Role Of Vortices Research Articles

Laboratory Modeling of Geophysical Vortices

Investigators have modeled oceanic and atmospheric vortices in the laboratory in a number of different ways, employing background rotation, density effects, and geometrical confinement. In this article, we address barotropic vortices in a rotating fluid, emphasizing generation techniques, instability issues, and topography effects in particular. We then review work on vortices in shallow fluid layers, including topography effects on vortices in coastal areas and the role of vortices in tidal exchange between two connected basins.

Role of vortices in voice production: Normal versus asymmetric tension

Decreasing the closing speed of the vocal folds can reduce loudness and energy in the higher frequency harmonics, resulting in reduced voice quality. Our aim was to study the correlation between higher frequencies and the intraglottal vorticity (which contributes to rapid closing by producing transient negative intraglottal pressures). Using six excised canine larynges (three with symmetric and three with asymmetric, periodic vocal fold motion), intraglottal vorticity was calculated from 2D velocity fields measured using particle imaging velocimetry. There is a strong correlation between intraglottal vorticity and acoustic energy in the higher frequencies; in periodic asymmetric motion, the vorticity and higher frequencies are both reduced. For unilateral vocal fold paralysis, these findings suggest one reason why periodic, asymmetric motion, may produce an abnormal voice. Further study will help determine when and why reinnervation, as opposed to medialization, may result in better voice quality.

3D coherent vortices in the turbulent near wake of a square cylinder

Large Eddy Simulations of a constant-density flow carrying a passive scalar around a square cylinder at Reynolds numbers Re = 1000 and Re = 1500 are performed. We describe the three-dimensional topology of the turbulent flow in terms of intense longitudinal counter-rotating coherent vortices. We show also a change of regime concerning spanwise Kelvin–Helmholtz (KH) vortices. At the lower Reynolds number, they roll up downstream of the obstacle to form the von Kármán vortex street (VK). At higher Reynolds, they form immediately in the separating shear layer on the two sides of the cylinder. The role of vortices in the passive-scalar mixing is also looked at. To cite this article: C. Brun, T. Goossens, C. R. Mecanique 336 (2008).

Introduction of Special Edition “Role of Vortex on Bluff Body Aerodynamics”

Introduction of Special Edition “Role of Vortex on Bluff Body Aerodynamics”

Role of Vortices in Growth of Microbubbles at Mitral Mechanical Heart Valve Closure

This study is aimed at refining our understanding of the role of vortex formation at mitral mechanical heart valve (MHV) closure and its association with the high intensity transient signals (HITS) seen in echocardiographic studies with MHV recipients. Previously reported numerical results described a twofold process leading to formation of gas-filled microbubbles in-vitro: (1) nucleation and (2) growth of micron size bubbles. The growth itself consists of two processes: (a) diffusion and (b) sudden pressure drop due to valve closure. The role of diffusion has already been shown to govern the initial growth of nuclei. Pressure drop at mitral MHV closure may be attributed to other phenomena such as squeezed flow, water hammer and primarily, vortex cavitation. Mathematical analysis of vortex formation at mitral MHV closure revealed that a closing velocity of approximately 12 m/s can induce a strong regurgitant vortex which in return can instigate a local pressure drop of about 0.9 atm. A 2D experimental model of regurgitant flows was used to substantiate the impact of vortices. At simulated flow and pressure conditions, a regurgitant vortex was observed to drastically enlarge micron size hydrogen bubbles at its core.

A39 水泳のプル動作による推進力発生のメカニズム(水泳)

Generation of propulsive force in swimming was investigated. In knowing the mechanism of the generation unsteady character of flow is impotant, and the conventional quasi-unsteady theory cannot give the correct understandings on the the generating mechanism of the propulsive force. The role of vortex is critical in the generation of momentum and therefore in that of force. Swimmers generate strong vortices by the motion of pitching and heaving motion of hand. In the motion the spped of hand is not necessarily fast, the magnitude and the direction of the induced velocity due to vortices are crucial.

Magnetic ground states and the role of vortices in ferromagnetic hollow nanospheres

We have analytically calculated and compared the total magnetic energies of remanent magnetization states of hollow particles to find conditions for single-domain behavior. The size of the particles and the thickness of the spheroidal shell have been systematically varied for different hard and soft magnetic materials and the corresponding phase diagrams of the lowest-energy configurations are derived.

