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DOE OSTI2021

Mechanisms of Dynamic Near-Wake Modulation of a Utility-Scale Wind Turbine

The current study uses large eddy simulations to investigate the transient response of a utility-scale wind turbine wake to dynamic changes in atmospheric and operational conditions, as observed in previous field-scale measurements. Most wind turbine wake investigations assume quasi-steady conditions, but real wind turbines operate in a highly stochastic atmosphere, and their operation (e.g. blade pitch, yaw angle) changes constantly in response. Furthermore, dynamic control strategies have been recently proposed to optimize wind farm power generation and longevity. Therefore, improved understanding of dynamic wake behaviours is essential. First, changes in blade pitch are investigated and the wake expansion response is found to display hysteresis as a result of flow inertia. The time scales of the wake response to different pitch rates are quantified. Next, changes in wind direction with different time scales are explored. Under short time scales, the wake deflection is in the opposite direction of that observed under quasi-steady conditions. Finally, yaw changes are implemented at different rates, and the maximum inverse wake deflection and time scale are quantified, showing a clear dependence on yaw rate. To gain further physical understanding of the mechanism behind the inverse wake deflection, the streamwise vorticity in different parts of the wake is quantified. The results of this study provide guidance for the design of advanced wake flow control algorithms. The lag in wake response observed for both blade pitch and yaw changes shows that proposed dynamic control strategies must implement turbine operational changes with a time scale of the order of the rotor time scale or slower.

flow-structure interactions↗

DOE OSTI2021

Dynamic Mode Decomposition of Random Pressure Fields over Bluff Bodies

Fluctuating surface pressures on a bluff body exposed to a boundary layer flow generally are characterized as a spatiotemporally varying random field. In this paper, a dynamic mode decomposition (DMD) was applied to extract dominant features embedded in these random pressure fields. Utilizing an unsupervised machine learning algorithm, spatial modes and their temporal variations were grouped into different clusters at scales, e.g., macro, meso, and micro. A proper orthogonal decomposition (POD) of the experimental data was carried out to observe commonalities and distinctive perspectives each decomposition offers. Here, a comprehensive examination of the DMD/POD for their convergence criteria, data sufficiency, and modal components analysis was conducted. The physical interpretation of the spatiotemporal pressure field based on these decomposition schemes was discussed. At different scales, the DMD modes can capture the evolution of aerodynamic features, e.g., convection of vortices (or vortex tubes) and other structures. The distribution of energy among these three broad scales also reflects an energy cascade in pressure fluctuations akin to turbulence.

97 MATHEMATICS AND COMPUTING↗

DOE OSTI2022

Magnetic state switching in FeGa microstructures

This work demonstrates that magnetoelectric composite heterostructures can be designed at the length scale of 10µms that can be switched from a magnetized state to a vortex state, effectively switching the magnetization off, using electric field induced strain. This was accomplished using thin film magnetoelectric heterostructures of Fe 81.4 Ga 18.6 on a single crystal (011) [Pb(Mg 1/3 Nb 2/3 )O 3 ] 0.68 -[PbTiO 3 ] 0.32 (PMN-32PT) ferroelectric substrate. Here, the heterostructures were tripped from a multi-domain magnetized state to a flux closure vortex state using voltage induced strain in a piezoelectric substrate. FeGa heterostructures were deposited on a Si-substrate for superconducting quantum interference device magnetometry characterization of the magnetic properties. The magnetoelectric coupling of a FeGa continuous film on PMN-32PT was characterized using a magneto optical Kerr effect magnetometer with bi-axial strain gauges, and magnetic multi-domain heterostructures were imaged using x-ray magnetic circular dichroism—photoemission electron microscopy during the transition to the vortex state. The domain structures were modelled using MuMax 3 , a micromagnetics code, and compared with observations. The results provide considerable insight into designing magnetoelectric heterostructures that can be switched from an ‘on’ state to an ‘off’ state using electric field induced strain.

