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At least 289 records · Page 16

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NASA NTRS1988

Experimental studies in vortex pair motion coincident with a liquid reaction

An experimental examination of the coincidence of a liquid reaction (acid/base) with the formation of a vortex pair structure is described in which emphasis is placed on the evolution of the strained diffusion layer and reacted core structures. Flow visualization of the reaction process is achieved via the technique of chemically sensitive LIF. The observed growth of reacted core structures associated with each vortex is compared with theoretically predicted behavior (Marble, 1983; Karagozian and Marble, 1986). Vortex pair separation is also compared with theoretical correlations, and the relevance of the analogy between a fast liquid reaction and a gaseous reaction is discussed.

Karagozian, A. R.↗

NASA NTRS1994

Climatology of large-scale isentropic mixing in the Arctic winter stratosphere from analyzed winds

Dynamic isolation of the winter Arctic circumpolar vortex is studied using analyzed winds derived from geopotential height fields. Isentropic trajectories are calculated for assemblages of particles initialized on uniform latitude-longitude grids. Transport across isolines of Ertel potential vorticity (PV) is used to characterize the mixing processes of ejection of vortex air and entrainment of midlatitude air into the vortex. During January and February a barrier to mixing, where exchange of air is inhibited, typically forms near the vortex boundary. At 450 K, transport across the barrier is predominantly in the form of thin filaments of particles ejected from the vortex. These filaments tend to wrap around the vortex, creating a layered structure of vortex and midlatitude air at the vortex edge. Near or total splits of the vortex into two or more distinct vortex fragments are quite common based on these trajectory calculations. Significant entrainment deep into the vortex is rare and results from only a limited number of the splitting events. During December and March the mixing barrier is less evident due to nonconservative factors during the spin-up and breakdown of the vortex, respectively. In December both ejection and entrainment are only weakly inhibited by the mixing barrier. Exchange in March is dominated by ejection of air from the vortex.

Dahlberg, Steven P.↗

NASA NTRS2021

Transition Analysis for Isolated Trips on BOLT-II Wind-Tunnel and Flight Configuration

The BOLT-II configuration designed for an upcoming flight experiment exemplifies the complexities of hypersonic boundary layer transition in the presence of multiple and potentially interacting instability mechanisms. The present paper is related to the NASA-led roughness experiment on the secondary side of the BOLT-II configuration. First, computations are performed to aid mean-flow based correlations for the transition measurements acquired during an experimental campaign in the Ludwieg Tube facility at the United States Air Force Academy. To that end, both laminar and turbulent flow computations are performed for selected conditions of the experiment with a 1:3 scale model of the slightly shorter BOLT configuration. The variation in well-known correlation parameters for transition due to isolated roughness elements with respect to element position, height, and flow Reynolds number is delineated. Furthermore, the wake instability theory is used to examine the amplification of unstable disturbances behind the centerline trip at the design conditions of the BOLT-II flight experiment and for its dynamically scaled version on the 1:3 scale model of the wind tunnel experiment. To our knowledge, this study represents the first analysis of the wake instabilities in the presence of a prominent entropy layer outside the boundary layer. At the nominal flight design condition of Re ∞ ) = 5.44 and Re ∞ = 2.5 x 10 6 /m, a diamond planform (“pizza-box”) trip with k/δ = 0.70 and planform-halfwidth-to-height ratio of b/k = 3.0 at X/L = 0.254 induces a strong distortion of the boundary-layer flow in the vicinity of the symmetry plane, such that the wake structure includes a centerline streak produced by the flow retardation behind the trip along with multiple additional, prominent vorticity structures representing the necklace vortex system. Stability results reveal that each of these streaks can support a strong amplification of wake instabilities within a frequency band of 95 kHz–255 kHz, with a peak N-factor of greater than 15 beyond X/L ≈ 0.42. The dynamically scaled wind tunnel configuration with Re ∞ = 20.25 x 10 6 /m is also found to support N > 15 at similar axial stations.

