Vortex Wall Research Articles

NUMERICAL ANALYSIS OF THE FLUID DYNAMICS IN A HELICAL STENT

Motivation: The implantation of a stent into a native coronary artery represents a bold intrusion of a totally foreign substance into the circulatory system. Although most of the clinically implanted coronary stents are uneventful in patients, many however failed due to disturbed flow caused by the stent. It is always desirable to continuously carry out research that may eventually lead to the development of an ideal stent for the future. Methods: The flow dynamics of a simple helical stent is numerically analyzed by computational fluid dynamics (CFD), (CFD-ACE+ 2004, ESI-CFD INC., USA). Based on the physiological character of coronary artery, the parameters of helical stent, pitch, roughness, leading angle, and diameter are manipulated to derive flow information of wall shear stress, pressure distribution, and vortices. Results and Conclusions: The results show that the flow is dominated by the contact geometry between the blood flow and the wall when the wall of stent is thick. Swirling flow is also observed when the helical stent is implanted into the artery with large wall effect and small screw angle. At the same stent thickness, the helical stent exhibits larger wall shear stress than mesh stent. Endothelium proliferation is thus expected to decrease.

Large-scale vortical structures produced by an impinging density jet in shallow crossflow

The large-scale vortical structures produced by an impinging density jet in shallow crossflow were numerically investigated in detail using RNG turbulence model. The scales, formation mechanism and evolution feature of the upstream wall vortex in relation to stagnation point and the Scarf vortex in near field were analyzed. The computed characteristic scales of the upstream vortex show distinguished three-dimensionality and vary with the velocity ratio and the water depth. The Scarf vortex in the near field plays an important role in the lateral concentration distributions of the impinging jet in crossflow. When the velocity ratio is relatively small, there exists a distinct lateral high concentration aggregation zone at the lateral edge between the bottom layer wall jet and the ambient crossflow, which is dominated by the Scarf vortex in the near field.

Fabrication of curved-line nanostructures on membranes for transmission electron microscopy investigations of domain walls

We have fabricated curved-line ferromagnetic nanostructures on membranes for transmission electron microscopy investigations of the equilibrium magnetic spin configurations. The magnetic elements were fabricated using electron-beam lithography and a lift-off procedure for pattern transfer. Due to the fragile nature of the membranes, the design of the elements was chosen to ensure that the lift-off was possible without using ultrasound. The elements included three-quarter rings, zig-zag lines and wavy lines with notches resulting in constrictions with widths down to 30 nm. Magnetic configurations were observed using electron holography and Lorentz microscopy. In particular, the details of the spin structure of vortex and transverse walls, and its dependence on the local geometry were obtained. While Lorentz microscopy provided qualitative information about magnetic spin orientations and the positions of domain walls, electron holography gave quantitative high-resolution images of the magnetic induction allowing the direct measurement of the stray field between adjacent domain walls.

Multi-vortices are wall vortices: A numerical proof

We study the Abrikosov–Nielsen–Olesen multi-vortices. Using a numerical code we are able to solve the vortex equations with winding number up to n = 25 000 . We can thus check the wall vortex conjecture previously made in [S. Bolognesi, Nucl. Phys. B 730 (2005) 127; S. Bolognesi, Nucl. Phys. B 730 (2005) 150]. The numerical results show a remarkable agreement with the theoretical predictions.

Observation of thermally activated domain wall transformations

The spin structure of head-to-head domain walls in Ni80Fe20 structures is studied using high-resolution photoemission electron microscopy. The quantitative phase diagram is extracted from these measurements and found to exhibit two phase boundaries between vortex and transverse domain walls. The results are compared with available theoretical predictions and micromagnetic simulations and differences to the experiment are explained, taking into account thermal excitations. Temperature-dependent measurements show a thermally activated transformation of transverse to vortex domain walls in 7 nm thick and 730 nm wide structures at a transition temperature between 260 °C and 310 °C, which corresponds to a nucleation barrier height for a vortex wall between 6.7×10−21J and 8.0×10−21J.