36 MATERIALS SCIENCE↗

DOE OSTI2015

Vortices in normal part of proximity system

It is shown that the order parameter Δ induced in the normal part of superconductor-normal-superconductor proximity system is modulated in the magnetic field differently from vortices in bulk superconductors. Whereas Δ turns zero at vortex centers, the magnetic structure of these vortices differs from that of Abrikosov's.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2017

Equilibrium properties of superconducting niobium at high magnetic fields: A possible existence of a filamentary state in type-II superconductors [Possible existence of a filamentary state in type-II superconductors]

The standard interpretation of the phase diagram of type-II superconductors was developed in the 1960s and has since been considered a well-established part of classical superconductivity. However, upon closer examination a number of fundamental issues arises that leads one to question this standard picture. To address these issues we studied equilibrium properties of niobium samples near and above the upper critical field H c2 in parallel and perpendicular magnetic fields. The samples investigated were very high quality films and single-crystal disks with the Ginzburg-Landau parameters 0.8 and 1.3, respectively. A range of complementary measurements has been performed, which include dc magnetometry, electrical transport, muon spin rotation spectroscopy, and scanning Hall-probe microscopy. Contrary to the standard scenario, we observed that a superconducting phase is present in the sample bulk above H c2 and the field H c3 is the same in both parallel and perpendicular fields. Our findings suggest that above H c2 the superconducting phase forms filaments parallel to the field regardless of the field orientation. Near H c2 the filaments preserve the hexagonal structure of the preceding vortex lattice of the mixed state, and the filament density continuously falls to zero at H c3 . Finally, our paper has important implications for the correct interpretation of the properties of type-II superconductors and can be essential for practical applications of these materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2022

Zinc oxide (ZnO) hybrid metasurfaces exhibiting broadly tunable topological properties

Extreme light confinement observed in periodic photonic structures, such as the vortex singularities in momentum (k) space, has been associated with their topological nature. Consequently, by exploiting and tuning their topological properties, optical metasurfaces have been demonstrated as an attractive platform for active photonics. However, given the fact that most active media under external excitations can only provide limited refractive index change, the potential advancements offered by the topological character of active metasurfaces have remained mostly unexplored. Zinc oxide (ZnO), which has recently exhibited optically-induced extraordinarily large permittivity modulations at visible and near-infrared frequencies, is an excellent active material for dynamic metasurfaces exhibiting strong tuning. This work demonstrates that a hybrid metasurface consisting of an array of ZnO nanodisks on a silver backplane displays broadly tunable topological properties. In particular, by performing k-space scattering simulations using measured pump-fluence-dependent material properties of ZnO, we study in detail the light reflection from the hybrid metasurface. Our results validate that the large k-space topology tuning of the metasurface can result in enormously strong polarization manipulation of near-infrared light in the vicinity of the topological features. The observed polarization switching effect is highly sensitive to the polarization and wavelength of an incident wave, owing to the symmetry and dispersion characteristics of the proposed system. Our study indicates that leveraging a combination of the extraordinary material properties and the k-space topology, hybrid metasurfaces based on ZnO may open new avenues for creating all-optical switchable metadevices.

36 MATERIALS SCIENCE↗

DOE OSTI2019

Manipulating topological transformations of polar structures through real-time observation of the dynamic polarization evolution

Topological structures based on controllable ferroelectric or ferromagnetic domain configurations offer the opportunity to develop microelectronic devices such as high-density memories. Despite the increasing experimental and theoretical insights into various domain structures (such as polar spirals, polar wave, polar vortex) over the past decade, manipulating the topological transformations of polar structures and comprehensively understanding its underlying mechanism remains lacking. By conducting an in-situ non-contact bias technique, here we systematically investigate the real-time topological transformations of polar structures in PbTiO 3 /SrTiO 3 multilayers at an atomic level. The procedure of vortex pair splitting and the transformation from polar vortex to polar wave and out-of-plane polarization are observed step by step. Furthermore, the redistribution of charge in various topological structures has been demonstrated under an external bias. This provides new insights for the symbiosis of polar and charge and offers an opportunity for a new generation of microelectronic devices.

Science & Technology - Other Topics↗

DOE OSTI2019

Experimental investigation of cross flow mixing in a randomly packed bed and streamwise vortex characteristics using particle image velocimetry and proper orthogonal decomposition analysis

