Vortex Core Research Articles

Mutual friction and diffusion of two-dimensional quantum vortices

We present a microscopic open quantum systems theory of thermally-damped vortex motion in oblate atomic superfluids that includes previously neglected energy-damping interactions between superfluid and thermal atoms. This mechanism couples strongly to vortex core motion and causes dissipation of vortex energy due to mutual friction, as well as Brownian motion of vortices due to thermal fluctuations. We derive an analytic expression for the dimensionless mutual friction coefficient that gives excellent quantitative agreement with experimentally measured values, without any fitted parameters. Our work closes an existing two orders of magnitude gap between dissipation theory and experiments, previously bridged by fitted parameters, and provides a microscopic origin for the mutual friction and diffusion of quantized vortices in two-dimensional atomic superfluids.

Non-Abelian topological superconductivity in maximally twisted double-layer spin-triplet valley-singlet superconductors

Recent studies point to an exotic spin-triplet valley-singlet (STVS) superconducting phase in certain two-valley electron liquids, including rhombohedral trilayer graphene, Bernal bilayer graphene and ZrNCl, which nevertheless admits only trivial topology. Here, we predict that upon twisting two layers of STVS superconductors, a chiral fpm {{{{{{{rm{i}}}}}}}}{f}^{{prime} }-wave superconducting phase emerges near the ‘maximal’ twist angle of 30∘ where the system becomes an extrinsic quasi-crystal with 12-fold tiling. The resulting composite hosts an odd number of chiral Majorana edge modes and a single non-Abelian Majorana zero mode (MZM) in the vortex core. Through detailed symmetry analysis and microscopic modelling, we demonstrate that the non-Abelian topological superconductivity (TSC) forms robustly near the maximal twist when the isolated Fermi pockets coalesce into a single connected Fermi surface in the moiré Brillouin zone. Our results establish the large-twist-angle engineering, with distinct underlying moiré physics from magic-angle graphene, as a viable route toward non-Abelian TSC.

Synchronization of the Self-Oscillations of the Magnetic Vortices in Exchange-Coupled Ferromagnetic Disks

The low-frequency (gyrotropic) self-oscillations of the magnetic vortices in interacting ferromagnetic disks, which are caused by a spin-polarized current, are studied by numerical simulation. Various magnetization oscillation modes depending on the configuration of the magnetic state of the system are considered. The influence of the pumping current nonuniformity on the phase difference of the vortex gyration in neighboring disks is investigated. The overlap of the disks is shown to increase the interaction between the vortices and, hence, to decrease the dephasing of the vortex core oscillations. The prospects of using overlapping disks to ensure phase synchronization of arrays of spin-transfer vortex oscillators are discussed.

Vortex generation in stirred binary Bose-Einstein condensates

Binary Bose-Einstein condensates of two distinct atomic species are studied, with interesting effects being verified due to the imbalance of atomic masses. We study the formation of vortices and associated turbulence flows created by stirring two mass-imbalanced coupled Bose-Einstein condensates in the miscible regime. We consider the mixture in the laboratory frame, confined in a pancake-like harmonic trap slightly perturbed elliptically by a time-dependent periodic potential, which introduces turbulent dynamics with final stable patterns of vortices, induced by the periodicity of the perturbation. Derived rotational frequencies are shown to be associated with the observed vortex patterns in the asymptotic regime, with a correspondence between the stirring procedure and a direct time-independent approach being indicated. It was also confirmed that a larger pattern of vortices is supported by the higher mass condensed species at the same frequency. For the experimentally accessible mixtures $^{85}\mathrm{Rb}\text{\ensuremath{-}}^{133}\mathrm{Cs}$ and $^{85}\mathrm{Rb}\text{\ensuremath{-}}^{87}\mathrm{Rb}$, we numerically solve the equations of motion and analyze the system dynamics using a range of measures including energy spectra. In the transient turbulent regime, spectral analysis shows evidence for the characteristic ${k}^{\ensuremath{-}5/3}$ Kolmogorov classical law associated with turbulence, modified by the universal ${k}^{\ensuremath{-}3}$ vortex core scaling in the ultraviolet regime.