Large N, [formula omitted] strings and bag models

We study Z N strings in non-Abelian gauge theories, when they can be considered as domain walls compactified on a cylinder and stabilized by the flux inside. To make the wall vortex approximation reliable, we must take the 't Hooft large N limit. Our construction has many points in common with the phenomenological bag models of hadrons.

Domain walls and textured vortices in a two-component Ginzburg–Landau model

We look for domain wall and textured vortex solutions in a two-component Ginzburg–Landau model inspired by two-band superconductivity. The two-dimensional two-component model, with equal coherence lengths and no magnetic field, shows some interesting properties. In the absence of a Josephson type coupling between the two order parameters a “textured vortex” is found by analytical and numerical solution of the Ginzburg–Landau equations. With a Josephson type coupling between the two order parameters we find the system to split up in two domains separated by a domain wall, where the order parameter is depressed to zero.

Quantitative Analysis of Magnetic Excitations in Landau Flux-Closure Structures Using Synchrotron-Radiation Microscopy

We have measured the excitation spectrum of six micron magnetic squares using x-ray magnetic circular dichroism. We observe all three excitations expected in a Landau flux-closure pattern. High temporal and spatial resolution allows quantitative analysis of the excitations. A short magnetic in plane pulse excites the magnetic element and we observe precessional motion of the magnetization within the domains as well as a domain wall mode and vortex motion. The vortex moves perpendicular to the excitation field and relaxes without showing a circulating orbit.

The normal and oblique collision of a dipole with a no-slip boundary

Benchmark results are reported of two separate sets of numerical experiments on the collision of a dipole with a no-slip boundary at several Reynolds numbers. One set of numerical simulations is performed with a finite differences code while the other set concerns simulations conducted with a Chebyshev pseudospectral code. Well-defined initial and boundary conditions are used and the accuracy and convergence of the numerical solutions have been investigated by inspection of several global quantities like the total kinetic energy, the enstrophy and the total angular momentum of the flow, and the vorticity distribution and vorticity flux at the no-slip boundaries. It is found that the collision of the dipole with the no-slip wall and the subsequent flow evolution is dramatically influenced by small-scale vorticity produced during and after the collision process. The trajectories of several coherent vortices are tracked during the simulation and show that in particular underresolved high-amplitude vorticity patches near the no-slip walls are potentially responsible for deteriorating accuracy of the computations in the course of time. Our numerical simulations clearly indicate that it is extremely difficult to obtain mode- or grid-convergence for this seemingly rather simple two-dimensional vortex–wall interaction problem.

Magnetization reversal in individual micrometer-sized polycrystalline Permalloy rings

The magnetization reversal of individual 2 μm and 5 μm diameter polycrystalline Permalloy rings, with respective widths 0.75 μm and 1 μm, thickness 45 nm, has been investigated by focused magneto-optic Kerr effect (MOKE) magnetometry. Micromagnetic simulation of the reversal in the 2 μm diameter ring reveals that the onion-to-vortex state switching occurs by nucleation and subsequent annihilation of vortex walls that span the width of the ring, and that the vortex-to-reverse-onion state switching occurs by expansion of a reverse domain. The hysteresis loop shows good agreement with the experimental MOKE loop. Measurements of the switching through one-half of a 5 μm diameter ring enable the determination of the circulation of the vortex states accessed during one applied field cycle. The rings switch via one vortex state (either clockwise or anticlockwise) on both downward and upward applied field sweeps. The number of applied field cycles spent switching via one vortex state before changing to switch via the opposite vortex state is random, likely to be due to the history of the spin configuration and thermal fluctuations.

Influence of freelayer in magnetic tunnel junction on switching of submicrometer magnetoresistive random access memory arrays

As magnetic tunnel junction (MTJ) cells for magnetoresistive random access memory (MRAM) are reduced in size, the presence of a magnetization vortex seriously interferes with switching selectivity. We prepared 0.3 /spl mu/m/spl times/0.8 /spl mu/m, nearly rectangular shape MTJs consisted of PtMn/CoFe/Ru/CoFe/AlOx/NiFe t (t=3,4.5, and 6 nm). Both at-field and remanent state measurements at 0.4 V were conducted to distinguish kinks originating from vortex and domain wall pinning. In addition, we measured samples at various temperatures (from room temperature to 500 K), and with various hard axis fields (from 0 to 90 Oe). As temperature increased, average switching fields decreased, more rapidly for t=6 nm junctions, but kinks were not eliminated completely. When the hard axis field reached about 40-60 Oe, nearly kink-free switching was possible for t=6 nm MTJs.