The study of flow and heat transfer through porous media or randomly packed beds is important as these configurations are widely used in many engineering applications, for example, heat energy storage, chemical catalytic reactors, and nuclear reactors. The flow mixing characteristics in a cross-flow plane of a facility with randomly packed spheres at an aspect ratio of 6.3 were experimentally investigated. The velocity fields at several regions of the cross-flow plane located in the vicinity of the wall and in the pores between spheres were obtained by applying the matched-index-of-refraction and time-resolved particle image velocimetry (TR-PIV) techniques for Reynolds numbers ranging from 700 to 1700. The TR-PIV results revealed various flow patterns in the transverse plane of the packed spheres, including swirling flow structures aligned with the axial flow direction, a strong bypass flow near the enclosure wall, and a circulation region created when the bypass flow ejected into a large spatial gap. When the Reynolds number was increased, the peaks of root-mean-square fluctuating velocities, $u'_{rms}$ and $v'_{rms}$, were found to increase approximately at the same ratio as the increase in Reynolds number, and the magnitude of the Reynolds stress increased considerably. In addition, the characteristics of flow mixing in different flow regions were investigated via the two-point cross-correlation of fluctuating velocities. Using Taylor’s hypothesis, the vorticity iso-surfaces were constructed. Thus, constructed iso-surfaces showed that shear layers generated from the bypass flow gaps were stretched, broken into smaller flow structures, and then evolved as vortex pairs when entering the neighboring gaps. The results obtained by applying proper orthogonal decomposition (POD) analysis to the velocity fields showed that the statistically dominant flow structures had approximately the same size and shape as those depicted by Taylor’s hypothesis. Vortex characteristics, such as populations, spatial distributions, and strengths, for various spatial regions and Reynolds numbers were obtained by a combination of POD analysis and vortex identification.

42 ENGINEERING↗

DOE OSTI2014

On the evolution of vortex rings with swirl

A laminar vortex ring with swirl, which has the meridional velocity component inside the vortex core, was experimentally generated by the brief fluid ejection from a rotating outlet. The evolution of the vortex ring was investigated with flow visualizations and particle image velocimetry measurements in order to find the influence of swirling flow in particular upon the transition to turbulence. Immediately after the formation of a vortex ring with swirl, a columnar strong vortex along the symmetric axis is observed in all cases of the present experiment. Then the characteristic fluid discharging from a vortex ring with swirl referred to as “peeling off” appears. The amount of discharging fluid due to the “peeling off” increases with the angular velocity of the rotating outlet. We conjectured that the mechanism generating the “peeling off” is related to the columnar strong vortex by close observations of the spatio-temporal development of the vorticity distribution and the cutting 3D images constructed from the successive cross sections of a vortex ring. While a laminar vortex ring without swirl may develop azimuthal waves around its circumference at some later time and the ring structure subsequently breaks, the swirling flow in a vortex ring core reduces the amplification rate of the azimuthal wavy deformation and preserved its ring structure. Then the traveling distance of a vortex ring can be extended using the swirl flow under certain conditions.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2024

The Role of the Toroidal Vortex in Cumulus Clouds' Entrainment and Mixing

Abstract Shallow convective clouds play a crucial role in Earth's energy budget, as they modulate the radiative transfer in the atmosphere and participate in the vertical transport of aerosols, energy, and humidity. The parameterizations representing these complex, vital players in weather and climate models are mostly based on a description of steady‐state plumes and are a source of major uncertainty. Recently, several studies have shown that buoyant thermals are inherent in atmospheric convection and contain a toroidal (ring) vortex. This work studies those vortices in growing shallow cumulus (Cu) clouds using high‐resolution (10 m) Large Eddy Simulations that resolve these vortices in much detail. Recent analysis of such data showed that small‐scale turbulent diffusion is unable to explain the large diluted portion of the cloud. Here we advocate for the important role of the Cu toroidal vortex (TV) in cloud dilution and present the complex dynamics and structure of a Cu TV. Nevertheless, since the vortex dominates the cloud's dilution, simplicity emerges when considering the cloud's lateral mass flux profile. The cloud mixing is quantified using direct flux calculations and Eulerian tracers. In addition, Lagrangian tracers are used to identify the origin of the entrained air and its thermodynamic properties. It shows that most of the air entrained by the vortex is not recycled by the vortex, yet is significantly more humid than the environment. We suggest that the development of new models describing thermals, together with their toroidal vortices, might improve cloud parameterizations in weather and climate models.