Surface Wind Speeds and Enthalpy Fluxes During Tropical Cyclone Formation From Easterly Waves: A CYGNSS View

AbstractWe examined the Cyclone Global Navigation Satellite System (CYGNSS) retrievals of surface wind speeds and enthalpy fluxes in African easterly waves that led to the formation of 30 Atlantic tropical cyclones during 2018–2021. Lag composites show a cyclonic proto‐vortex as early as 3 days prior to tropical cyclogenesis. The enthalpy flux distribution does not vary substantially before cyclogenesis, but subsequently, there is a marked increase in the extreme upper values. In the composites, a negative radial gradient of enthalpy fluxes becomes apparent 2–3 days before cyclogenesis. These results—based on novel data blending satellite retrievals and global reanalysis—support the findings from recent studies that the spin‐up of tropical cyclones is associated with a shift of peak convection toward the vortex core and an inward increase of enthalpy fluxes.

Majorana-Weyl cones in ferroelectric superconductors

Topological superconductors are predicted to exhibit outstanding phenomena, including non-Abelian anyon excitations, heat-carrying edge states, and topological nodes in the Bogoliubov spectra. Nonetheless, and despite major experimental efforts, we are still lacking unambiguous signatures of such exotic phenomena. In this context, the recent discovery of coexisting superconductivity and ferroelectricity in lightly doped and ultraclean ${\mathrm{SrTiO}}_{3}$ opens new opportunities. Indeed, a promising route to engineer topological superconductivity is the combination of strong spin-orbit coupling and inversion-symmetry breaking. Here we study a three-dimensional parabolic band minimum with Rashba spin-orbit coupling, whose axis is aligned by the direction of a ferroelectric moment. We show that all of the aforementioned phenomena naturally emerge in this model when a magnetic field is applied. Above a critical Zeeman field, Majorana-Weyl cones emerge regardless of the electronic density. These cones manifest themselves as Majorana arc states appearing on surfaces and tetragonal domain walls. Rotating the magnetic field with respect to the direction of the ferroelectric moment tilts the Majorana-Weyl cones, eventually driving them into the type-II state with Bogoliubov Fermi surfaces. We then consider the consequences of the orbital magnetic field. First, the single vortex is found to be surrounded by a topological halo and is characterized by two Majorana zero modes: one localized in the vortex core and the other on the boundary of the topological halo. Based on a semiclassical argument we show that upon increasing the field above a critical value the halos overlap and eventually percolate through the system, causing a bulk topological transition that always precedes the normal state. Finally, we propose concrete experiments to test our predictions.

Driving a magnetic texture by magnon currents

Thermally induced spin dynamics in solids have sparked broad interest in both fundamental physics and spintronic applications. As theoretically proposed, thermally excited magnons created by temperature gradients can be used to manipulate spin textures such as topological magnetic solitons. However, so far, the effectiveness of such thermomagnonic torques remained a problem in practice. Here, the dynamics of magnetic vortex cores in thin ferromagnetic platelets driven by thermomagnonic torques are investigated using high-resolution Lorentz transmission electron microscopy. Large deflections of the magnetic vortex core transverse to the direction of the temperature gradient are observed. A generalized Thiele equation model is used to identify the magnitude of the contribution of the involved torques. Our results pave the way for the manipulation of magnetic domains on the nanoscale by thermomagnonic currents and provide insights into the interplay of temperature and spin.

