Study Of Vortex Dynamics Research Articles

Shape optimization of the caudal fin of the three-dimensional self-propelled swimming fish

Shape optimization of the caudal fin of the three-dimensional self-propelled swimming fish, to increase the swimming efficiency and the swimming speed and control the motion direction more easily, is investigated by combining optimization algorithms, unsteady computational fluid dynamics and dynamic control in this study. The 3D computational fluid dynamics package contains the immersed boundary method, volume of fluid method, the adaptive multi-grid finite volume method and the control strategy of fish swimming. Through shape optimizations of various swimming speeds, the results show that the optimal caudal fins of different swimming modes are not exactly the same shape. However, the optimal fish of high swimming speed, whose caudal fin shape is similar to the crescent, also have higher efficiency and better maneuverability than the other optimal bionic fish at low and moderate swimming speeds. Finally, the mechanisms of vorticity creation of different optimal bionic fish are studied by using boundary vorticity-flux theory, and three-dimensional wake structures of self-propelled swimming of these fish are comparatively analyzed. The study of vortex dynamics reveals the nature of efficient swimming of the 3D bionic fish with the lunate caudal fin.

Transport Properties of Two-dimensional Snowballs Below the Surface of 4He-II Under Rotation

AbstractKelvin-waves play an important role for the dissipation of quantum turbulence at low temperatures. Here the plasma resonance of two-dimensional (2D) snowballs trapped below the surface of rotating superfluid 4He are measured for the first time in order to examine whether 2D snowballs could be a new probe for study of vortex dynamics. Below 200 mK, a positive shift of the resonant frequency f1 and linewidth broadening are observed as small variations in the absorption spectra under rotation. Both f1 and the linewidth Δf increase linearly with the rotation speed, and the slopes of f1 and Δf against the rotation speed have no temperature dependence. The increase of Δf suggests that an additional dissipation is caused by the coupling between the snowballs and vortices. We provide a qualitative explanation for the linear increase of Δf in the context of Kelvin-waves excited by the motion of snowballs.

Electric and magnetic fields inside the superconducting core of neutron stars

Feynman's approach has been used to derive the equation of dynamics for type II superconductors from the Schrodinger equation. A closed set of equations for the study of vortex dynamics has been obtained. These equations have been used for calculating electric and magnetic fields inside the core of neutron stars. In particular, the contribution of vortices to the generation of electric and magnetic fields inside the core of the star is explicitly displayed.

Creep and depinning of vortices in a nontwinned YBa2Cu3O6.87 single crystal

We present the results of a transport study of vortex dynamics in YBa2Cu3O6.87 crystals in magnetic field H∥c. At low magnetic fields, H<4kOe, the measurements were performed in the range of vortex velocities v=10−4–2m∕s, which covers the thermal creep and flux flow modes. The pinning force Fp depends nonmonotonically on magnetic field in both modes, though the low-field minimum in the Fp(H) curve shifts to higher fields with increasing velocity v, which is interpreted as partial ordering of the vortex lattice. The increase of the pinning force Fp with increasing field, which is observed in the flux flow mode in fields H⩾3kOe, is interpreted by the presence of finite transverse barriers. The barriers result in preserving the entangled vortex solid phase for the above-barrier vortex motion along the action of the Lorentz force. We also show that field variation of the depinning current has a single maximum, while the field variation of the pinning force in the deep creep mode has two maxima. The appearance of two maxima is associated with nonmonotonic field variation of the activation energy Upl, which corresponds to plastic vortex creep mediated by the motion of dislocations.

Magnetic vortex oscillator driven by d.c. spin-polarized current

Transfer of angular momentum from a spin-polarized current to a ferromagnet provides an efficient means to control the dynamics of nanomagnets. A peculiar consequence of this spin-torque, the ability to induce persistent oscillations of a nanomagnet by applying a dc current, has previously been reported only for spatially uniform nanomagnets. Here we demonstrate that a quintessentially nonuniform magnetic structure, a magnetic vortex, isolated within a nanoscale spin valve structure, can be excited into persistent microwave-frequency oscillations by a spin-polarized dc current. Comparison to micromagnetic simulations leads to identification of the oscillations with a precession of the vortex core. The oscillations, which can be obtained in essentially zero magnetic field, exhibit linewidths that can be narrower than 300 kHz, making these highly compact spin-torque vortex oscillator devices potential candidates for microwave signal-processing applications, and a powerful new tool for fundamental studies of vortex dynamics in magnetic nanostructures.