Domain wall motion in nanowires

We investigated the motion of domain walls in ferromagnetic cylindrical nanowires by solving the Landau–Lifshitz–Gilbert equation numerically for a classical spin model in which energy contributions from exchange, crystalline anisotropy, dipole–dipole interactions, and a driving magnetic field are considered. Depending on the diameter, either transverse domain walls or vortex walls are found. A transverse domain wall is observed for diameters smaller than the exchange length of the given system. In this case, the system effectively behaves one dimensionally and the domain wall velocity agrees with the result of Slonczewski for one-dimensional walls. For larger diameters, a crossover to a vortex wall sets in which enhances the domain wall velocity drastically. For a vortex wall the domain wall velocity is described by the Walker formula.

Head-to-head domain-wall phase diagram in mesoscopic ring magnets

The nanoscale spin structure of head-to-head domain walls in mesoscopic ferromagnetic rings has been studied by high-resolution nonintrusive photoemission electron microscopy as a function of both ring width (100–730 nm) and film thickness (2–38 nm). Depending on the geometry, two types of head-to-head domain walls are found (vortex and transverse walls). The experimental phase diagram, which identifies the transition between the wall types, is compared to analytical calculations of the energy and micromagnetic simulations, which are found to agree well with the experimental results.

Structural development of vortical flows around a square jet in cross-flow

The present computational study is devoted to unfolding the complex process of three–dimensional flow interaction around a square jet in cross–flow. The aim is to provide a clear understanding about the structural development of the entire vortical flow field, which may immensely enhance our knowledge regarding mutual interaction among various vortical structures that takes place around the jet. Careful attempts have been made to capture the detailed mechanism of formation of the near–field horseshoe–vortex system and the roll–up process of the hovering vortices. The rolled–up shear–layer hovering vortices, which wrap around the front and the lateral jet–cross–flow interface, are observed to initiate the Kelvin–Helmholtz–like instability. The present study also clearly displays the inception process of the counter–rotating vortex pair (CVP) from the shear layers that develop on the two lateral side walls of the jet pipe. In order to better understand the complete flow–interaction process and the governing flow physics, the simulation was performed for a moderate value of the Reynolds number ( Re = 225), and for a jet–to–free–stream velocity ratio of 2.5. The interaction process between the streamwise wall vortices and the developed upright (or spin–off, or zipper) vortices in the downstream boundary layer is observed to contribute substantially in the structural development of the jet wake. The upright vortices were seen to originate from the tornado–like critical points on the channel floor shear layer, and subsequently the vortices lift themselves away from the channel floor to merge ultimately with the evolving CVP. Importantly, such merging processes are observed to locally enhance the CVP strength. Following the topological theory of Legendre, the depicted map of computed critical points and the separation lines helps to provide additional insight into the flow mechanism. The computed results clearly demonstrate the entire vortical flow–interaction process to its totality, including all the recent experimental predictions that are made for such flows. Notably, as it was experimentally verified for round jets in cross–flow, in the present configuration too, the flow separation on the channel floor is found to be the basic source of inception of the wall and the upright vortices. The separated flow in the vicinity of different wall vortical corelines joins to form the upright vortices.

