Vortex Core Region Research Articles

Vortex states in the Ising superconductors on honeycomb lattice

By self-consistently solving the Bogoliubov-de Gennes equations of a model Hamiltonian built on the honeycomb lattice, we study the roles played by the Ising spin-orbital coupling (ISOC) pertinent to the transition metal dichalcogenides (TMDs) in the presence of a magnetic field. It is shown that the presence of ISOC enables the superconductivity to survive in a high in-plane magnetic field, otherwise it would be destroyed completely by the magnetic field. For the vortex states produced by a perpendicular magnetic field, the ISOC will induce a weak ferromagnetic order within the vortex core region. The rapid increase of the ferromagnetic order at very low temperatures prevents the vortex core size from further shrinks with temperature decreasing, and results in a larger saturation value at low temperatures. More importantly, the ISOC also has the effect in shifting the energy levels of the finite energy vortex bound states and tends to merge them into the near zero-energy bound state, and thus obscures the discrete energy level feature for the energy dependence of the local density-of-state. The results in some way account for the experimental observations, and suggest that the ISOC in the TMDs may be an intrinsic factor apart from the thermal broadening to give rise to the near zero-energy peak observed in experiments on the vortex states of TMDs’ superconductors.

Investigation of vortex in pump sump by V3V measurements

The aims, scope and conclusions of the paper must be in a self-contained abstract of a single paragraph with 60-120 words. The abstract must be informative and not just indicative and contain a summary of the significant results reported in the paper. No references should be cited. Avoid mathematical expressions as far as possible. Just below the abstract, up to six keywords should be provided. The vortex may appear in the sump when the pump inlet is not properly designed or the pump suction pipe is not deeply submerged enough, not only will the pump’s energy performance and cavitation performance be significantly reduced, but it may also cause the pump unit to vibrate and fail to operate properly. The water surface vortex is the most common vortex into the sump, and it is also a vortex that has a great influence on the operation of the pump. According to the development process of the water surface vortex, it is divided into four shapes: A-shape vortex: the water flow rotates slowly, forming only a shallow sunken; the B-shape vortex: the water flow rotates faster, the sunken becomes deeper as a funnel, and the air intermittently enters the water pump. It has only influence on the performance of the pump; C-shape vortex: the rotation speed of the water flow is very fast, the funnel has penetrated into the suction pipe, the air continuously enters the water pump, and the pump unit generates strong vibration, which has a serious impact on the operation of the water pump; D-shape vortex: The water flow rotates sharply around the suction pipe. A large amount of air enters the water pump, and the pump unit generates severe vibration, which is impossible to operate. The pump unit generates severe vibration and cannot run. In this paper, the vortex of the sump is measured by the V3V volumetric field three-dimensional laser velocimetry to study the mechanism and development of vortex under the flow rate at the BEP of the pump. The measurement results show that the distribution of the circumferential velocity Vu of the vortex region in the vortex core is characterized by the value at the center of the core being close to zero, and the circumferential velocity of the vortex increasing with the increase of the radius, At a certain value the circumferential velocity is gradually reduced as the radius increased, and there is a maximum value during this change. At each stage of the development of the vortex, the regularity of the circumferential velocity of the vortex changes with the radius is the same that circumferential velocity increases in the vortex core, then decreases gradually outside of vortex core. The 3D vortex is obtained. The circumferential velocity gradient is quite large in the vortex core region. The maximum value is not on the water surface, but at a small depth below the water surface. The dissipation of vorticity is much rapid than its accumulation, which is shown as Instantaneous collapse of the vortex (less than 1 s). When the vorticity of the vortex reaches a certain value, it often causes severe vibration of the pump unit.

Aerodynamic Characteristics of the X-Tail Stabilizer in a Hybrid Unmanned Aircraft

The paper presents the results of the numerical calculation of the flow around hybrid unmanned aircraft with X-tail stabilizers. This aircraft design is an innovative combination of a gyroplane and a multi-rotor. The aerodynamic properties of the two-part X-tail stabilizer were analysed. The aerodynamic forces and moments on this element of the aircraft were specified from the CFD simulation calculations for the defined values of the angle of attack and the slip angle, and particular attention was paid to the differences between the upper and lower parts of the stabilizer. In addition, the turbulence kinetic energy on the surface of the vortex core region were analysed to conduct a qualitative and quantitative analysis of energy losses in flow, resulting from local separation and vortices. The obtained results allow for conducting further aerodynamic comparative analyses on other types of stabilizers.