Imaging ac losses in superconducting films via scanning Hall probe microscopy

Various local probes have been applied to understanding current flow through superconducting films, which are often surprisingly inhomogeneous. Here we show that magnetic imaging allows quantitative reconstruction of both current density, J, and electric field, E, resolved in time and space, in a film carrying subcritical ac current. Current reconstruction entails inversion of the Biot-Savart law, while electric fields are reconstructed using Faraday's law. We describe the corresponding numerical procedures, largely adapting existing work to the case of a strip carrying ac current, but including new methods of obtaining the complete electric field from the inductive portion determined by Faraday's law. We also delineate the physical requirements behind the mathematical transformations. We then apply the procedures to images of a strip of YBa2Cu3O7 (YBCO) carrying an ac current at 400 Hz. Our scanning Hall probe microscope produces a time-series of magnetic images of the strip with 1 micron spatial resolution and 25 microsecond time resolution. Combining the reconstructed J and E, we obtain a complete characterization including local critical current density, E-J curves, and power losses. This analysis has a range of applications from fundamental studies of vortex dynamics to practical coated conductor development.

On the Lamb vector and the hydrodynamic charge

This work is an attempt to test the concept of hydrodynamic charge (analogous to the electric charge in Electromagnetism) in the simple case of a coherent structure such as the Burgers vortex. We provide experimental measurements of both the so-called Lamb vector and its divergence (the charge) by two-dimensional Particle Image Velocimetry. In addition, we perform a Helmholtz-Hodge decomposition of the Lamb vector in order to explore its topological features. We compare the charge with the well-known Q criterion in order to assess its interest in detecting and characterizing coherent structure. Usefulness of this concept in studies of vortex dynamics is demonstrated.

Magnetic soft X-ray microscopy: Imaging spin dynamics at the nanoscale

Time-resolved magnetic X-ray microscopy combines in a unique way spatial resolution down to 15 nm with a temporal resolution of less than 100 ps. The former is enabled through state-of-the-art Fresnel zone plates used as X-ray optical elements and the latter is limited by the inherent time structure of current synchrotron radiation X-ray sources. Using a stroboscopic pump and probe scheme spin dynamics in magnetic systems of confined geometries can be imaged with great detail. The magnetization of the elements is excited with fast electronic pulses with a rise time of 100 ps that are launched into waveguide structures. Varying the delay time between the pump and the X-ray probing pulses one can image in real space the temporal evolution of the magnetic configuration in nanoscale elements. This paper is based on an invited talk given at the ICM2006 conference and provides an overview of the current status of time-resolved soft X-ray microscopy. Studies of vortex dynamics in rectangular structures are reported as a specific example.

Mixed-state microwave resistivity in RE–BaCuO films

We present measurements of the microwave complex resistivity at 48 GHz in REBa 2Cu 3O 7− δ thin films (RE=Y, Sm) in moderate magnetic fields ( μ 0 H<0.8 T) and above 60 K. The measured complex resistivity gives valuable information for the study of vortex dynamics. We show that the field dependence of the microwave resistivity has rather different behaviours in YBa 2Cu 3O 7− δ and SmBa 2Cu 3O 7− δ . By applying the conventional model of vortex motion, the data in SmBa 2Cu 3O 7− δ would yield exotic behaviours for the vortex parameters. However, taking into account the enhanced field-induced change in quasiparticle density in a superconductor with lines of nodes in the gap, the vortex part of the response behaves similarly in YBa 2Cu 3O 7− δ and SmBa 2Cu 3O 7− δ .

Analysis of Dust Vortex Dynamics in Gas Discharge Plasma

Results of experimental and numerical studies of vortex dynamics are presented for macroparticles in complex dusty plasma. Experimental data have been obtained for grains with radius ap = 1.7 µm over a wide range of plasma parameters in capacitive RF discharge under microgravity conditions on the International Space Station (ISS).

Field Analysis of Two-Dimensional Dynamics of Non-neutral Plasma by Imaging Diagnostics and Examination by Sector Probing

The linear response of the luminosity of the charge-coupled-device (CCD) camera image to the electron-flux distribution on the phosphor screen is demonstrated as the basis of two dimensional (2D) analyses of non-neutral plasma dynamics. We present a fast and sufficiently accurate procedure to construct the potential and the electric field distribution from the observed 2D images. Such field analyses are essential for deep and extensive studies of vortex dynamics or turbulence. Using this procedure, we quantitatively compare the image diagnostics by sector probing for the first time to show that core dynamics which is observed clearly by imaging is severely obscured by probing; thus, its application should be limited to simple dynamics of a small number of discrete distributions of particles.