BOLT↗

DOE OSTI2022

Tuning of oscillation modes by controlling dimensionality of spin structures

Abstract Harmonic oscillation of spin structures is a physical phenomenon that offers great potential for applications in nanotechnologies such as nano-oscillators and bio-inspired computing. The effective tuning of oscillations over wide frequency ranges within a single ferromagnetic nanoelement is a prerequisite to realize oscillation-based nanodevices, but it has not been addressed experimentally or theoretically. Here, utilizing a vortex core structure, one of spin structures, we report a drastic change of oscillation modes over the frequency range from MHz to sub-GHz in a 100 nm-thick permalloy circular disk. Oscillation mode was found to considerably depend on the shape and dimension of the vortex core structure and various oscillation modes over a wide range of frequencies appeared with dimensional change in the vortex core structure. This work demonstrates that oscillation modes of the vortex core structure can be effectively tuned and opens a way to apply spin structures to oscillation-based technology.

36 MATERIALS SCIENCE↗

DOE OSTI2014

Vortex-antivortex coexistence in Nb-based superconductor/ferromagnet heterostructures

Low-temperature magnetic force microscopy was used to study the threshold of nucleation of spontaneous vortex-antivortex structures in superconductor/ferromagnet (S/F) hybrid systems. We investigated S/F heterostructures composed of Py as the magnetic material and Nb as the superconductor, with different thicknesses of Py and Nb. The condition for nucleation of spontaneous vortex-antivortex structures depends on fundamental parameters such as the superconducting penetration depth and the coherence length, as well as on the thickness of the superconducting film and the magnetic domain width. Here, we compare our experimental results with those of existing theoretical models and provide an estimate of the threshold of the local out-of-plane component of the magnetization for different Py film thicknesses.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

NASA NTRS1992

The structure and development of streamwise vortex arrays embedded in a turbulent boundary layer

The results of an experimental investigation of the structure and development of streamwise vortices embedded in a turbulent boundary layer are presented. Measurements of secondary velocity in the crossplane are used to characterize the vortex array structure. Measurements in the crossplane at two streamwise locations characterize the influence of interactions among the vortices on the array structure when the initial spacing between vortices is varied. Evidence of the merging of counter-rotating cores is found in embedded arrays of closely spaced vortices. A model of vortex interaction and development is constructed from the experimental results. This model is based on the structure of the two dimensional Ossen vortex. The decay of vortex circulation due to the merging of the cores is correlated with the crossplane gradient in streamwise vorticity occurring between an embedded vortex and its adjacent counter-rotating neighbors.

Wendt, Bruce J.↗

DOE OSTI2020

Magnetic Vortex and Hyperthermia Suppression in Multigrain Iron Oxide Nanorings

Single-crystal iron oxide nanorings have been proposed as a promising candidate for magnetic hyperthermia application because of their unique shape-induced vortex-domain structure, which supports good colloidal stability and enhanced magnetic properties. However, the synthesis of single crystalline iron oxide has proven to be challenging. In this article, we showed that chemically synthesized multigrain magnetite nanorings disfavor a shape-induced magnetic vortex-domain structure. Our results indicate that the multigrain Fe 3 O 4 nanorings with an average outer diameter of ~110 nm and an inner to outer diameter ratio of ~0.5 do not show a shape-induced vortex-domain structure, which was observed in the single-crystal Fe 3 O 4 nanorings of similar dimensions. At 300 Ks, multigrain magnetite nanorings showed an effective anisotropy field of 440 Oe, which can be attributed to its high surface area and intraparticle interaction. Both calorimetric and AC loop measurements showed a moderate inductive heating efficiency of multigrain magnetite nanorings of ~300 W/g at 800 Oe. Our results shed light on the magnetic ground states of chemically synthesized multigrain Fe 3 O 4 nanorings.

36 MATERIALS SCIENCE↗

DOE OSTI2017

Study of the second magnetization peak and the pinning behaviour in Ba(Fe 0.935 Co 0.065 ) 2 As 2 pnictide superconductor

