure and evolution. The diagnostics offer a novel view of the well-known differences in the climatological structure of the polar vortices. Although both vortices form in autumn to early winter, the Arctic vortex has a shorter life span and breaks down over a month before the Antarctic vortex. There are substantial differences in the distortion of the vortices from zonal symmetry; the Arctic vortex is displaced farther off the pole and is more elongated than the Antarctic vortex. While there is a midwinter minimum in the distortion of the Antarctic vortex, the distortion of the Arctic vortex increases during its life cycle. There are also large differenc...

Helical vortices in swirl flow

Abstract: Helical vortices in swirl flow are studied theoretically and experimentally.A theory of helical vortices has been developed. It includes the following results: an analytical solution describing an elementary helical vortex structure – an infinitely thin filament; a solution for axisymmetrical vortices accounting for the helical shape of vortex lines and different laws of vorticity distribution; a formula for calculation of the self-induced velocity of helical vortex rotation (precession) in a cylindrical tube; an explanation of the zone with reverse flow (recirculation zone) arising in swirl flows; and the classification of vortex structures.The experimental study of helical vortices was carried out in a vertical hydrodynamical vortex chamber with a tangential supply of liquid through turning nozzles. Various vortex structures were formed owing to changing boundary conditions on the bottom and at the exit section of the chamber. The hypothesis of helical symmetry is confirmed for various types of swirl flow. The stationary helical vortex structures are described (most of them for the first time) the features of which agree with the results and predictions of the theoretical model developed. They are the following: a rectilinear vortex; a composite columnar vortex; helical vortices screwed on the right or on the left; a vortex with changing helical symmetry; a double helix – two entangled vortex filaments of the same sign.

Magnetoresistance of Ferromagnetic Nanowires

Abstract: Magnetoresistance of single Ni and Co nanowires, of about 60 nm in diameter and 6000 nm in length, was measured at room temperature. The full magnetoresistive hysteresis loops of single Ni nanowires, including the irreversible jump, are understood qualitatively, and major progress has been made towards their quantitative description, on the basis of anisotropic magnetoresistance. In contrast, the magnetoresistive hysteresis loops of single Co nanowires could not be described quantitatively, due to the presence of nucleation processes of domain walls or vortices.

Persistence of the lower stratospheric polar vortices

Abstract: The persistence of the Arctic and Antarctic lower stratospheric vortices is examined over the period from 1958 to 1999. Three different vortex-following diagnostics (two using potential vorticity and one based solely on the zonal winds) are compared and are shown to give very similar results for the breakup date. The variability in the timing of the breakup of both vortices is qualitatively the same: There are large interannual variations together with smaller decadal-scale variations and there is a significant increase in the persistence since the mid-1980s (all variations are larger for the Arctic vortex). Also, in both hemispheres, there is a high correlation between the persistence and the strength and coldness of the spring vortex, with all quantities having the same interannual and decadal variability. However, there is no such correlation between the persistence and the characteristics of the midwinter vortex. In the Northern Hemisphere, there is also a high correlation between the vortex persistence and the upper tropospheric/lower stratospheric eddy heat flux averaged over the 2 months prior to the breakup. This indicates that the variability in the wave activity entering the stratosphere over late winter to early spring plays a key role in the variability of the Arctic vortex persistence (and spring polar temperatures) on both interannual and decadal timescales. However, the extreme values of Arctic vortex coldness and persistence during the 1990's are not echoed as a similar extreme in the eddy heat flux. This suggests that the recent increase in vortex persistence is not solely due to changes in the wave activity entering the stratosphere.

Simulation of boundary layer transition induced by periodically passing wakes

Abstract: The interaction between an initially laminar boundary layer developing spatially on a flat plate and wakes traversing the inlet periodically has been simulated numerically. The three-dimensional, time-dependent Navier–Stokes equations were solved with 5.24×107 grid points using a message passing interface on a scalable parallel computer. The flow bears a close resemblance to the transitional boundary layer on turbomachinery blades and was designed following, in outline, the experiments by Liu & Rodi (1991). The momentum thickness Reynolds number evolves from Reθ = 80 to 1120. Mean and second-order statistics downstream of Reθ = 800 are of canonical flat-plate turbulent boundary layers and are in good agreement with Spalart (1988).In many important aspects the mechanism leading to the inception of turbulence is in agreement with previous fundamental studies on boundary layer bypass transition, as summarized in Alfredsson & Matsubara (1996). Inlet wake disturbances inside the boundary layer evolve rapidly into longitudinal puffs during an initial receptivity phase. In the absence of strong forcing from free-stream vortices, these structures exhibit streamwise elongation with gradual decay in amplitude. Selective intensification of the puffs occurs when certain types of turbulent eddies from the free-stream wake interact with the boundary layer flow through a localized instability. Breakdown of the puffs into young turbulent spots is preceded by a wavy motion in the velocity field in the outer part of the boundary layer.Properties and streamwise evolution of the turbulent spots following breakdown, as well as the process of completion of transition to turbulence, are in agreement with previous engineering turbomachinery flow studies. The overall geometrical characteristics of the matured turbulent spot are in good agreement with those observed in the experiments of Zhong et al. (1998). When breakdown occurs in the outer layer, where local convection speed is large, as in the present case, the spots broaden downstream, having the vague appearance of an arrowhead pointing upstream.The flow has also been studied statistically. Phase-averaged velocity fields and skin-friction coefficients in the transitional region show similar features to previous cascade experiments. Selected results from additional thought experiments and simulations are also presented to illustrate the effects of streamwise pressure gradient and free-stream turbulence.

