Vortex Displacement Research Articles

Numerical Simulation of Abrikosov Vortex at Columnar Defect in Superconductor

We report a numerical modeling of single vortex depinning and its subsequent dynamics in HTS film with extended linear defects under the influence of the transport current. Numerical simulation of stable pinned vortex state and its escape from a linear defect has been performed. The non-stationary dynamics of vortex escape has been investigated and time-dependent solution for vortex displacement from the defect has been obtained. The delay effect in vortex escape process has been studied and the time delay has been estimated. The impact of processes being studied on electrodynamic properties of a superconductor has also been discussed. The dynamics of vortex escape from columnar pinning site described in the present work is important both for understanding of vortex dynamics and applying high-Tc superconductors with columnar defects.

Modal switching of bubbly Taylor–Couette flow investigated by particle tracking velocimetry

Injection of bubbles into Taylor–Couette flow creates new flow modes which have not been experienced in single-phase flow conditions. An array of spiral vortices array is one such mode departing from the original toroidal Taylor vortices. The aim of the present study is to explore the mode transition of the two-phase flow using particle tracking velocimetry. Vertical-axis concentric cylinder with a rotating inner cylinder was used to measure the motion of both phases in the range 600 < Re < 4500. From time–space mapping of the experimentally measured stream functions, we found that bubble injection intensified the axial displacement of the wavy Taylor vortices in the toroidal array, and there was a smooth switching to a spiral array when the azimuthal traveling velocities coincided between two phases. During this vortical reconnection, the two counter-rotating vortices were attenuated asymmetrically. After the two phases co-organized a spiral array, the spiral-maintaining mechanism was weakened to restore the toroidal vortices. These forward and backward mode transitions occurred periodically as bubble injection flow rates increased. We, therefore, concluded that modal switching was an evidence of two-way bubble–vortex interaction for a maximized state, causing the largest drag reduction reported in this regime.

Comparison of Subseasonal‐to‐Seasonal Model Forecasts for Major Stratospheric Sudden Warmings

AbstractThis study explores forecasts for five distinctive major stratospheric sudden warmings (MSSWs) using subseasonal‐to‐seasonal prediction data with lead times up to about 2 weeks. Results reveal model‐to‐model variability of the forecasts, as some models, such as the European Centre for Medium‐Range Weather Forecasts and National Centers for Environmental Prediction models, forecast the MSSWs better. Results also demonstrate greater difficulty of forecasting the vortex split MSSWs (three of the five) than the vortex displacement MSSWs (the other two). It is also shown that the variable forecasts among different lead times and models for each MSSW are characterized by how well the models reproduce the weakenings and deformations of the stratospheric polar vortex around the MSSWs and the intensifications of tropospheric planetary waves before them. The variability is well captured by planetary wave activity, or poleward heat flux, in the lower stratosphere. This underlines the importance of enhanced planetary waves in the troposphere and lower stratosphere for successful forecasts of the forecasting MSSWs, although the causality between the MSSWs and enhanced planetary waves will deserve further investigation.

Mechanism of Vortex Perturbation via Unsteady Pitching

Experiments indicate that vortices trailing finite wings can be perturbed by periodic wing pitching, leading to rapid dissipation and bursting. To illustrate the perturbation mechanism, Betz vortex rollup relations are combined with the Theodorsen theory for unsteady lift response. A sinusoidal pitch motion on a rigid elliptic planform wing is computed in this study as one example. Pitching modifies the instantaneous lift due to the planform variations, via the reduced frequency, and the rollup relations are applied to low-frequency experimental conditions. The combined Betz–Theodorsen theory shows that relatively large spanwise perturbations of the vortex centers can be achieved and may accelerate the exponential growth associated with the Crow instability. In fact, 84.2% of the displacement of the unsteady trailing vortices due to unsteady pitching is oriented in the direction of the Crow instability. Furthermore, the axial velocity in the vortex center, calculated based on the Batchelor method, varies on the order of the flight speed. This forms two types of stagnation points produced by approaching and retreating axial core velocities; in the former case, conservation of mass leads to observations of “bursting.” This observation can be explained adequately on the basis of quasi-steady considerations.