Isothermal magnetic field dependence of magnetization and magnetic relaxation measurements were performed for the $H\parallel {\rm{c}}$ axis on a single crystal of Ba(Fe 0.935 Co 0.065 ) 2 As 2 pnictide superconductor having T c = 21.7 K. The second magnetization peak (SMP) for each isothermal M(H) was observed in a wide temperature range from T c to the lowest temperature of measurement (2 K). The magnetic field dependence of relaxation rate R(H), showed a peak (H spt) between H on (onset of SMP in M(H)) and H p (peak field of SMP in M(H)), which is likely to be related to a vortex-lattice structural phase transition, as suggested in the literature for a similar sample. In addition, the magnetic relaxation measured for magnetic fields near H spt showed some noise, which might be the signature of the structural phase transition of the vortex lattice. Analysis of the magnetic relaxation data using Maley's criterion and the collective pinning theory suggested that the SMP in the sample was due to the collective (elastic) to plastic creep crossover, which was also accompanied by a rhombic to square vortex lattice phase transition. The analysis of the pinning force density suggested a single dominating pinning mechanism in the sample, which did not showing the usual $\delta {l}$ and $\delta {T}_{{\rm{c}}}$ nature of pinning. Furthermore, the critical current density (J c ), estimated using the Bean critical state model, was found to be 5 × 10 5 A cm - 2 at 2 K in the zero magnetic field limit. Surprisingly, the maximum of the pinning force density was not responsible for the maximum value of the critical current density in the sample.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

NASA NTRS1985

Vortex-dominated flow with viscous core structure

Recent theoretical studies of vortex-dominated flows are reviewed with special emphasis on those for which the viscous core structures play an important role. The problems to be described are: The interaction and merging of two-dimensional vortices and of curved vortex filaments, the roll-up and decay of trailing far wakes, and the initiation of vortex breakdown. The analysis utilizes finite-difference solutions of the Navier-Stokes equations complemented by asymptotic expansion techniques.

Liu, C. H.↗

DOE OSTI2023

Saturation of Vortex Rings Ejected from Shock-Accelerated Interfaces

Structures evoking vortex rings can be discerned in shock-accelerated flows ranging from astrophysics to inertial confinement fusion. By constructing an analogy between vortex rings produced in conventional propulsion systems and rings generated by a shock impinging upon a high-aspect-ratio protrusion along a material interface, we extend classical, constant-density vortex-ring theory to compressible multifluid flows. Here, we further demonstrate saturation of such vortex rings as the protrusion aspect ratio is increased, thus explaining morphological differences observed in practice.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

NASA NTRS1987

Numerical simulation of channel flow transition, resolution requirements and structure of the hairpin vortex

Three-dimensional, nonlinear numerical simulations are presented for the K-type and H-type transitions for channel flow. There are two objectives. The first is to establish firmly the resolution requirements for the various stages in the transition process. Comparisons between calculations on various grids suggest a set of guidelines for maintaining a physically meaningful calculation. The second objective is to map out the structure of the hairpin vortices which arise in K-type and H-type transitions in channel flow, to the latest stage currently feasible. Flow field details are presented for both a subcritical Reynolds number of 1500 and a supercritical Reynolds number of 8000. The diagnostics include illustrations of the vertical shear, streamwise and spanwise vorticity, helicity, vortex stretching, and vortex diffusion fields.

Krist, Steven E.↗

NASA NTRS1977

Vortex pairing and organized structures in axisymmetric jets under controlled excitation

A description is given of experiments which have been carried out in a circular air jet facility consisting of two settling chambers in sequence. Sinusoidal perturbations in the exit profile are introduced at controlled frequencies and amplitudes with the aid of a loudspeaker attached to the wall of the first chamber. It was found that vortex pairing in circular jets can occur in two distinct modes, including the shear layer mode and the jet mode. Amplitude variations, the conditions for strong vortex pairing, and the spectral evolution downstream are illustrated with the aid of graphs.

Zaman, K. B. M. Q.↗

NASA NTRS2011

Reconstruction of Propagating Kelvin-Helmholtz Vortices at Mercury's Magnetopause

A series of quasi-periodic magnetopause crossings were recorded by the MESSENGER spacecraft during its third flyby of Mercury on 29 September 2009, likely caused by a train of propagating Kelvin-Helmholtz (KH) vortices. We here revisit the observations to study the internal structure of the waves. Exploiting MESSENGER s rapid traversal of the magnetopause, we show that the observations permit a reconstruction of the structure of a rolled-up KH vortex directly from the spacecraft s magnetic field measurements. The derived geometry is consistent with all large-scale fluctuations in the magnetic field data, establishes the non-linear nature of the waves, and shows their vortex-like structure. In several of the wave passages, a reduction in magnetic field strength is observed in the middle of the wave, which is characteristic of rolled-up vortices and is related to the increase in magnetic pressure required to balance the centrifugal force on the plasma in the outer regions of a vortex, previously reported in computer simulations. As the KH wave starts to roll up, the reconstructed geometry suggests that the vortices develop two gradual transition regions in the magnetic field, possibly related to the mixing of magnetosheath and magnetospheric plasma, situated at the leading edges from the perspectives of both the magnetosphere and the magnetosheath.