Meteorology & Atmospheric Sciences↗

DOE OSTI2018

Structural transition of vortices to nonlinear regimes in a dusty plasma

The structural transition of a steady state dust flow from linear to nonlinear regimes is analyzed using a two-dimensional hydrodynamic model for the dust fluid confined in an axisymmetric toroidal system along with an unbounded streaming plasma. Numerical solutions of the employed hydrodynamical model not only confirms the analytical structure of the driven dust vortex flow in the linear limit as reported in the previous analysis but also shows how the dust vortices are strongly affected by the nonlinear convective transport of the flow at the higher Reynolds number (Re) regime. Effects of various system parameters including external driving field and the Reynolds number are investigated within the linear to nonlinear transition regime 0.001 ≤ Re < 50. In agreement with relevant experimental observations, the steady flow structure which is symmetric around the center in the linear regime begins to turn asymmetric in the nonlinear regime. The structure of the steady dust flow is found to be influenced mainly by the dissipation scales due to kinematic viscosity, ion drag, and neutral collision in the nonlinear regime, whereas, in the linear regime, it is mainly controlled by the external driving field and the confining boundaries.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2022

Magnetic circuit for Abrikosov vortices: Vortex motion in a periodic labyrinth of magnetic T and I-shaped elements under a superconducting film

Here we explore tailored labyrinth-like vortex motion in a niobium film placed on top of periodic arrays of T and I -shaped permalloy elements by imaging the vortex distribution in magnetic fields applied perpendicular to the superconducting layer under different in-plane polarizations of the TI structure. At low temperatures, we observe pronounced meandering of vortex motion around the TI elements. Remarkably, vortices can easily penetrate the sample along the TI columns even though the average vortex pinning in the patterned area is larger than in a bare niobium film. Accordingly, at temperatures close to the superconducting transition temperature, T c , the voltagecurrent curves in the patterned area show an earlier departure from the zero-resistivity state at small currents, followed by slower growth with increasing current as compared to the un-patterned film. We present a model based on magnetostatic interactions between magnetic charges at the edges of the polarized TI-elements and single magnetic charges of induced vortices to account for this behavior. We expect that similar magnetic structures imposing labyrinth-like vortex motion could be used for collective entanglement of vortices envisioned in quantum circuit operations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2021

First Observations of an Ion Vortex in a Magnetic Hole in the Solar Wind by MMS

The coronal heating region is able to generate mirror mode structures by ion mirror instabilities. Linear magnetic holes are believed to be the remnants of mirror mode structures, thus they are believed to be messengers from the coronal heating region. They can be convected to ∼9 au with the solar wind flow, indicating that a stabilizing mechanism is necessary to make the magnetic holes survive for such a long time. Here, we investigate a magnetic hole with a size of ∼6.7 ρ {sub i} in the solar wind based on observations by the Magnetospheric Multiscale mission. The unprecedented high-resolution data enable us to reveal the existence of the ion vortex inside the structure for the first time. Such an ion vortex forms a ring-like current, which is consistent with the magnetic field depression. The self-consistent structure of the magnetic hole contributed by the ion vortex can help to further shed light on the mechanism of the long-term survival of magnetic holes in the astrophysical plasma.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2025

VorLap

SAND2025-10210O VorLap is a vortex-induced vibration overlap prediction tool for static structures, such as parked wind turbines and marine turbines encountering fluid flow (water or air movement around them) that may induce vortex-induced vibration. This tool uses precomputed frequency domain data for specific cross-sectional shapes or a generalized shedding model, along with geometric data and structural natural frequency data, to identify areas and conditions where vortex-induced vibration may occur. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

Moore, Kevin [Sandia National Lab. (SNL-CA), Liver↗

DOE OSTI2022

Ferroelectric incommensurate spin crystals

Ferroics, especially ferromagnets, can form complextopological spin structures such as vortices and skyrmions when subjected to particular electrical and mechanical boundary conditions. Simple vortex-like, electric-dipole-based topological structures have been observed in dedicated ferroelectric systems, especially ferroelectric-insulator superlattices such as PbTiO 3 /SrTiO 3 , which was later shown to be a model system owing to its high depolarizing field. To date, the electric dipole equivalent of ordered magnetic spin lattices driven by the Dzyaloshinskii-Moriya interaction (DMi) has not been experimentally observed. Here we examine a domain structure in a single PbTiO 3 epitaxial layer sandwiched between SrRuO 3 electrodes. In this work, we observe periodic clockwise and anticlockwise ferroelectric vortices that are modulated by a second ordering along their toroidal core. The resulting topology, supported by calculations, is a labyrinth-like pattern with two orthogonal periodic modulations that form an incommensurate polar crystal that provides a ferroelectric analogue to the recently discovered incommensurate spin crystals in ferromagnetic materials. These findings further blur the border between emergent ferromagnetic and ferroelectrictopologies, clearing the way for experimental realization of further electric counterparts of magnetic DMi-driven phases.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2022