Influence of vortex interactions on the exergy transfer and performance of OWC wells turbine

To provide deep dives about aerodynamic loss mechanisms in Wells turbines for wave energy conversion, a loss audit analysis was performed by numerical experiments in a monoplane Wells turbine with guide vanes. The interactions between the tip-leakage and leading-edge vortices during the stall process were captured by an improved vortex identification method, which revealed the relationship between vortex interactions and stall mechanisms by identifying coherent structures and tracking the vortex core trajectory. Finally, the influence of vortex interactions on exergy transfer was quantified. The results indicate that the lost kinetic energy and mixing losses dominate the loss generation in the Wells turbine stage under stall conditions. Under the beneficial effect of tip leakage flow, leading-edge separation first begins at the equilibrium region between the tip-leakage and leading-edge vortices. As the leading-edge vortices expand toward the blade tip, the intensified leading-edge vortex interacts with the casing suction-side corner vortex and accelerates the dissipation of the tip-leakage vortices. Consequently, the contributions of viscous irreversibilities outweigh those of shaft work, being the dominant factor in the decrease in flow exergy, leading to a decrease in exergy utilization by 38.46% from the pre-stall condition to the stall condition.

Generation of negative peak pressures under a separation bubble

This paper investigates the generation mechanism of negative peak pressures on a blunt plate under a separation bubble with Large Eddy Simulation (LES). Synchronized sampling of the wind pressures on and velocity fields around the plate is adopted to explore the relation between pressure fluctuations and vortex dynamics. The ordinary pressure fluctuation is found to be induced by high-frequency vortex convection. Formation of a negative peak pressure starts from the vortex head arriving at an observation point and ends up at the vortex tail leaving that location. A negative peak pressure occurs when the vortex core is right beneath/above the observation point. Spectral analysis of the wind pressures and Dynamic Modal Decomposition (DMD) for the velocity fields are combined to detect and visualize the low-frequency motion responsible for extreme pressure fluctuations. A counterpart of the low-frequency dominating component of the wind pressures is found in the velocity modal analysis, which is related to the shedding of an extreme large-scale vortical structure and demonstrates a flapping motion of the shear layer. The present study provides insights to the generation of negative peak pressures and the relation between vortex behaviors and the shear layer movement.

Enhancing the Resolution of Blade Tip Vortices in Hover with High-Order WENO Scheme and Hybrid RANS–LES Methods

The accurate prediction of blade tip vortices continues to be challenging for the investigation of rotor aerodynamics. In the current work, the blade tip vortex system of a Caradonna–Tung rotor in the hovering state is simulated on account of the framework of a high-order WENO scheme and hybrid RANS/LES method progressively. With the RANS method based on a fifth-order WENO–Roe scheme, the spatial resolution of wake age and capture the accuracy of tip vortices are improved significantly. However, the unsteadiness of the vortex system fails to be distinguished. Then, the DDES and IDDES methods, coupled with a fifth-order WENO–Roe scheme, are implemented to further enhance the resolution of the spatial turbulence. Compared with RANS, the improvement of spatial resolution is reflected in both the statistical and transient results with DDES. The identifiable vortex core distributions can be predicted at older wake ages. The asymmetrical characteristic of blade tip vortices is revealed along with the release of fluctuation while the predicted distribution of velocity profiles in the vortex core is enhanced. With the application of IDDES, the secondary turbulent structures are captured around the primary vortex core of blade tip vortices while the complex behaviors of the vortices are observed. However, the statistics of the averaged flow field are similar to DDES.

The Impact of Increasing the Length of the Conical Segment on Cyclone Performance Using Large-Eddy Simulation

In cyclone separators, the asymmetrical coherent structure significantly influences the velocity fluctuations and hence the cyclone performance. This asymmetric rotating vortex in the core region precesses around the cyclone axis with a frequency that depends on the cyclone geometry and operating conditions. In the present work, we studied the impact of increasing the length of the conical segment on the performance of cyclone separators as well as the precessing frequency of the asymmetrical structure. For this, five different cone lengths were considered such that the total cyclone length equalled 3.0D, 3.5D, 4.0D, 4.5D, and 5.0D (here, D is the main body diameter of the cyclone). The study was performed at three different inlet velocities, viz. 10, 15, and 20 m/s. Throughout the work, the angle of the conical segment was held fixed and resembled the reference model (which had a total cyclone length equal to 4.0D). The cyclone performance was evaluated using advanced closure large-eddy simulation with the standard Smagorinsky subgrid-scale model. Conclusive results indicate that with an increase in the cone length, the pressure losses reduce appreciably with small variations in the collection efficiency, followed by a reduction in the precessing frequency of the asymmetric vortex core. The results further indicate that the apex cone angle (or the bottom opening diameter) must be carefully adjusted when increasing the cone length.