流れにおける渦の役割(WS3うず : その本質と特性)

流れにおける渦の役割(WS3うず : その本質と特性)

Role of vortices in magnetization reversal of rectangular NiFeelements

Vortices are seen in the magnetization distributions of rectangular magneticelements in both experiments and micromagnetic simulations. To investigate therole of vortices during magnetization reversal, Ni80Fe20 elements 100 nm and200 nm wide and 5-60 nm thick were fabricated by electron beam lithography andstudied by high-resolution magnetic imaging in the transmission electronmicroscope. During reversal, vortices appeared near the ends of theelements, grew under an increasing reverse field, and disappeared after rapidswitching. Maximum switching fields of 400 Oe for 100 nm wide elements and200 Oe for 200 nm wide elements occurred for film thicknesses of 25-30 nm and above.Simulations showed that reversal in these elements always occurred by means ofvortices, however the simulated switching fields were much higher than theexperimentally observed values. Lower switching fields were obtained in thesimulations when vortex creation was assisted by `defects' at the edges of theelements. However, to successfully simulate the magnitude and thicknessdependence of the switching fields, it was necessary to start from an initialmagnetic state which already contained a vortex.

The Formation and Role of Vortices in Protoplanetary Disks

We carry out a two-dimensional, compressible, simulation of a disk, including dust particles, to study the formation and role of vortices in protoplanetary disks. We find that anticyclonic vortices can form out of an initial random perturbation of the vorticity field. Vortices have a typical decay time of the order of 50 orbital periods (for a viscosity parameter alpha=0.0001 and a disk aspect ratio of H/r = 0.15). If vorticity is continuously generated at a constant rate in the flow (e.g. by convection), then a large vortex can form and be sustained (due to the merger of vortices). We find that dust concentrates in the cores of vortices within a few orbital periods, when the drag parameter is of the order of the orbital frequency. Also, the radial drift of the dust induces a significant increase in the surface density of dust particles in the inner region of the disk. Thus, vortices may represent the preferred location for planetesimal formation in protoplanetary disks. We show that it is very difficult for vortex mergers to sustain a relatively coherent outward flux of angular momentum.

Discrete‐vortical elements simulation of air jet interaction with swirling air motion inside a 2D cylinder

A computational procedure based on a hybrid Lagrangian‐Eulerian discrete‐vortical element formulation and conformal transformation schemes are employed in this study to simulate the interaction of an air jet with swirling air flow inside a two‐dimensional cylinder. Such an investigation is of importance to many flow‐related industrial and environmental problems, such as mixing, cooling, combustion and dispersion of air‐borne or water‐borne contaminants because of the role of vortices in the global transport of matter and heat. The basis for the simulation is discussed and numerical results compared with theoretical results for the velocity field and streamfunction obtained by the method of images. The swirling air motion and the features of a real jet are well simulated and numerical results are validated by predictions of theory to within 20 per cent. To illustrate the merging and interaction processes of vortices and the formation of large eddies, velocity vectors, particle trajectories and streamline contours are presented.

Vortex Loop Distribution Near a Superfluid Phase Transition

The distribution of thermally excited vortex loops near a superfluid phase transition is calculated. The number density of loops with a given perimeter is found to change from an exponential decay with increasing perimeter to algebraic decay as Tcis approached from below, in agreement with recent simulations of cosmic strings and of the high-Tcsuperconducting transition in zero field. The total line density is calculated and compared with the simulations. A different viewpoint of the Kibble-Zurek scenario for rapidly quenched transitions is proposed, and the role of vortices above the transition temperature is briefly discussed.

Shape Evolution of Electrodeposited Copper Bumps with High Peclet Numbers

We report the shape evolution of initial copper bumps at Peclet numbers higher than a hundred. The role of vortices and of penetration flow within the cavity was discussed with numerical fluid dynamics computation to obtain a bump with a single hump at the center. The current distributions, or flux profiles, were calculated at the diffusion controlled overpotential and were compared with the electrodeposited bump shapes. For the 100 μm cavity width, the vortices increase at the upstream corners with Peclet numbers 1410 and 7311. The vortices are the local resistance of mass transfer to the cathode. These vortices cause the hollows in flux profiles at the upstream corner with these Peclet numbers. The penetration flow collides with the photoresist sidewall and the vortices decrease at downstream corners. These decreased vortices cause the increase in flux profile at downstream corners. For a 30 μm cavity width a single large vortex forms for the higher Peclet number 44,500 and a single hump in flux is achieved.

Chaos and dynamics of vortices in Josephson junction arrays

The approach to spatiotemporal chaos in two-dimensional (2D) Josephson junction arrays (JJAs) subjected to DC current drives is reviewed. The role of vortices in describing the various transitions and types of dynamical behaviour that occur is explored. Some analogies with hydrodynamics are found.