Sundberg, Torbjoern↗

DOE OSTI2014

Femtosecond laser surface structuring of silicon using optical vortex beams generated by a q-plate

We report on laser surface structuring of silicon using Ti:Sa femtosecond laser ablation with optical vortex beams. A q-plate is used to generate an optical vortex beam with femtosecond pulse duration through spin-to-orbital conversion of the angular momentum of light. The variation of the produced surface structures is investigated as a function of the number of pulses, N, at laser fluence slightly above the ablation threshold value. At low N (≈10), only surface corrugation of the irradiated, ring-shaped area is observed. This is followed by a progressive formation of regular ripples at larger N (≈100–500), which eventually transform in smaller columnar structures for N ≈ 1000. Moreover, the central, non-ablated part is gradually decorated by nanoparticles produced during laser ablation, a process which eventually leads to the formation of a central turret of assembled nanoparticles. Our experimental findings suggest the importance of a feedback mechanism and a cumulative effect on the formation of ripples with interesting patterns not achievable by the more standard beams with a Gaussian intensity profile.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2016

Spin Vortex Resonance in Non-planar Ferromagnetic Dots

In planar structures, the vortex resonance frequency changes little as a function of an in-plane magnetic field as long as the vortex state persists. Altering the topography of the element leads to a vastly different dynamic response that arises due to the local vortex core confinement effect. In this work, we studied the magnetic excitations in non-planar ferromagnetic dots using a broadband microwave spectroscopy technique. Two distinct regimes of vortex gyration were detected depending on the vortex core position. The experimental results are in qualitative agreement with micromagnetic simulations.

36 MATERIALS SCIENCE↗

NASA NTRS1994

Flow structure generated by perpendicular blade vortex interaction and implications for helicopter noise predictions

Activities carried out in support of research on flow structure generated by perpendicular blade vortex interaction and implications for helicopter noise prediction are summarized. Progress in the following areas is described: (1) construction of 8 inch-chord NACA 0012 full-span blade; (2) Acquisition of two full-span blades; (3) preparation for hot wire measurements; (4) related work on a modified Betz's theory; and (5) work related to helicopter noise prediction. In addition, a list of publications based on the results of prior experimentation is presented.

Devenport, William J.↗

NASA NTRS1992

The structure of the polar vortex

The paper develops a comparative picture of the 1987 Southern Hemisphere and 1989 Northern Hemisphere lower stratospheric, polar vortex circulation and constituent distributions as observed by the Airborne Antarctic Ozone Experiment, August 17-September 22, 1987, and Airborne Arctic Stratospheric Expedition, January 3-February 19, 1989 aircraft campaigns. Overall, both polar vortices define a region of highly isolated air, where the exchange of trace gases occurs principally at the vortex edge through erosional wave activity. Aircraft measurement showed that between 50 and 100 mbar, horizontally stratified long-lived tracers such as N2O are displaced downward 2-3 km on the cyclonic (poleward) side of the jet with the meridional tracer gradient sharpest at the jet core. Eddy mixing rates, computed using parcel ensemble statistics, are an order of magnitude or more lower on the cyclonic side of the jet compared to those on the anticyclonic side. Poleward zonal mean meridional flow on the anticyclonic side of the jet terminates in a descent zone at the jet core.

Schoeberl, Mark R.↗

NASA NTRS1993

Turbulent structure of a wingtip vortex in the near field

The turbulent rollup of a vortex generated by a rectangular wing has been investigated. Extensive mean and turbulence measurements of the flowfield on a wingtip and in the near field have been completed. Velocity fluctuation measurements show that the near-field core is not laminar. A large axial velocity excess was found to exist in the core of the vortex. A momentum balance in the near-field of the wingtip showed that the magnitude of the core Reynolds-stress gradient terms are the same order as the largest terms in the governing equations. Navier-Stokes computations of the identical configuration, including wind tunnel walls and using measured inflow and outflow boundary conditions, reproduced many of the features of the experiment. Inherent limitations of the Baldwin-Barth turbulence model combined with limited grid resolution caused the computed vortex core to be more diffuse than desired. The momentum balance also demonstrated that the level of numerically generated false diffusion in the vortex core is relatively high.

Zilliac, Gregory G.↗