New Parameter to Characterize Rankine Vortex Formation in Liquid Draining Tanks

Air core vortexing is a subject of topical interest in aerospace engineering because it causes many undesirable effects such as cavitation in propellant feed pumps, reduced flow rate in liquid propulsion tanks, etc. Propellant tanks used in spacecraft and rocket applications are often exposed to environmental disturbances. This may cause rotating motions in draining liquid propellant and initiate the development of an air core vortex during the draining process. The air core enters the drain port and blocks the same, resulting in a reduced discharge of draining liquid propellant. Previous researchers have made use of the parameters (namely, critical submergence and time to empty the tank ) to quantify vortexing, wherein contradictions are found in the effect of these parameters on the phenomenon in the light of the present results. Based on the said contradictions, it is inferred that these parameters are not capable of properly characterizing the vortexing phenomenon. Motivated to solve this serious issue, the current study has attempted to formulate a new parameter, namely, the “characteristic volume of air core,” which could meaningfully characterize the vortexing phenomenon. The new parameter is found to bring to light both the unsteady as well as the time-averaged characters of the vortexing phenomenon.

Large Eddy Simulation of Hydrofoil Tip Leakage Vortex

The axial-flow turbomachinery is important in water supply and drainage. However, there is a gap between the blade tip and the casing, which is easy to cause tip leakage vortex. Tip leakage of axial-flow turbomachinery has bad impact on the operation stability. In this case, the numerical simulation is used to study the simplified hydrofoil model with clearance. Then the change of tip leakage vortex core position and the pressure distribution are analyzed. Results show that with the fluid flows downstream, the longitudinal position of the tip leakage vortex core first drops and then rises. On different streamwise planes, the vortex core has the lowest pressure. From the downstream to the vortex generating position, the vortex core pressure decreases gradually, and the cavitation is most likely to occur at the location where the vortex occurs.

Experimental investigations on central vortex core in swirl spray flames using high-speed laser diagnostics

Centrally staged swirl combustion can effectively reduce NOx emission. However, the complex combustion field is susceptible to producing large-scale coherent structures, such as precessing vortex core and central vortex core (CVC). This study mainly investigates the effect of CVC on the flow and flame in a centrally staged swirl spray combustor at elevated temperature and pressure using 10 kHz high-speed CH* chemiluminescence (CL), 20 kHz particle image velocimetry, and CH2O planar laser-induced fluorescence (PLIF). For the pilot flame, both CH* CL and CH2O PLIF flame are fork-shaped with three long parts, and the middle parts of flame dynamics indicate CVC structure. For the stratified flame, the CVC structure exists in an extended strip area of strong vorticity near the centerline of the combustor. The analysis of proper orthogonal decomposition modes shows that the motion of CVC is mainly swing, followed by precessing. Simultaneous diagnostics indicates that the entrainment of CVC leads to CH2O transport from the shear layer to the central region of the combustor. In general, the CH2O signal is mainly distributed in two positive velocity regions, the pilot/main jet and around CVC. Taking advantage of the CVC effect on radical transportation is a potential method to improve the mixing of the combustor, such as temperature distribution.