Construction of the magnetic phase diagram in small-volume HTS by an OFC magnetometer

We applied the OFC magnetometer for studies of vortex dynamics in small-volume HTS. The substitution of the solenoid coil by a flat one in a tunnel diode oscillator made the coil's filling factor maximal for flat specimens enabling to reach 1-3 /spl Aring/ absolute and 10/sup -6/ relative changes of the penetration depth, /spl lambda/, of RF magnetic field into HTS films. Our technique operates in high magnetic fields and measures linear changes of /spl lambda/ in 1 mm range with high resolution. It is also an excellent Q-meter for detection of changes /spl sim/10/sup -9/ W of the absorption in HTS samples. Such abilities allow it to be applied in many fields of science and technology. In particular, detection of the magnetic field-dependent changes of the frequency and amplitude of a 23 MHz oscillator enabled lines on the magnetic phase diagram for Y-Ba-Cu-O film to be constructed. It seems to have many similarities as compared to the diagrams defined for bulk HTS by other methods. However, how close the relation is in fact, may be revealed after additional tests on the same quality specimens.

Modifications of pinning properties induced by introduction of columnar defects in a (Hg 0.8Cu 0.2)Ba 2CuO 4+ δ single crystal

Columnar defects have been introduced along the c axis of a (Hg 0.8Cu 0.2)Ba 2CuO 4+ δ (Hg-1201) single crystal by irradiation with 6 GeV Pb ions. The nature of the induced latent tracks has been characterized by high-resolution electron microscopy (HREM). The resulting modifications of pinning properties have been inferred from magnetic measurements carried out, before and after irradiation, with the field applied along the c-axis. We have investigated the induced effects on the critical temperature, the field and temperature dependences of the persistent current density, and the irreversibility line. A detailed study of vortex dynamics in the presence of columnar defects has been carried out on the basis of relaxation experiments. It is demonstrated that, with correlated disorder, Hg-1201 exhibits features very similar to those previously reported in YBa 2Cu 3O 7 (YBCO), despite a much larger electronic anisotropy.

Two Dimensional Josephson Junction Arrays

Two dimensional Josephson junction arrays (JJAs) offer theopportunity to study a variety of basic physical concepts. Thepresent review focuses on recent experimental work on thedynamics of JJAs, as characterized by ac conductancemeasurements. The review starts with a discussion of basicphysics necessary to describe JJAs. Some experimentalissues, array fabrication and measurement techniques areconsidered next. In a perpendicular magnetic field, a JJA isan experimental realization of the frustrated XY model, withthe frustration parameter f, corresponding to the number offlux quanta in a unit cell of the array, adjusted by themagnetic field. It is thereby possible to investigate thenature of the ground states at arbitrary frustrations. Phasetransitions are the next topic: the vortex unbindingtransition is observed at integer f-values, while, if thejunction coupling energies are appropriately varied across thearray, at half-integer f-values the Ising transition,associated with chiral symmetry breaking, may be observed.Some aspects of vortex dynamics, a subject which is not yetcompletely understood, are then considered. Under certainconditions there is virtually no pinning in JJAs, they aretherefore ideally suited for the study of vortex dynamics. Thenext topic of this review is concerned with the influence ofdisorder on the ground states and on the phase transitions inJJAs. Site percolation in JJAs has provided some insightsinto the physics of disordered systems and allowed to verifysome theoretical predictions on percolation in two dimensions.A quick look at JJAs in the underdamped regime concludesthis review.

Vortex-dynamics study of the frequency dependence of the superfluid onset temperature

The superfluid transition in a thin ${}^{4}\mathrm{He}$ film on an oscillating Mylar substrate at two frequencies was studied by using a two-torsional-oscillators technique. An onset in superfluid and a peak in dissipation in the same coverage film were observed at higher temperatures at high frequency than at low frequency. The frequency dependence agrees quantitatively with the prediction in the dynamic Kosterlitz-Thouless theory. In interpretation of the frequency dependence we found a peak in the inverse of the diffusion length of a vortex at a coverage of 1.0 atomic layer. A comparison with the isothermal compressibility of ${}^{4}\mathrm{He}$ film adsorbed on graphite suggests that the diffusion length increases with compressibility in the vicinity of 1.0 atomic layer. We also report the ratio of the diffusion constant to the square of the core radius of the vortex in the submonolayer region as well as at coverages above 1 atomic layer.

Vortex dynamics in three-dimensional continuous myocardium with fiber rotation: Filament instability and fibrillation.