Near Critical Swirling Flow in a Slightly Contracting Pipe

The development of an inviscid, incompressible, axisymmetric, and near-critical swirling jet entering a slightly contracting finite-length pipe is studied. Certain flow conditions that can reflect the physical situation are prescribed along the pipe inlet, outlet, centerline, and wall. To understand the nature of the flows near the critical swirl level, a nonlinear small-disturbance analysis is developed from the governing equations of motion. It shows that a small but finite pipe contraction causes the flow to accelerate near the centerline and decelerate along the wall. For certain inlet swirl levels and pipe contractions, the flow on the wall at the pipe outlet can stagnate. The analysis indicates for the first time that a wall separation zone can appear in pipes at rotation levels below the critical swirl for vortex breakdown and compete with the appearance of the breakdown zones. Increasing the vortex core radius, the speed of the axial jet, and the pipe length promotes the wall separation phenomenon at swirl levels below the critical swirl for a fixed pipe contraction or at a smaller pipe contraction for a fixed swirl level. Based on the present results and previous studies, the interaction between wall separation zones and vortex breakdown zones is discussed. This investigation might be relevant for high-Reynolds-number laminar flows in pipes and as long as the boundary layers remain thin and attached to the pipe wall.

Effects of Wall Rotation on Heat Transfer to Annular Turbulent Flow: Outer Wall Rotating

AbstractSimulations were conducted for air flowing upward in a vertical annular pipe with a rotating outer wall. Simulations concentrated on the occurrence of laminarization and property variations for high heat flux heat transfer. The compressible filtered Navier-Stokes equations were solved using a second-order accurate finite volume method. Low Mach number preconditioning was used to enable the compressible code to work efficiently at low Mach numbers. A dynamic subgrid-scale stress model accounted for the subgrid-scale turbulence. When the outer wall rotated, a significant reduction of turbulent kinetic energy was realized near the rotating wall and the intensity of bursting appeared to decrease. This modification of the turbulent structures was related to the vortical structure changes near the rotating wall. It has been observed that the wall vortices were pushed in the direction of rotation and their intensity increased near the nonrotating wall. The consequent effect was to enhance the turbulent kinetic energy and increase the Nusselt number there.

Mean streamline analysis for performance prediction of cross-flow fans

This paper presents the mean streamline analysis using the empirical loss correlations for performance prediction of cross-flow fans. Comparison of overall performance predictions with test data of a cross-flow fan system with a simplified vortex wall scroll casing and with the published experimental characteristics for a cross-flow fan has been carried out to demonstrate the accuracy of the proposed method. Predicted performance curves by the present mean streamline analysis agree well with experimental data for two different cross-flow fans over the normal operating conditions. The prediction method presented herein can be used efficiently as a tool for the preliminary design and performance analysis of general-purpose cross-flow fans.

Anomalous switching in submicrometer magnetic tunnel junction arrays arising from magnetic vortex and domain wall pinning

We present switching characteristics of patterned submicrometer magnetic tunnel junction arrays containing NiFe and CoFe free layers. The resemblance of magnetization and magnetoresistance (MR) curves was studied by micromagnetic calculations and experimental measurements. Upon analyzing the MR transfer curves, the magnetic vortex and domain wall pinning effects on anomalous switching of each magnetic tunnel junction can be distinguished by remanent states. Data indicates that the low saturation magnetization of the free layer increases domain wall pinning and decreases trapped magnetization vortices.

Control of unstable oscillations in a confined and bounded injecting wall channel

The analysis of a confined flow field generated through two separated injecting walls was carried out by studying the effect of a mass flow rate difference between the two walls. Such a parameter has been found to play a major role in unstable flow field behaviour. Concretely speaking, we have identified two vortex shedding phenomena, i.e. the main flow and the wall vortex shedding phenomena. Results clearly show that when the mass flow rate is increased at the second injecting wall, wall vortex coherence is enhanced and impinging of such structures forces a coupling phenomenon to develop between flow field dynamics and acoustics. On the other hand, only a 15% mass flow rate difference of the first injecting block is sufficient to prevent such coupling between acoustics and vortex shedding phenomenon. Consequently, the resonance phenomenon is pronouncedly weakened and significant oscillation reduction is achieved.

The enhancement of flux-line pinning in all-oxide superconductor/ferromagnetheterostructures

We have studied the local critical current density,jc, in the superconductor thin film of bilayer structures consisting ofYBa2Cu3O7 and theferromagnets La2/3Ca1/3MnO3 and SrRuO3, respectively, by means of quantitative magneto-optics. A pronounced hysteresis ofjc was observed which is ascribed to the magnetization state of the ferromagnetic layer.The results are discussed within the frame of magnetic vortex–wall interactions.