Multiple magnetic orders in LaFeAs 1-x P x O uncover universality of iron-pnictide superconductors

The iron-pnictide superconductors have generated tremendous excitement as the competition between magnetism and superconductivity has allowed unique in-roads towards elucidating a microscopic theory of unconventional high-temperature superconductivity. In addition to the stripe spin density wave ($C^a_{2M}$) phase observed in the parent compounds of all iron-pnictide superconductors, two novel magnetic orders have recently been discovered in different parent structures: an out-of-plane collinear double-Q ($C^c_{4M}$) structure in the hole-doped (Ca, Sr, Ba) 1-x (Na) x Fe 2 As 2 and Ba 1-x K x Fe 2 As 2 families, and a spin vortex crystal “hedgehog” ($C^{ab}_{4M}$) structure in the CaKFe 4 As 4 family. Using neutron diffraction, we demonstrate that LaFeAs 1-x P x O contains all three magnetic orders within a single-phase diagram as a function of substitution, all of which compete strongly with superconductivity. Our experimental observations combined with theoretical modeling demonstrate how the reduction in electronic correlations by chemical substitution results in larger Fermi surfaces and the sequential stabilization of multiple magnetic anisotropies. Our work presents a unified narrative for the competing magnetic and superconducting phases observed in various iron-pnictide systems with different crystal structures and chemistry.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2019

Instability of flux flow and production of vortex-antivortex pairs by current-driven Josephson vortices in layered superconductors

We report numerical simulations of the nonlinear dynamics of Josephson vortices driven by strong dc currents in layered superconductors. Dynamic equations for interlayer phase differences in a stack of coupled superconducting layers were solved to calculate a drag coefficient η(J) of the vortex as a function of the perpendicular dc current density J. It is shown that Cherenkov radiation produced by a moving vortex causes significant radiation drag increasing η(v) at high vortex velocities v and striking instabilities of driven Josephson vortices moving faster than a terminal velocity v c . The steady-state flux flow breaks down at v > v c as the vortex starts producing a cascade of expanding vortex-antivortex pairs evolving into either planar macrovortex structures or branching flux patterns propagating both along and across the layers. This vortex-antivortex pair production triggered by a rapidly moving vortex is most pronounced in a stack of underdamped planar junctions where it can occur at J > J s well below the interlayer Josephson critical current density. Both v c and J s were calculated as functions of the quasiparticle damping parameter, and the dc magnetic field was applied parallel to the layers. The effects of vortex interaction on the Cherenkov instability of moving vortex chains and lattices in annular stacks of Josephson junctions were considered. It is shown that a vortex driven by a current density J > J s in a multilayer of finite length excites self-sustained large-amplitude standing waves of magnetic flux, resulting in temporal oscillations of the total magnetic moment. We evaluated a contribution of this effect to the power W radiated by the sample and showed that W increases strongly as the number of layers increases. Furthermore, these mechanisms can result in nonlinearity of the c-axis electromagnetic response and contribute to THz radiation from the layered cuprates at high dc current densities flowing perpendicular to the ab planes.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2019

Switchable polar spirals in tricolor oxide superlattices

There are increasing evidences that ferroelectric states at the nanoscale can exhibit fascinating topological structures including polar vortices and skyrmions, akin to those observed in the ferromagnetic systems. Here we report the discovery of a new type of polar topological structure, an ordered array of nanoscale spirals, in the PbTiO 3 /BiFeO 3 /SrTiO 3 tricolor ferroelectric superlattice system obtained via phase-field simulations. This polar spiral structure is composed of fine ordered semi-vortex arrays with vortex cores forming a wavy distribution. It is demonstrated that the tricolor system has an ultrahigh Curie temperature of ~1000 K and a temperature of ~800 K for the phase transformation from spiral structure to in-plane orthorhombic domain structure, demonstrating a great thermal stability. The periodicity phase-diagram is constructed, showing multiple phases from inplane domain, polar spiral, and polar vortex to flux-closure with increasing ferroelectric layer thickness. Moreover, the spiral structure has a net in-plane polarization that could be switched by an experimentally-feasible irrotational in-plane field. The switching process involves a metastable vortex state and is fully reversible. This discovery could open up a new routine to design novel multiferroic topological structures with enhanced stability and tunability towards potential future applications in next-generation electronics.

36 MATERIALS SCIENCE↗