Vortex Dynamics Research Articles

Arctic polar vortex dynamics during winters 2014/2015 and 2020/2021

The dynamic barrier of the polar vortex contributes to lowering the temperature inside the vortex in the lower stratosphere and prevents the penetration of air masses into the vortex. The presence of a dynamic barrier during winter is one of the criteria determining the possibility of ozone depletion from late winter to spring. We considered the dynamics of the Arctic polar vortex in the winters of 2014/2015 and 2020/2021 at the 50, 30 and 10 hPa levels by the vortex delineation method using the geopotential. In early January 2015 and 2021, sudden stratospheric warmings were recorded as a result of the splitting (4 January 2015) and the significant displacement (5 January 2021) of the polar vortex. In both cases, the weakening of the dynamic barrier of the polar vortex was observed. The polar vortex is characterized by the presence of a dynamic barrier, when the wind speed along the entire edge of the vortex is more than 20, 24 and 30 m/s at the 50, 30 and 10 hPa levels, respectively. A decrease in the average wind speed along the vortex edge below 30, 36 and 45 m/s, at the 50, 30 and 10 hPa levels, respectively, usually indicates a local decrease in the wind speed below 20, 24 and 30 m/s at these levels, i.e., indirectly indicates a weakening of the dynamic barrier.

Complex AC Magnetic Susceptibility as a Tool for Exploring Nonlinear Magnetic Phenomena and Pinning Properties in Superconductors.

The versatile AC magnetic susceptibility technique offers a detailed insight into the complex electrodynamic phenomena in superconductors. In the present study, we outline the key effects related to the temperature, AC field amplitude and frequency variations of the fundamental and harmonic components for an investigation of the vortex dynamics in a flux-grown FeSe crystal. By means of higher harmonic (nonlinear) analysis, we have explored certain atypical, asymmetric features in the AC magnetic response. These effects were identified through the detection of an even (second) harmonic and an unusual temperature shift in the odd (third) harmonic, possibly due to the complex interactions related to the composite superconducting/magnetic morphology of the crystal. Using the high-frequency sensitivity of the third harmonic, the basic functional dependencies of the pinning activation energy, as the main mixed state parameter, were determined with the implementation of the Kim-Anderson Arrhenius relation in the framework of the collective creep theory.

Investigation of vortex dynamics diagnosis in the stall state of mixed-flow pump with blade gap size effect

Investigation of vortex dynamics diagnosis in the stall state of mixed-flow pump with blade gap size effect

Comparing energy dissipation mechanisms within the vortex dynamics of gap and gapless nano-sized superconductors

The presence of magnetic fields and/or transport currents can cause penetration of vortices in superconductors. Their motion leads to dissipation and resistive state arises, which in turn strongly affects the performance of superconducting devices such as single-photon and single-electron detectors. Therefore, an understanding of the dissipation mechanisms in mesoscopic superconductors is not only of fundamental value but also very important for further technological advances. In the present work, we analyzed the contributions and interplay of the dissipative mechanisms due to the locally induced electric field and an intrinsic relaxation of the superconducting order parameter, Ψ, in mesoscopic samples by using the time-dependent Ginzburg–Landau theory. Although often neglected, we show that the dissipated energy due to relaxation of Ψ must be taken into account for an adequate description of the total dissipated energy. The local increase of the temperature due to vortex motion and its diffusion in the sample were also analyzed, where the joint effect of thermal relaxation and vortex dynamics plays an important role for the dissipative properties presented by the superconducting systems.

Numerical Simulation of Hydrodynamics Characteristics in a Tank with an Up-down Reciprocating Agitator

The hydrodynamics characteristics in a tank with an up-down reciprocating disc agitator were numerically investigated using computational fluid dynamics (CFD) simulations. A dynamic mesh technique along with a user-defined function (UDF) was used to solve the reciprocating movement of the disc agitator in the tank. The dynamic flow field and averaged values of some important parameters were obtained. The verification of the simulation was completed by comparing its results with experimental data in the literature. It is found that the flow pattern in the tank with a reciprocating agitator is characterized by two dynamic vortices, which are above and below the disc, respectively. Flow field, force acting on the disc and power requirement change periodically in the tank. The increase of frequency, amplitude and disc diameter leads to the increase of the averaged velocity in the tank as a whole. Nevertheless, the uniformity of velocity distribution is slightly improved, worsened and greatly improved respectively under the above operating conditions. The averaged values of the force acting on the disc and power consumption are in the second and the third power relations with the reciprocating frequency and amplitude, respectively. The ratio of fluid averaged velocity in one cycle to averaged disc speed, and Newton number are hardly affected by reciprocating frequency and amplitude, while they increase with the enlarged size of the disc agitator.