Wave propagation in ventricular muscle is rendered highly anisotropic by the intramural rotation of the fiber. This rotational anisotropy is especially important because it can produce a twist of electrical vortices, which measures the rate of rotation (in degree/mm) of activation wavefronts in successive planes perpendicular to a line of phase singularity, or filament. This twist can then significantly alter the dynamics of the filament. This paper explores this dynamics via numerical simulation. After a review of the literature, we present modeling tools that include: (i) a simplified ionic model with three membrane currents that approximates well the restitution properties and spiral wave behavior of more complex ionic models of cardiac action potential (Beeler-Reuter and others), and (ii) a semi-implicit algorithm for the fast solution of monodomain cable equations with rotational anisotropy. We then discuss selected results of a simulation study of vortex dynamics in a parallelepipedal slab of ventricular muscle of varying wall thickness (S) and fiber rotation rate (theta(z)). The main finding is that rotational anisotropy generates a sufficiently large twist to destabilize a single transmural filament and cause a transition to a wave turbulent state characterized by a high density of chaotically moving filaments. This instability is manifested by the propagation of localized disturbances along the filament and has no previously known analog in isotropic excitable media. These disturbances correspond to highly twisted and distorted regions of filament, or "twistons," that create vortex rings when colliding with the natural boundaries of the ventricle. Moreover, when sufficiently twisted, these rings expand and create additional filaments by further colliding with boundaries. This instability mechanism is distinct from the commonly invoked patchy failure or wave breakup that is not observed here during the initial instability. For modified Beeler-Reuter-like kinetics with stable reentry in two dimensions, decay into turbulence occurs in the left ventricle in about one second above a critical wall thickness in the range of 4-6 mm that matches experiment. However this decay is suppressed by uniformly decreasing excitability. Specific experiments to test these results, and a method to characterize the filament density during fibrillation are discussed. Results are contrasted with other mechanisms of fibrillation and future prospects are summarized. (c)1998 American Institute of Physics.

13C and 1H NMR study of vortex dynamics in a layered organic superconductor at selective nuclear sites

The quasi-two-dimensional superconductor, κ-(BEDT-TTF)2Cu(NCS)2, under a perpendicular field was investigated by NMR relaxation rate measurements at the three different nuclear sites. The vortex dynamics contribution to the relaxation rate was separated from the quasiparticle contribution and found to show a peak formation around 2 K with a site-dependent peak height. The analysis supports that the peak behavior does not come from the simple crossover but from some qualitative change in the vortex dynamics.

Quantum interference of a single vortex in a mesoscopic superconductor.

We propose an Aharonov-Bohm-type interference experiment for a vortex in a superconductor to directly test the vortex velocity part of the Magnus force. Based on the recent advances in the nanofabrication technology as well as the progresses in both theoretical and experimental studies of vortex dynamics we show that this conceptually simple experiment can be easily realized. If established, the proposed vortex interference experiment not only demonstrates the quantum coherence phenomenon at a macroscopic level, but also has a potential for technical applications.

Vortex dynamics of viscous fluid flows. Part 1. Two-dimensional flows

The method of product integration is applied to the vortex dynamics of two-dimensional incompressible viscous media. In the cases of both unbounded and bounded flows under the no-slip boundary condition, the analytic solutions of the Cauchy problem are obtained for the Helmholtz equation in the form of linear and nonlinear product integrals. The application of product integrals allows the generalization in a natural way of the vortex dynamics concept to the case of viscous flows. However, this new approach requires the reconsideration of some traditional notions of vortex dynamics. Two lengthscales are introduced in the form of a micro- and a macro-scale. Elementary ‘vortex objects’ are defined as two types of singular vortex filaments with equal but opposite intensities. The vorticity is considered as the macro-value proportional to the concentration of elementary vortex filaments inhabiting the micro-level. The vortex motion of a viscous medium is represented as the stochastic motion of an infinite set of elementary vortex filaments on the micro-level governed by the stochastic differential equations, where the stochastic velocity component of every filament simulates the viscous diffusion of vorticity, and the regular component is the macro-value induced according to the Biot–Savart law and simulates the convective transfer of vorticity.In flows with boundaries, the production of elementary vortex filaments at the boundary is introduced to satisfy the no-slip condition. This phenomenon is described by the application of the generalized Markov processes theory. The integral equation for the production intensity of elementary vortex filaments is derived and solved using the no-slip condition reformulated in terms of vorticity. Additional conditions on this intensity are determined to avoid the many-valuedness of the pressure in a multi-connected flow domain. This intensity depends on the vorticity in the flow and the boundary velocity at every time instant, together with boundary acceleration.As a result, the successive and accurate application of the product-integral method allows the study of vortex dynamics in a viscous fluid according to the concepts of Helmholtz and Kelvin.

Vortex dynamics in YBaCuO single crystals with point- and line-like defects-flux creep studies

Studies of vortex dynamics and pinning were conducted in irradiated Y 1Ba 2Cu 3O 7 single crystals by observing the decay of supercurrents with time. The materials contained either point-like defects from proton irradiation or line-like defects from irradiation with energetic heavy ions. Analysis of the first case gives clear support for collective creep theory. With linear defects, the experiments give evidence for thermal smearing of the vortex pinning potential at higher temperatures, as predicted theoretically.