Steady Flows in an Oscillating Spheroidal Cavity with Elastic Wall

The steady flow arising in a spheroidal cavity with periodically-deformed elastic wall is studied experimentally. It is found that average flows whose intensities and structures depend on the wall oscillation frequency and amplitude can develop in the fluid. The average flow is generated in the Stokes boundary layer whose relative thickness is characterized by the dimensionless frequency of the vibrational action. Flow in the form of a pair of toroidal vortices which occupy the entire cavity volume can be observed over the range of low dimensionless frequencies when the boundary layer thickness is comparable with the characteristic cavity dimension. Increase in the dimensionless frequency (decrease in the relative thickness of the Stokes layers) leads to a displacement of the primary vortices towards the cavity boundary. In this case secondary vortices with opposite swirling are formed in the central part of the cavity above the primary vortices. The further increase in the dimensionless frequency leads to development of the secondary vortices and growth of the flow intensity. The large-scale secondary vortices occupy almost the entire cavity volume over the range of high dimensionless frequencies. The dependences of the regimes of average flows and their intensities on the control dimensionless parameters, the oscillation amplitude and frequency, are found on the basis of the results of the investigation.

Interaction of a trailing vortex with an oscillating wing

A technique of particle image velocimetry is employed to characterize the flow structure of a trailing vortex incident upon the tip region of an oscillating wing (plate). The amplitude and velocity of the wing are nearly two orders of magnitude smaller than the wing chord and free stream velocity, respectively. Depending upon the outboard displacement of the incident vortex relative to the wing tip, distinctive patterns of upwash, downwash, and shed vorticity are observed. These patterns are a strong function of the phase of the wing motion during its oscillation cycle. At a given phase, the wing oscillation induces upwash that is reinforced by the upwash of the incident vortex, giving a maximum net upwash. Conversely, when these two origins of upwash counteract, rather than reinforce, one another during the oscillation cycle, the net upwash attains minimum value. Analogous interpretations hold for regions of maximum and minimum net downwash located outboard of the regions of upwash. The magnitude and scale of the vorticity shed from the tip of the wing are directly correlated with the net upwash, which takes different forms related to the outboard displacement of the incident vortex. As the location of the incident vortex is displaced towards the wing tip, both the maximum upwash and the maximum vorticity of the tip vortex initially increase and then decrease. For the limiting case where the incident vortex impinges directly upon the tip of the wing, there is no tip vortex or induced region of upwash. Furthermore, at small values of vortex displacement from the wing tip, the position of the incident vortex varies significantly from its nominal position during the oscillation cycle. All of the foregoing features are interpreted in conjunction with the flow topology in the form of streamlines and critical points, superposed on patterns of vorticity. It is shown that despite the small amplitude of the wing motion, the flow topology is fundamentally different at maximum positive and negative values of the velocity of the wing tip, that is, they are not symmetric.

Electro-optically induced topological reactions of optical indicatrix orientation and polarization state defects.

We have studied the behavior of topological defects (TDs) of the optical indicatrix orientation and polarization state, which appear when a divergent optical beam propagates through LiNbO3 crystals under an external electric field. We have found that the topological reaction of the splitting of a central TD of optical indicatrix orientation, which occurs under the electric field EX, proceeds according to a scheme 1=1/2+1/2. This reaction splits a doubly charged optical vortex into two singly charged vortices. An increase in the electric field strength leads to angular displacement of the optical vortices, thus making it possible to operate spatial positions of the optical vortices via a Pockels effect. We have found that the lateral TDs with the strengths ±½ represent the defects of the polarization state and are not caused by defects of optical indicatrix orientation. They form topological dipoles and generate optical vortices. These topological dipoles undergo no topological reactions in the external electric field.

Strong driving of a single coherent spin by a proximal chiral ferromagnet

We experimentally investigate the influence of a driven, dynamic vortex magnetization state on an individual nitrogen-vacancy (NV) spin in diamond. The vortex core can be translated within the ferromagnet using an applied magnetic field, allowing us to map out the spatial dependence of the interaction. The vortex displacement is determined using magneto-optical microscopy, while the vortex's influence on the spin is probed using optically detected magnetic resonance to measure the Rabi oscillation frequency between spin levels. We find that the close proximity of the vortex core to the NV (within about 200 nm) leads to more than an order of magnitude enhancement of the Rabi frequency. The NV/vortex interaction differs significantly for transitions to the ${m}_{s}=+1$ and ${m}_{s}=\ensuremath{-}1$ spin states, which we attribute to the chiral nature of the vortex state dynamics. We compare the results with micromagnetic simulations and a simple analytical model to shed light on the mechanisms behind the observed effects.