Performance and Loads of a Wing-Offset Compound Helicopter

Analysis of a hingeless rotor with a single wing on the retreating side for lift compounding was conducted. The goals included validation of performance and load predictions with wind tunnel test data, study of the impact of different aerodynamic inflow models, and understanding of benefits by lift compounding with a single wing on the retreating side. The three primary test cases include collective sweeps of the isolated rotor, and the rotor with the wing, at two different incidence angles. The comprehensive analysis was able to accurately predict the performance and blade structural loads of both the isolated rotor and rotor plus wing configurations. Overprediction of propulsive force leads to underprediction of lift-to-drag ratio in several cases. The normal bending moments were well captured for all cases, while the chord bending moment predictions had a phase offset from the test data, but magnitude and harmonics were captured. Comparing inflow models found that dynamic inflow and vortex wake (prescribed and free) models provided similar results. At these advance ratios, prescribed and free wake models showed almost no differences. Additionally, the vortex particle method showed an overprediction of thrust and greater rotorto-wing aerodynamic interference compared to test data. The addition of the wing on the retreating side provided dual benefits of increasing maximum lift-to-drag ratio and reduction of structural loads for a given total thrust. These effects are a result of both lift share between the rotor and wing, and lift offset, the rotor carries a roll moment to balance the wing's roll moment.

Generation Model of a Spatially Limited Vortex in a Stratified Unstable Atmosphere

This paper presents a new model for the generation of axisymmetric concentrated vortices. Thesolution of a nonlinear equation for internal gravity waves in an unstable stratified atmosphere is obtained andanalyzed within the framework of ideal hydrodynamics. The corresponding expressions describing thedependences on the radius for the radial and vertical velocity components in the inner and outer regions ofthe vortex include combinations of Bessel functions and modified Bessel functions. The proposed new nonlinearanalytical model makes it possible to study the structure and nonlinear dynamics of vortices in theradial and vertical regions. The vortex is limited in height. The maximum vertical velocity component isreached at a certain height. Below this height, radial flows converge towards the axis, and above it, an outflowoccurs. The resulting instability in the stratified atmosphere leads to an increase in the radial and verticalvelocity components according to the hyperbolic sine law, which turns into exponential growth. The characteristicgrowth time is determined by the inverse growth rate of the instability. The formation of vortices withfinite velocity components, which increase with time, is analyzed. The radial structure of the azimuthalvelocity is determined by the structure of the initial perturbation and can change with height. The maximumrotation is reached at a certain height. The growth of the azimuth velocity occurs according to a super-exponentiallaw.

Ring Vortex Dynamics Following Jet Formation of a Bubble Expanding and Collapsing Close to a Flat Solid Boundary Visualized via Dye Advection in the Framework of OpenFOAM

A bubble expanding and collapsing near a solid boundary develops a liquid jet toward the boundary. The jet leaves a torus bubble and induces vortices in the liquid that persist long after the bubble oscillations have ceased. The vortices are studied numerically in axial symmetry and compared to experiments in the literature. The flow field is visualized with different methods: vorticity with superimposed flow-direction arrows for maps at a time instant and colored-liquid-layer flow-field maps (dye advection) for following the complete long-term fluid flow up to a chosen time since bubble generation. Bubbles with equal energy—maximum radius in a free liquid Rmax∞= 500 µm—are studied for different distances Dinit from the solid boundary. The interval of normalized distances D* = Dinit/Rmax∞ from 0.4 to 1.8 is covered. Two types of vortices were reported in experiments, one moving toward the solid boundary and one moving away from it. This finding is reproduced numerically with higher resolution of the flow field and in more detail. The higher detail reveals that the two types of vortices have different rotation directions and coexist with individually varying vorticity amplitude throughout the interval studied. In a quite narrow part of the interval, the two types change their strength and extent with the result of a reversal of the dominating rotational direction of the fluid flow. Thereby, the experimentally found transition interval could be reproduced and refined. It is interesting to note that in the vortex transition interval, the erosion of a solid surface is strongly augmented.

Model of relativistic superconductivity

We propose a simplified 2D effective scalar electrodynamics model of relativistic type-II superconductivity. Within this framework, we discuss possible electromagnetic (EM) behaviors of superconductivity along the lines of the nonrelativistic Ginzburg–Landau (GL) theory. This model may be of significance to study very diverse physical phenomena such as a nonlinear version of the electron–hole conversion by a superconductor and electron-positron dynamics around neutron stars. We briefly comment on extending the model to study the 3D dynamics of vortex–vortex interactions and Abrikosov fluxonics.