Structure and dynamics of the wake of bubbles and its relevance for bubble interaction

Abstract: The flow in the wake of single and two interacting air bubbles freely rising in water is studied experimentally using digital-particle-image-velocimetry in combination with high-speed recording The experiments focus on ellipsoidal bubbles of diameter of about 04–08 cm which show spiraling, zigzagging, and rocking motion during their rise in water, which was seeded with small tracer particles for flow visualization Under counterflow conditions in the vertical channel, the bubbles are retained in the center of the observation region, which allows the wake oscillations and bubble interaction to be observed over several successive periods By simultaneous diffuse illumination in addition to the light sheet, we were able to record both the path and shape oscillations of the bubble, as well as the wake structure in a horizontal and vertical cross section The results show that the zigzagging motion is coupled to a regular generation and discharge of alternate oppositely oriented hairpin-like vortex structures Associated with the wake oscillation, the bubble experiences a strong asymmetric deformation in the equatorial plane at the inversion points of the zigzag path The zigzag motion is superimposed on a small lateral drift of the bubble, which implies the existence of a net lift force This is explained by the observed different strength of the hairpin vortices in the zig and zag path; a seemingly familiar phenomenon was found in recent numerical results of the sphere wake flow For spiraling bubbles the wake is approximately steady to an observer moving with the bubble It consists of a twisted pair of streamwise vortex filaments which are wound in a helical path and are attached to the bubble base at an asymmetrical position The minor axis of the bubble is tilted in the tangential plane as well as in the radial plane toward the spiral center Due to the pressure field induced by the asymmetrically attached wake two components of the lift force exist, one that causes the lateral motion and the other a centripetal force that keeps the bubble on a circular path A mechanism is proposed to explain the reason for one bubble to spiral or to zigzag Experiments with two simultaneous released bubbles show that bubble interaction is strongly triggered by the wake dynamics Once a bubble is captured in the wake of a rocking bubble, it accelerates and rises via successive jumps until they collide The jumps are explained by the upwards induction effect of the ring-like heads of the hairpin vortices being shed from the leading bubble The final collision and repulsion thereafter abruptly enlarges the wake for a short moment, which is suggested to be one major contribution to the amplification of turbulence production in bubbly flows

The Wave-Driven Ocean Circulation

Abstract: Oceanic surface gravity waves have a mean Lagrangian motion, the Stokes drift. The dynamics of winddriven, basin-scale oceanic currents in the presence of Stokes drift are modified by the addition of so-called vortex forces and wave-induced material advection, as well by wave-averaged effects in the surface boundary conditions for the dynamic pressure, sea level, and vertical velocity. Some theoretical analyses previously have been made for the gravity wave influences on boundary-layer motions, including the Ekman currents. The present paper extends this theory to the basin-scale, depth-integrated circulation in a bounded domain. It is shown that the Sverdrup circulation relation, with the meridional transport proportional to the curl of the surface wind stress, applies to Lagrangian transport, while the associated Eulerian transport is shown to have a component opposite to the Stokes-drift transport. A wave-induced correction to the relation between sea level and surface dynamic pressure is also derived. Preliminary assessments are made of the relative importance of these influences using a global wind climatology and an empirical relationship between the wind and wave fields. Recommendations are made for further development and testing of this theory and for its inclusion in general circulation models.

Local magnetic probes of superconductors

Abstract: Investigations of the magnetic properties of high temperature superconductors (HTSs) have revealed the existence of striking new vortex phenomena due, in part, to their strong crystalline anisotropy, very short coherence lengths and the much larger thermal energies available at high temperatures Some of these phenomena, for example vortex lattice 'melting', pose serious problems for technological applications of the most anisotropic HTS materials and a fuller understanding of them is of considerable importance The most direct information regarding vortex structures and dynamics is obtained through local measurement of the magnetic field within or at the surface of a superconducting sample A detailed review of such local magnetic probes is presented here including Lorentz microscopy, magnetic force microscopy, Bitter decoration, scanning Hall probe microscopy, magneto-optical imaging, and scanning superconducting quantum interference device microscopy In each case the principles underpinning the techni