Response of a Streamwise Vortex–Wall Interaction to Unsteady Forcing

Streamwise vortices exist in a number of engineering flows in both external and internal aerodynamics, often with undesirable effects. An experimental investigation of a streamwise vortex–wall interaction is performed through the use of a vortex-generating wing section and a suspended splitter plate. A synthetic jet actuator is implemented below the vortex, and the response of the vortex to unsteady forcing is evaluated with a focus on spatial amplification through convective instabilities. At low forcing frequencies, a large displacement of the vortex is recorded near the actuator, but with almost no spatial amplification. This behavior agrees qualitatively with transient growth studies of columnar vortices. Within a specific frequency range, it is shown that spatial amplification of the vortex displacement occurs. The range of amplified forcing frequency agrees well with predictions for the Crow instability. The vortex–wall interaction generates a secondary vortex structure that rolls up from the boundary layer. When the actuator displaces the primary vortex structure, this causes a strong dynamic reaction in the secondary structure. This occurs due to a mutual induction mechanism between the two vortices as well as a spatial oscillation of the separation point from which the secondary structure originates.

Study of Stratospheric Sudden Warming (SSW) over the tropical and subtropical regions of India using Rayleigh lidar

ABSTRACTThe Stratospheric Sudden Warming (SSW) is one of the most spectacular phenomena in the atmosphere and has impacts on the Earth’s lower, middle, and upper atmospheres. In this study, two major SSW episodes associated with vortex displacement and vortex splitting that occurred in the years 1998 and 1999, respectively, are investigated for the first time over Mt. Abu using lidar observations. Analyses show that ground-based lidar and satellite observations from the Halogen occultation experiment (HALOE) on board the upper atmospheric research satellite (UARS) can capture the effect of SSW events. Lidar measurements are able to capture SSW warming and its decay very accurately. The impact of SSW is further investigated in the European Centre for Medium-Range Weather Forecasts (ECMWF) Interim-reanalysed potential vorticity. Moreover, a detailed study has been presented to understand the latitudinal variation of SSW warming and the associated mesospheric cooling over the Indian region. The results showed that warming is higher over the northern Indian region (35° N, 77° E) compared with the southern Indian region (5° N, 77° E).

Stratospheric precursor of non-uniform variation in early spring surface temperature over Eurasia

The stratospheric influences on the non-uniform variation in early spring (March–April, MA) surface temperature over Eurasia is investigated based on the ERA-Interim, NCEP-1, and NCEP-2 reanalysis data for the period 1980–2016. A lead–lag correlation is found between preceding winter (December–February, DJF) stratospheric polar vortex displacements (SPVD) and the MA west–east seesaw pattern in surface temperature over Eurasia. Further analysis reveals that the East Asian jet stream may act as a bridge linking DJF SPVD and MA surface temperature over Eurasia. A positive change in SPVD is associated with a decelerated polar jet stream and an accelerated East Asian jet stream in the troposphere in DJF. The East Asian jet stream signal can persist into MA. As a result, anomalous southerly/northerly winds prevail over western/eastern Eurasia, accounting for the west–east surface temperature seesaw over Eurasia.

A strong phase reversal of the Arctic Oscillation in midwinter 2015/2016: Role of the stratospheric polar vortex and tropospheric blocking

AbstractIn January 2016, Asia and North America experienced unusual cold temperatures, although the global average of surface air temperature broke the warmest record during a strong El Niño event. This was closely related to the remarkable phase transition of the Arctic Oscillation (AO), which can be explained by stratosphere‐troposphere interactions. First, the quasi‐biennial oscillation changed to its westerly phase in summer 2015 and the stratospheric polar vortex was stronger in early to midwinter 2015/2016. As blocking did not occur in December, the associated downward propagation signal resulted in a strongly positive AO in late December 2015. Second, after late December, the positive phase of Pacific‐North America pattern became apparent in El Niño event, which strengthened the Aleutian anticyclone in the stratosphere. In addition, an equivalent barotropic (“blocking”) anticyclone was established in the troposphere over Asia. The coexistence of blocking over Asia and North America characterized the negative AO and a strong zonal wave number 2 pattern. Due to stronger zonal wave number 2 signals from the troposphere, the stronger stratospheric polar vortex was elongated, with two cyclonic centers over Asia and the North Atlantic in January. The resultant southward displacement of polar vortices was followed by rare snowfall in the subtropical region of East Asia and a heavy snowstorm on the East Coast of the United States.