Numerical study on the scattering of acoustic waves by a compact vortex

A new family of compact vortex models is developed and taken as base vortical flows to numerically study the acoustic scattering by solving the two-dimensional Euler equations in the time domain with high-order accurate finite-difference methods and nonreflecting boundary conditions. The computations of scattered fields with very small amplitude are found to be in excellent agreement with a benchmark provided by previous studies. Simulations for the scattering from a Taylor vortex reveal that the amplitude of the scattered fields is strongly influenced by two dimensionless quantities, the vortex strength Mv based on the maximal velocity of the vortex, and the acoustic length-scale ratio λ/R defined as the acoustic wavelength relative to the vortex core size. To have a deep understanding of the roles played by these two quantities, another significant quantity used for describing quantitatively the total amount of scattering, namely, scattered sound power, is introduced. Thereupon, on the basis of a global analysis of scale effects of these two dimensionless quantities on the scattered sound power, the scattering defined in a physical coordinate system with Mv and λ/R is divided into three domains, long-wave domain, resonance domain, and geometrical-acoustics domain. For each domain, we examine the influence of Mv and λ/R in detail and derive the explicit scaling laws involved in the strength of the scattered field and these two dimensionless quantities separately. Furthermore, the computations for the scattering from a high-order compact vortex are conducted at a wide range of Mv and λ/R and compared with the results from the Taylor vortex in each domain to gain some insights into the acoustic scattering by a compact vortex.

Influence of dual-axial swirler configuration on hydrodynamic stability in combustor

To improve hydrodynamic stability in a combustor, an unsteady flow analysis method is needed. Hence, the proper orthogonal decomposition (POD) method based on a large eddy simulation (LES) unsteady flow field and corresponding experimental verification were utilized to analyze and assess the influence of the precession vortex core (PVC) motion law on the pulsation downstream of different swirler configurations. The pulsation outcomes of the unsteady simulation match the experimental data quite well, with case 1 having the highest pulsation quantity. The POD analysis reveals that the majority of pulsation energy is concentrated in the first two modes. The results of the motion state, regularity of the time coefficient, and frequency characteristics also demonstrate that the combustor's PVC features are compatible with modes 1 and 2. There is an optimum value of swirl number, 0.72, for which the flow field's stability has the lowest degree of disturbance. Moreover, the airfoil vane's stability is beyond that of the straight vane. The mean flow field and the coherent flow field in the pulsation flow field of case 3 are the most beneficial factors concerning combustion stability, and the unstable aspects of the transition flow field and the turbulent flow field are the least in shape and energy ratio. The data procured from the experiment certify that there is no characteristic frequency of pulsation within 5000 Hz that exists in case 3.

Resonator-like behavior of a wall-bounded precessing vortex core in a diffuser with wall asymmetries

This paper reports a detailed investigation of the interaction between a wall-bounded precessing vortex core (PVC) occurring in swirling flows after vortex breakdown and a wall asymmetry. Experiments are carried out in an axisymmetric diffuser downstream of an axial swirl generator inducing a swirling flow with a swirl number of S = 1.1. Wall pressure measurements and two-component particle image velocimetry (PIV) are conducted for Reynolds numbers (Re) ranging from 20 000 to 76 000 in the initial axisymmetric configuration and several asymmetric configurations, with an additional cylindrical protrusion placed on the diffuser wall at different streamwise and circumferential positions. It is first confirmed that synchronous pressure fluctuations at the PVC frequency are only produced in asymmetric configurations. Furthermore, the analysis of the pressure data in several asymmetric configurations revealed for the first time a resonator-like behavior of a wall-bounded PVC. While a change of the protrusion circumferential position in a given cross section of the diffuser only affects the phase of the synchronous pressure fluctuations, the amplitude of the latter features successive minima (pressure node) and maxima (pressure anti-node) as the protrusion is moved along the diffuser in the streamwise direction. In addition, as the protrusion is moved closer to a pressure node, the phase of the synchronous pressure fluctuations exhibits a sudden variation of ±π. Similar results are observed for all tested values of Reynolds number, whereas the PVC frequency linearly increases with Re. A reconstruction of the PVC helical structure based on PIV measurements showed that these consecutive pressure nodes are spaced by a distance equal to approximately one third of the PVC helical pitch. Finally, it also revealed that two different states are observed, depending on the position of the protrusion along the diffuser: the synchronous pressure component reaches its maximum value as the PVC center is approaching either its closest or farthest angular position with respect to the protrusion. The transition from one state to another one depends on the streamwise position of the protrusion with respect to the pressure nodes. These unprecedented experimental observations pave the way to novel theoretical developments for a better understanding and modeling of synchronous pressure fluctuations induced by wall-bounded PVC in asymmetric geometries.