Contribution to the analysis of temporal and spatial structures near the lift-off region of a turbulent hydrogen diffusion flame

The upstream region of a turbulent H 2 /air jet diffusion flame near the lift-off position was investigated with respect to the local flame structure and its temporal development. Single- and double-pulse one-dimensional Raman/Rayleigh imaging was used to measure quantitative profiles of major species concentrations and temperature with high accuracy and good spatial resolution. With the experimental setup consisting of two narrowband tunable excimer lasers and the respective detection systems, new diagnostic approaches were demonstrated that permitted analysis of details of turbulent transport and flame stabilization in this region of the flame. From single-pulse experiments, the role of vortices in the flame stabilization process was examined; also, the momentary location of flame fronts in individual 1D images was determined. Two different types of double-pulse experiments were performed in addition. Using two consecutive pulses of varying delay, information on typical times for transport and chemical reaction was obtained. In specific cases, the local flame speed could also be determined. In the second type of double-pulse experiments, adjacent one-dimensional traces were monitored from which temperature gradients and scalar dissipation rates were determined simultaneously in two perpendicular directions. These diagnostic approaches provide quantitative multiscalar and structural information that is not available to point-wise or 2D measurements; they were combined in an attempt to reveal details of the combustion process under investigation.

The role of vortices in the magnetization process of small dimension permalloy films

A micromagnetic model has been developed for small dimensional permalloy films, such as those being considered for high density recording. The results show that variations in the magnetization occur through the formation of vortices which give rise to Barkhausen noise in the transfer curve.

Role of vortices in the mutual coupling of superconducting and normal-metal films.

I propose a possible explanation to a recently observed ``cross-talk'' effect in metal-insulator-metal trilayers, indicating a sharp peak near a superconducting transition in one of the metal films. Coulomb interactions are excluded as a dominant coupling mechanism, and an alternative is suggested, based on the local fluctuating electric field induced by mobile vortices in the superconducting layer. This scenario is compatible with the magnitude of the peak signal and its shape; most importantly, it addresses the nonreciprocity of the effect in exchanging the roles of the films.

The role of tidal vortices in material transport around straits

The strong tidal current (tidal jet) in straits generates tidal vortices with a scale of several kilometers. The role of the vortices in material transport was investigated in the Neko Seto Sea, located in the western part of the Seto Inland Sea of Japan. A clockwise vortex with a diameter of about 0.8 km was observed in Nigata Bay (lying between two straits, the Neko Seto Strait and the Meneko Seto Strait). It was concluded that the clockwise vortex was the tidal vortex which was generated by the tidal jet in the Meneko Seto Strait. The vortex moved into the bay with the tide, but tended to stay on the sand bank in the bay. It was confirmed by current measurement with an ADCP and turbidity measurement that the secondary convergent flow was generated in the bottom layer of the vortex. This secondary flow seemed to contribute to the formation of the sand bank. It was suggested that tidal vortices may play an important role in the sediment transport and formation of topography in and around straits.

Voltage turbulence and vortex dynamics in dc-driven two-dimensional arrays of Josephson junctions.

Chaos and the role of vortices in turbulence are investigated in the context of a two-dimensional Josephson-junction-array model. A dc current is injected perpendicular to one edge of the array. The current varies linearly from ${\mathit{I}}_{\mathrm{max}}$ at one corner of the edge to ${\mathit{I}}_{\mathrm{min}}$ at the other corner. The two control parameters are ${\mathit{I}}_{\mathrm{max}}$ and the transverse gradient of the current \ensuremath{\Delta}I=${\mathit{I}}_{\mathrm{max}}$-${\mathit{I}}_{\mathrm{min}}$ that is equivalent to an injected vorticity. The kinetics, determined by current conservation, is in terms of phase angles, but vortices and their dynamic behavior can be monitored. The main results are the following: (a) a \ensuremath{\Delta}I-vs-${\mathit{I}}_{\mathrm{max}}$ phase diagram for the steady-state, periodic, quasiperiodic, and various spatially varying chaotic or ``turbulent'' regimes; (b) an identification of the voltage-spectrum frequencies with appearance rates of the vortex ``collective variables''; (c) a physical picture of turbulence as a spatial rearrangement and mixing of positive and negative vortices at the drive edge, with irregular vortex appearances producing voltage chaos. The idea of collective variables for turbulence may have applications elsewhere.

Vortices, Superfluidity and Phase Transition in 4He Film Adsorbed on Porous Materials

We investigate the superfluid transition of 'He thin film adsorbed on porous materials with emphasis on the role of vortices. Considering the multi· connectivity of the film, we show that the interaction among the vortices excited in the film is essentially one dimensional due to the quantum behavior of the flow. As a result, it is generally shown that the vortices do not dissociate at TA where the long range order (LRO) vanishes in contrast to the two dimensional case. We also study the mechanism of dissipation for applied AC flow with the frequency (J), noting that a finite dimension of the film, the pore size a, plays an important role. The crossover phenomena between the Kosterlitz- Thouless transition and the I! transition are predicted: The superfiuid onset is accompanied with the disappearance of the free vortices in large a or high (J) cases, whereas with the appearance of the LRO in small a or low (J) cases. Our theory qualitatively explains various experimental results with the torsion pendulum.