Exploiting bistable pinning of a ferromagnetic vortex for nitrogen-vacancy spin control

The strong, localized magnetic field produced by the core of a ferromagnetic vortex provides a platform for addressing and controlling individual nitrogen-vacancy (NV) center spins in diamond. Translation of a vortex state in a thin ferromagnetic disk or wire can be understood as motion through an effective pinning potential, arising from the defects in the material. Coupling an NV spin to a vortex state in a proximal ferromagnet imprints the pinning landscape onto the spin transitions. Quantitative characterization of the pinning potential is necessary to control the spin-vortex system. First, we map the effective pinning potential by raster scanning the vortex core through a permalloy disk and measuring the hysteretic vortex displacement vs. magnetic field using differential magneto-optical microscopy. Second, we demonstrate that the interaction between the vortex and a nearby NV spin can be characterized using the pinning map and the path taken by the vortex core through the landscape. Finally, we identify locations of bistability in the pinning landscape, and use them to manipulate the nitrogen vacancy spin in a controlled bimodal fashion by switching the spin on and off resonance with a driving field on a ∼ 10 ns timescale at room temperature.

Sudden stratospheric warmings: statistical characteristics and influence on NO2 and O3 total contents

Statistical characteristics of major and minor sudden stratospheric warmings (SSWs) in the Northern Hemisphere (NH) for 1958–2015 are analyzed using data of NCEP-NCAR, ERA 40, and ERA-Interim reanalyses. Dependencies of the number of major SSWs with the displacement of the circumpolar stratospheric vortex and the number of minor SSWs on the phase of the quasi-biennial oscillation (QBO) of the equatorial stratospheric wind and on the level of solar activity (SA) in the 11-year solar cycle have been revealed. Major SSWs accompanied by a displacement of the polar vortex occur more often at a high level of SA and at the easterly phase of the QBO in the 50–40 hPa layer, while minor SSWs occur more often at a low SA level and at the westerly phase of the QBO. An analysis of spatiotemporal dynamics of the stratospheric polar vortex at major SSWs is performed. The most probable directions of vortex displacement caused by SSWs have been revealed. Influences of the major SSWs on the total contents of NO2 and ozone, as well as on stratosphere temperature, are analyzed.

All-electrical magnetic vortex array sensing

Vortex sensing magnetometers based on arrays of soft magnetic dots are good candidates for high-resolution and accurate spatial magnetic-field estimation. When the arrays are laid out along different spatial directions they can perform tensor gradiometry allowing the measurement of field components and their spatial derivatives as a function of orientation. Detection is based on using spin-polarized currents to counteract vortex displacements or to excite vortex oscillation modes triggered by magnetic-field application. Sensor linearization, field detection range and conditions to obtain large sensitivity electronic compatibility and scalability are discussed.

Abrikosov fluxonics in washboard nanolandscapes

Abrikosov fluxonics, a domain of science and engineering at the interface of superconductivity research and nanotechnology, is concerned with the study of the properties and dynamics of Abrikosov vortices in nanopatterned superconductors, with particular focus on their confinement, manipulation, and exploitation for emerging functionalities. Vortex pinning, guided vortex motion, and the ratchet effect are three main fluxonic “tools” which allow for the dynamical (pinned or moving), the directional (angle-dependent), and the orientational (current polarity-sensitive) control of the fluxons, respectively. Thanks to the periodicity of the vortex lattice, several groups of effects emerge when the vortices move in a periodic pinning landscape: Spatial commensurability of the location of vortices with the underlying pinning nanolandscape leads to a reduction of the dc resistance and the microwave loss at the so-called matching fields. Temporal synchronization of the displacement of vortices with the number of pinning sites visited during one half ac cycle manifests itself as Shapiro steps in the current-voltage curves. Delocalization of vortices oscillating under the action of a high-frequency ac drive can be tuned by a superimposed dc bias. In this short review a set of experimental results on the vortex dynamics in the presence of periodic pinning potentials in Nb thin films is presented. The consideration is limited to one particular type of artificial pinning structures — directly written nanolandscapes of the washboard type, which are fabricated by focused ion beam milling and focused electron beam induced deposition. The reported results are relevant for the development of fluxonic devices and the reduction of microwave losses in superconducting planar transmission lines.