Lagrangian analysis of the fluid transport induced by the interaction of two co-axial co-rotating vortex rings

In this paper, the fluid transport in the interaction of two co-axial co-rotating vortex rings are investigated. Vortex rings are generated using the piston-cylinder apparatus, and the resulting velocity fields are measured using digital particle image velocimetry. The interaction process is analysed by means of vorticity contour, as well by investigation of the Lagrangian coherent structures (LCSs) defined by the ridges of the finite-time Lyapunov exponent (FTLE). Experimental results demonstrate that two types of vortex interaction are identified, namely strong and weak interactions, respectively. For the strong interaction, the Lagrangian boundaries of the two vortex rings are merged together and form a flux window for fluid transport. For weak interaction, only the Lagrangian drift induced by the motion of the front vortex ring is observed and affects the Lagrangian boundary of the rear vortex ring. Moreover, the fluids transported in the strong interaction carry considerable momentum but no circulation. By contrast, there are nearly no fluxes occurring in the weak interaction. By tracking the variations of circulation and impulse occupied by the separated regions distinguished by the LCSs, it is found that the circulation nearly has no change, but the impulse occupied by vortex core region has significant change. In the strong interaction, the impulse of rear vortex ring decreases but the impulse of the front vortex ring increases. Based on the impulse law, it is speculated that the fluid force generated by the formation of the rear vortex rings can be enhanced. Therefore, the strong interaction between wake vortices can actually improve the propulsive efficiency of the biological systems by operating the formation of large-scale vortices.

Experimental analysis of tip vortex cavitation mitigation by controlled surface roughness

This study presents results of experiments where roughness applications are evaluated in delaying the tip vortex cavitation inception of an elliptical foil. High-speed video recordings and laser doppler velocimetry (LDV) measurements are employed to provide further details on the cavitation behavior and tip vortex flow properties in different roughness pattern configurations. The angular momentum measurements of the vortex core region at one chord length downstream of the tip indicate that roughness leads to a lower angular momentum compared with the smooth foil condition while the vortex core radius remains similar in the smooth and roughened conditions. The observations show that the cavitation number for tip vortex cavitation inception is reduced by 33% in the optimized roughness pattern compared with the smooth foil condition where the drag force increase is observed to be around 2%. During the tests, no obvious differences in the cavitation inception properties of uniform and non-uniform roughness distributions are observed. However, the drag force is found to be higher with a non-uniform roughness distribution.

Three-Dimensional Development of Coherent Structures in a Two-Dimensional Laminar Separation Bubble

Three-dimensional flow development is experimentally assessed in a convectively unstable laminar separation bubble formed over a NACA 0018 airfoil at a chord Reynolds number of 125,000, an angle of attack of 4 deg, and a freestream turbulence intensity of 0.07%. The flow is weakly excited in a spanwise uniform manner at a frequency matching that of the most unstable disturbances in the natural separation bubble, leaving the base flow unmodified while enabling three-dimensional reconstructions of the dominant coherent structures using phase-locked planar particle image velocimetry measurements. Time-averaged flowfield reconstructions show a strongly two-dimensional topology of the separation bubble for both the natural and weakly excited cases. Analysis of the flow development demonstrates that, for both the natural and excited flows, spanwise-oriented and strongly two-dimensional shear-layer vortices form in the separation bubble upstream of the mean maximum height location. Spanwise undulations develop in the vortex filaments that continually intensify with downstream convection as a result of the streamwise forward sections of the filaments lifting away from the surface. This motion reorients the vorticity of the primary structures from the spanwise direction into the streamwise and wall-normal directions, forming hairpinlike structures above the vortex core region. These findings offer new quantitative insight into the vortex dynamics and breakdown process of the shear-layer vortices in two-dimensional, convectively unstable laminar separation bubbles subject to low freestream turbulence levels.

Free-stream turbulence interaction with a wing-tip vortex

The interaction of a wing-tip vortex of a rectangular, square-tipped wing having a NACA 0012 airfoil section with a grid-generated turbulent flow was investigated in this paper. The experiments were conducted in the near and mid-wake regions at three free stream turbulence (FST) intensities of 0.5%, 3% and 6%, and at two Reynolds numbers, Re, based on the wing chord length, cw, of 2×105 and 3×105. Stereoscopic Particle Image Velocimetry (SPIV) and hot-wire measurements were carried out at four downstream positions, namely x/cw=1.25,3.25,6.25 and 7.75. Streamwise velocity contours showed that the wing-tip vortex decayed with increased FST and downstream distance. In the vortex core region, the streamwise velocity decelerated while the vortex adopted a wake-like profile. FST was found to decrease the vortex circulation and to increase the vortex radius and vortex meandering amplitude. When increasing Reynolds number, the grid cases showed little variation of the vortex radius and peak vorticity levels, particularly at larger downstream positions, suggesting that the effects of FST and Re number on the vortex development are nearly independent. The measured total turbulent kinetic energy (TKE) was found to be mostly due to vortex meandering. In that, total TKE levels devoid of meandering showed a virtually turbulence-free vortex core.