Role of grain boundary networks in vortex motion in superconducting films

We study the vortex dynamics of the polycrystalline superconductors in the presence of both random point defects and the generated grain boundary (GB) networks with Voronoi diagram. The synergistic effect of adjacent GBs on restricting the vortex motion in intragranular region is proposed and the corresponding intensity factor of the synergistic effect which characterizes the strength of the synergistic restriction of adjacent grain boundaries is also determined in the present work. The interconnected GBs offer easy-flow channels for vortices in addition to pinning effects on the vortices. The combined channels and the vortex flow patterns in the superconducting film are analyzed in detail from molecular dynamics simulations. Furthermore, it is discovered that the critical current increases with the decrease of magnetic field intensity, temperature, and the average grain size. The large number of vortices results in the enhanced repulsive interaction forcing the vortices to move out from the GBs. The thermal depinning from GBs leads to the lower Lorentz force range. The increase of the grain size causes the number of GBs to decrease. In summary, these effects leads the critical current to become a decreasing function of magnetic field, temperature, and grain size.

Manipulating vortices with a rotating laser beam in Bose–Einstein condensates

We investigate the influence of a rotating laser beam on the dynamics of a topological vortex excitation in single-component and binary two-dimensional Bose–Einstein condensates loaded in harmonic traps. The vortex dynamics show different characters by considering the relative position of the vortex and the laser beam and the excitations in the condensates with respect to the rotating frequency and the size of the laser beam. We show the dynamic phase diagrams of single-component condensates with radial isotropy and anisotropy and binary condensates with the vortex and the laser beam being in the same component and different components. Interesting dynamical states such as bound-state, free-state, chaotic-state, free-chaotic-state and bound-chaotic-state are identified for different systems within certain parameter regions.

Effect of Column Defects on the Vortex Dynamics of Superconducting Thin Film with Ginzburg–Landau Parameters

Effect of Column Defects on the Vortex Dynamics of Superconducting Thin Film with Ginzburg–Landau Parameters

Temporal design of flow intermittency in grooved channel for advanced heat transfer enhancement

The present numerical work is a continuing exploration of heat transfer enhancement by flow intermittency in grooved channels based on the energy buffer mechanism elucidated in our previous study, and aims to quantitatively optimize the time domain design of the intermittency pattern. The open-source computational fluid dynamics code OpenFOAM is employed to resolve the intermittent channel flow with triangular surface grooves. The main conclusion is that the optimal thermal performance is achieved when the time duration of each intermittency stage (acceleration, pulse-on, and deceleration) matches the characteristic time of flow dynamics (vortex development and heat transportation). The averaged surface Nusselt number is increased by 120% under the optimized intermittency pattern with the thermal performance factor reaching 1.9. The pulse-off stage of the flow intermittency expands further flexibility to meet diverse prerequisites in practical applications (constant coolant consumption, energy cost, or cooling capacity). By revealing the characteristic flow physics that governs the thermal performance of intermittent flow in grooved channels, the present work novelly proposes quantitative guidance on the temporal design of flow intermittency to achieve effective heat transfer enhancement.

Microwave Flux-flow Hall Effect in a Multi-band Superconductor FeSe

We have measured the flux-flow Hall effect in a multi-band superconductor FeSe pure single crystal to investigate the nature of the vortex core state by means of the cross-shaped bimodal cavity technique. We found that the flux-flow Hall angle of FeSe at low temperatures is about 0.5, which is equal to or smaller than that evaluated by the effective viscous drag coefficient measurements. This feature is in contrast to the cuprate superconductors. The conductivity tensor of multi-band superconductors that are contributed from holes and electrons shows partial cancellation of the flux-flow Hall voltage by the electrons and holes, which can explain the observed feature. Therefore, our study suggests the appearance of the multi-band nature in the vortex dynamics.

Vortex dynamics induced by scanning SQUID susceptometry

We measured the local magnetic response of a niobium thin film by applying a millitesla-scale AC magnetic field using a micron-scale field coil and detecting the response with a micron-scale pickup loop in a scanning superconducting quantum interference device (SQUID) susceptometry measurement. Near the film's critical temperature, we observed a step-like nonlinear and dissipative magnetic response due to the dynamics of a small number of vortex-antivortex pairs induced in the film by the local applied AC field. We modeled the dynamics of the measurement using a combined two-dimensional London-Maxwell and time-dependent Ginzburg-Landau approach, allowing us to construct a detailed real-space picture of the vortex motion causing the observed dissipative response. This work pushes scanning SQUID susceptometry of two-dimensional superconductors beyond the regime of linear response and lays the foundation for microscopic studies of vortex dynamics and pinning in superconducting devices and more exotic materials systems.