Assessment of a Differential Subgrid Stress Model for Large-Eddy Simulations of Turbulent Unconfined Swirling Flames

Swirling flames are widely used in engineering to intensify mixing and stabilize combustion in gas turbine power plants and industrial burners. Swirling induces new instability modes, leading to intensification of coherent structures, asymmetric geometry, vortex core precession, and flame oscillations. Large-Eddy Simulation (LES) has the capability to furnish more accurate and reliable results than the simulations based on Reynolds-averaged Navier–Stokes equations (RANS). Subgrid-scale models in LES need to describe the backscatter (local transfer of kinetic energy from small scales to larger scales) that is intensified in swirling flames. In this paper, the Differential Subgrid Stress Model (DSM), previously developed by the authors, is assessed using an experimental database from Sydney University on swirl-stabilized turbulent unconfined non-premixed methane-air flame. Regime without vortex precession is simulated numerically using the DSM and Smagorinsky subgrid-scale model. Experimental measurements of mean velocity, profiles of mass fractions, and temperature are used for comparison with the simulation data. The standard Smagorinsky model is considered the basic approach. Differences in the flow field statistics obtained in both subgrid-scale LES models are analyzed and discussed. The importance of taking the backscatter into account is highlighted.

Modification of three-magnon splitting in a flexed magnetic vortex

We present an experimental and numerical study of three-magnon splitting in a micrometer-sized magnetic disk with a vortex state strongly deformed by static in-plane magnetic fields. Excited with large enough power at frequency fRF, the primary radial magnon modes of a cylindrical magnetic vortex can decay into secondary azimuthal modes via spontaneous three-magnon splitting. This nonlinear process exhibits selection rules leading to well-defined and distinct frequencies fRF/2±Δf of the secondary modes. Here, we demonstrate that three-magnon splitting in vortices can be significantly modified by deforming the magnetic vortex with in-plane magnetic fields, leading to a much richer three-magnon response. We find that, with increasing field, an additional class of secondary modes is excited, which are localized to highly flexed regions adjacent to the displaced vortex core. While these modes satisfy the same selection rules of three-magnon splitting, they exhibit much lower three-magnon threshold power compared to regular secondary modes of a centered vortex. The applied static magnetic fields are small (≃10 mT), providing an effective parameter to control the nonlinear spectral response of confined vortices. Our work expands the understanding of nonlinear magnon dynamics in vortices and advertises these for potential neuromorphic applications based on magnons.

Coherent and incoherent structures in fuzzy dark matter haloes

ABSTRACTWe show that fuzzy dark matter haloes exhibit spatial differentiation in the coherence of the field configuration, ranging from completely coherent in the central solitonic core to incoherent outside it, with a significant drop of the dimensionless phase-space density with increasing radius. The core is a pure condensate, overlapping perfectly with the Penrose-Onsager mode corresponding to the largest eigenvalue of the one-particle density matrix. The virialized outer halo exhibits no clear coherence as a whole upon radial and temporal averaging but can be described as a collection of local, short-lived quasi-condensate lumps, exhibiting suppressed fluctuations, which can be identified with the structures commonly referred to as granules. These localized regions are separated by vortices that form a dynamical web, inhibiting phase coherence across the entire halo. We further examine the core oscillations, finding that they are accurately described by two time-dependent parameters characterizing the size of the core, rc(t), and a crossover region, rt(t). For the haloes in our merger simulations, this feature is reflected in the (anti-)correlated oscillation of the peak value of the density power-spectrum. The halo’s turbulent vortex tangle appears to reach a quasi-equilibrium state over probed time-scales, with the incompressible component of the kinetic energy exhibiting a characteristic k−3 tail in its spectrum, indicative of a ρ ∼ r2 density profile around the quantum vortex cores. Comparison of the peak wavenumbers in the corresponding power-spectra shows the inter-vortex spacing and the granule length scale in the outer halo to be very similar and slightly above the core size.