Circular polarization reversal of half-vortex cores in polariton condensates

Vortices are topological objects carrying quantized orbital angular momentum and have been widely studied in many physical systems for their applicability in information storage and processing. In systems with spin degree of freedom the elementary excitations are so called half-vortices, carrying a quantum rotation only in one of the two spin components. We study the spontaneous formation and stability of localized such half-vortices in semiconductor microcavity polariton condensates, non-resonantly excited by a linearly polarized ring-shaped pump. The TE-TM splitting of optical modes in the microcavity system leads to an effective spin-orbit coupling, resulting in solutions with discrete rotational symmetry. The cross-interaction between different spin components provides an efficient method to realize all-optical half-vortex core switching inverting its circular polarization state. This switching can be directly measured in the polarization resolved intensity in the vortex core region and it can also be applied to higher order half-vortex states.

Effect of URANS and Hybrid RANS-Large Eddy Simulation Turbulence Models on Unsteady Turbulent Flows Inside a Side Channel Pump

Abstract In this work, the unsteady Reynolds-averaged Navier–Stokes (URANS) and three hybrid Reynolds-averaged Navier–Stokes-large eddy simulation (RANS-LES) models are employed to resolve the vortical flows in a typical single-stage side channel pump, to evaluate the suitability of these advanced turbulence models in predicting the pump hydraulic performance and unstable swirling flows. By the comparison of the overall performance, it can be observed that the results obtained by scale-adapted simulation (SAS) are closer to test data than shear stress transport (SST), detached eddy simulation (DES) and filter-based model (FBM). Simultaneously, the distribution of axial velocity on the plane near the interface is used to describe the position and intensity of internal fluid exchange between impeller and side channel. It is obvious that the intensity of mass flow exchange is strong near the inner and outer edges. Then, the vortex core region illustrates that the vortex is easily produced near the interface due to internal fluid exchange. Finally, the evolutions of circumferential in-plane vortical structures are presented to further account for the process of fluid exchange and the main vortex flows. It reveals that the recirculation flow presents a strong instability during 6–7 blade pitches as the fluid enters into the impeller and the flow is stable in downstream 7–8 blade pitches. Besides, the flow turns to be unsteady near outlet affected by the sudden change of fluid direction. This work could provide some suggestions for the choice of appropriate turbulence model in simulating strong swirling flows.

Numerical and experimental analysis on the non-uniform inflow characteristics of a reactor coolant pump with a steam generator channel head

ABSTRACT Distorted inflow caused by the complex structure of the steam generator channel head (SGCH) occurs at the entrance to the reactor coolant pump (RCP) in the AP1000 system, which may generate impeller cracks and result in deterioration of performance and stability. In this paper, to analyze the influence of distorted inflow on flow properties of the RCP, the inflow distortion characteristics are studied by a full three-dimensional steady and unsteady numerical simulation. The simulation model was verified experimentally by testing the RCP with a straight pipe and a channel head at different flow rates. Using this model, the inlet distortion was quantified and the variation law of different distortion was obtained. The results showed that a non-uniform swirling flow under lateral pressure differences was formed when the pump was coupled with a channel head instead of a straight pipe, leading to the destruction of circular symmetry of the inflow field. Further increasing the tube length alone was not effective in improving the inflow field of the RCP, and the inlet swirl effect always existed. As the distorted flow entered the pump with a pair of vortices, the chaotic flow field imposed a non-uniform stress on each blade of the impeller, resulting in the unevenly distributed flow rate of each flow-path of the blade. Then, in the flow channels of the impeller, there were obvious asymmetrical vortex core regions, even in opposite directions, which originally formed in the inflow field of the RCP and raised the risk of blade fatigue failure. Owing to the significant effects of inflow distortion produced by the SGCH, understanding the mechanism of non-uniform inflow characteristic is of great value to the design and optimization of the RCP. Abbreviations: RCP: reactor coolant pump; SG: steam generator; SGCH: steam generator channel head

Dynamics of spiral spin waves in magnetic nanopatches: Influence of thickness and shape

We explore the dynamics of spiral spin waves in Permalloy nano-elements with variable aspect ratio of geometric dimensions, and their potential use as improved spin wave emitters with no or little biasing field required. Numerical results show that above a certain thickness, propagating spiral waves can be obtained in circular and square shaped elements in a flux closure state. VNA-FMR experiments on 20 nm (thin) and 80 nm (thick) samples confirm two type of spectra corresponding to different dispersions for thinner and thicker elements. We show that, for the thicker films, the vortex core region acts as a source of large amplitude spiral spin waves, which dominate over other modes. In case of the thinner elements, these modes are critically damped. For different shapes of the patch, we show that a rich collection of confined propagating modes can also be excited, modifying the final wave front and enriching the potential of the nano-dot as a spin wave emitter. We give an explanation for the intense spiral modes from the perspective of a balance of dipolar and exchange energies in the sample.