Nonlinear two-dimensional water waves with arbitrary vorticity

We consider the two-dimensional water-wave problem with a general non-zero vorticity field in a fluid volume with a flat bed and a free surface. The nonlinear equations of motion for the chosen surface and volume variables are expressed with the aid of the Dirichlet-Neumann operator and the Green function of the Laplace operator in the fluid domain. Moreover, we provide new explicit expressions for both objects. The field of a point vortex and its interaction with the free surface is studied as an example. In the small-amplitude long-wave Boussinesq and KdV regimes, we obtain appropriate systems of coupled equations for the dynamics of the point vortex and the time evolution of the free surface variables.

PIV Snapshot Clustering Reveals the Dual Deterministic and Chaotic Nature of Propeller Wakes at Macro- and Micro-Scales

This study investigates the underlying mechanisms governing the evolution of tip vortices in the far field of a naval propeller wake. To achieve this, a novel approach utilizing data clustering applied to particle image velocimetry snapshots is employed. The clustering of data is carried out using the k-means algorithm, with the optimal number of clusters determined by evaluating two metrics: the within-cluster sum of squares and the average silhouette. The clustering of phase-locked propeller wake data is focused on the vorticity associated with the regions containing tip vortices. Additionally, techniques such as proper orthogonal decomposition, t-distributed stochastic neighbor embedding, and kernel density estimation are employed to visually represent the data clusters in a two-dimensional space, facilitating their assessment and subsequent discussion. This paper shows how the application of data clustering enables a comprehensive understanding of the complex mechanisms driving the dynamics of propeller wake vortices in both the transitional and far fields. Specifically, it reveals the dual nature of the propeller wake flow, characterized by deterministic and chaotic behavior at macro- and micro-scales.

Size effect of randomly distributed pinning centers on magnetic vortices motion in type-Ⅱ superconductor

We consider the size effect of randomly distributed pinning centers on magnetic vortices motion in a mesoscopic type-Ⅱ superconductor in the present of magnetic field using the time-dependent Ginzburg-Landau (TDGL) equations. The pinning centers distributed randomly in various contents in the system are characterized by a distribution function p(r) which fixes the size and positions of them. We investigate the magnetization curves and the magnetic induction as a function of the external magnetic field along the z-axis. In addition, the density of superconducting electrons and the maximum magnetization values for various sizes of the pinning centers is considered. The size of the pinning centers obviously affects the dynamics of the magnetic vortices in the superconductor. The numerical simulations show that Magnetization values and the stable vortex configuration depend on not only content but also size of the pinning centers. The smaller size R (or w) by which the shape of them is characterized results in an increase (or decrease) of both the lower critical magnetic field H1 and the maximum magnetization value. The present work will show good results into understanding and studying the size effect of pinning centers on dynamics of the magnetic vortices in a mesoscopic type-Ⅱ superconductor.

Vortex dynamics in NbTi films at high frequency and high DC magnetic fields

We report on the characterization of NbTi films at sim 11 GHz and in DC magnetic fields up to 4 T, performed by means of the coplanar waveguide resonator technique, providing quantitative information about the penetration depth, the complex impedance, and the vortex-motion-induced complex resistivity. This kind of characterization is essential for the development of radiofrequency cavity technology. To access the vortex-pinning parameters, the complex impedance was analyzed within the formalism of the Campbell penetration depth. Measurements in this frequency range allowed us to determine the complete set of vortex-pinning parameters and the flux flow resistivity, both analyzed and discussed in the framework of high-frequency vortex dynamics models. The analysis also benefits from the comparison with results obtained by a dielectric-loaded resonator technique on similar samples and by other ancillary structural and electromagnetic characterization techniques that provide us with a comprehensive picture of the material. It turns out that the normalized flux flow resistivity follows remarkably well the trend predicted by the time dependent Ginzburg-Landau theory, while the pinning constant exhibits a decreasing trend with the field which points to a collective pinning regime.

Holographic dissipation prefers the Landau over the Keldysh form

Although holographic duality has been regarded as a complementary tool in helping understand the nonequilibrium dynamics of strongly coupled many-body systems, it still remains a remarkable challenge how to confront its predictions quantitatively with real experimental scenarios. By matching the holographic vortex dynamics with the phenomenological dissipative Gross-Pitaevskii models, we find that the holographic dissipation mechanism can be well captured by the Landau form rather than the Keldysh one, although the latter is much more widely used in numerical simulations. Our finding is expected to open up novel avenues for facilitating the quantitative test of holographic predictions against upcoming experimental data. Our result also provides a prime example how holographic duality can help select proper phenomenological models to describe far-from-equilibrium nonlinear dynamics beyond the hydrodynamic regime.