Mapping the Landscape of Domain-Wall Pinning in Ferromagnetic Films Using Differential Magneto-Optical Microscopy

The propagation of domain walls in a ferromagnetic film is largely determined by domain wall pinning at defects in the material. In this letter we map the effective potential landscape for domain wall pinning in Permalloy films by raster scanning a single ferromagnetic vortex and monitoring the hysteretic vortex displacement vs. applied magnetic field. The measurement is carried out using a differential magneto-optical microscopy technique which yields spatial sensitivity $\sim 10$ nm. We present a simple algorithm for extracting an effective pinning potential from the measurement of vortex displacement vs. applied field. The resulting maps of the pinning potential reveal distinct types of pinning sites, which we attribute to quasi-zero-, one-, and two-dimensional defects in the Permalloy film.

Semi-empirical modeling of fuselage–rotor interference for comprehensive codes: influence of side-slip angle

A semi-empirical and physics-based analytical formulation of the induced velocities generated by the fuselage shell of the Bo105 wind tunnel model in the volume around the rotor is derived from velocity data computed by a panel code. The reduced-order analytical model is several orders of magnitude faster than the panel code and thus is predestinated for use in comprehensive rotor codes. Angle of attacks investigated include vertical descent, shallow descent, level flight and climb to vertical ascent. Side-slip angles range from forward to quartering flight. The analytical induced velocity model can be directly used to account for the inflow at the blade elements and also allows for analytical or numerical integration of rotor wake convection to compute the associated displacements of rotor blade tip vortices travelling downstream within this velocity field. This model will be used to replace a fully panelized fuselage (and thus significantly reduce the computational effort) throughout a simulation with an aeromechanics code to account for the influence of the fuselage (e.g., in a design stage). The usage within an aerodynamics code (e.g., a panel code) reduces the panelization to the rotor blades only, leaving the computation of the fuselage-induced velocities to the model. The focus of this paper is the analytical evaluation of fuselage–rotor interference in side-slip angles on rotor trim controls, using blade element momentum theory. The results are compared to the influence of thrust-induced inflow gradients on rotor trim.

Magnetic neutron scattering by magnetic vortices in thin submicron-sized soft ferromagnetic cylinders.

Using analytical expressions for the magnetization textures of thin submicron-sized magnetic cylinders in vortex state, we derive closed-form algebraic expressions for the ensuing small-angle neutron scattering (SANS) cross sections. Specifically, for the perpendicular and parallel scattering geometries, we have computed the cross sections for the case of small vortex-center displacements without formation of magnetic charges on the side faces of the cylinder. The results represent a significant qualitative and quantitative step forward in SANS-data analysis on isolated magnetic nanoparticle systems, which are commonly assumed to be homogeneously or stepwise-homogeneously magnetized. We suggest a way to extract the fine details of the magnetic vortex structure during the magnetization process from the SANS measurements in order to help resolving the long-standing question of the magnetic vortex displacement mode.

Stratospheric polar vortex splits and displacements in the high‐top CMIP5 climate models

AbstractSudden stratospheric warming (SSW) events can occur as either a split or a displacement of the stratospheric polar vortex. Recent observational studies have come to different conclusions about the relative impacts of these two types of SSW upon surface climate. A clearer understanding of their tropospheric impact would be beneficial for medium‐range weather forecasts and could improve understanding of the physical mechanism for stratosphere‐troposphere coupling. Here we perform the first multimodel comparison of stratospheric polar vortex splits and displacements, analyzing 13 stratosphere‐resolving models from the fifth Coupled Model Intercomparison Project (CMIP5) ensemble. We find a wide range of biases among models in both the mean state of the vortex and the frequency of vortex splits and displacements, although these biases are closely related. Consistent with observational results, almost all models show vortex splits to occur barotropically throughout the depth of the stratosphere, while vortex displacements are more baroclinic. Vortex splits show a slightly stronger North Atlantic surface signal in the month following onset. However, the most significant difference in the surface response is that vortex displacements show stronger negative pressure anomalies over Siberia. This region is shown to be colocated with differences in tropopause height, suggestive of a localized response to lower stratospheric potential vorticity anomalies.