Large‐Scale Experiments on the Formation of Surface Vortices with and without Vortex Suppression

AbstractExperiments were carried out in a 50‐m3 cylindrical tank to determine the influence of strong momentum on the formation of large‐scale gas‐core vortices. Gas‐core lengths were measured for varying volume flow rates and submergence depths. The critical Froude numbers were also determined and the efficiency of different vortex suppressors on the gas‐core formation was investigated. The horizontal velocity field inside the vortex core region was additionally recorded using particle image velocimetry. The experimental results were used to verify numerical simulations and compared to vortex models and correlations from literature.

Characterisation and analysis of indoor tornado for contaminant removal and emergency ventilation

As an essential emergency management strategy, innovative emergency ventilation schemes that can quickly remove infectious and fatal contaminants without further spreading are highly demanded for public and commercial buildings. This study numerically investigated a vortex flow driven ventilation in a model room to explore the dynamic characteristics and 3D visualisation of vortex-driven indoor tornados. Four approaches to identify the core region of the indoor tornado were developed and compared against each other. By successfully capturing the continuously changing centre of the vortex and significant core region size variations at different heights, the swirl vector method was recommended as a quantifiable approach to identify the core region of indoor tornados. The numerical outcomes also revealed a strong connection between the lift angle, vortex intensity, overall size of indoor tornado and maximum size of core region. The best contaminants control and removal was achieved at lift angle of 20° in this study and an optimum lift angle ranging from 10° to 20° was recommended for future study.

LES study of transient behaviour and turbulent characteristics of free-surface and floor-attached vortices in pump sump

ABSTRACTIn this work, large eddy simulation combined with volume of fluid method is performed to simulate the flow and the associated vortices in a pump sump. The numerical results are compared with experimental data. Particular interest is paid to discovering the temporal evolution of the free-surface and floor-attached vortices. Both vortices undergo periodic enhancement and attenuation due to the advection and stretching/tilting effects generated by the background flow. The primary vortex core is accompanied by spirally wrapped secondary vortex tubes, resulting in anisotropic turbulence structure around the vortex core regions. The findings provide a deep insight into the application of large eddy simulation in pump sump and the underlying mechanism of vortex formation and evolution in the sump.

Effect of Cr atoms in vortex dynamics of NbSe2 superconductor and study of second magnetization peak effect

We report on the high quality single crystal of CrxNbSe2 growth to study the effect of Cr impurity on the vortex dynamics of NbSe2 superconductor using M–H curves with different relaxation rates. The field dependence nature of critical current density (Jc) is well investigated using Larkin-Ovchinnikov collective pinning model and Anderson-Kim’s plastic creep model. We have observed significant improvement in the field dependence nature of Jc with increasing Cr impurities in NbSe2. This reveals the crossover of vortex lattice from elastic (E) to plastic (P) creep region causing second magnetization peak (SMP) or fishtail effect at highest Cr concentration, which has not been yet observed. Also addition of Cr impurity enhances the Jc up to 4 × 105 A/cm2. However, the interaction between core region of vortices and pinning centres ensure the δl pinning in Cr0.0005NbSe2 and significantly improved with increment of Cr impurity, which allowing both δl and δTc mechanism. Both, δl and δTc core pinning strongly enhance the coupling between vortices in pinning potential of nonstatistical distribution of Cr impurity, which makes 3D magnetic fluxes into 2D nature. Furthermore, the point pining and surface pinning mechanisms are responsible for pinning at low magnetic field region in Cr0.0005NbSe2. The appearance of prominent peak in pinning force density near upper critical field (Hc2) indicates the lattice softening at higher Cr impurity. These studies not only provide fundamental study of vortex dynamic but also open the door for further study in low Tc superconducting materials.

Local NMR relaxation rates T1−1 and T2−1 depending on the d -vector symmetry in the vortex state of chiral and helical p -wave superconductors

Local NMR relaxation rates in the vortex state of chiral and helical $p$-wave superconductors are investigated by the quasiclassical Eilenberger theory. We calculate the spatial and resonance frequency dependences of the local NMR spin-lattice relaxation rate ${T}_{1}^{\ensuremath{-}1}$ and spin-spin relaxation rate ${T}_{2}^{\ensuremath{-}1}$. Depending on the relation between the NMR relaxation direction and the $d$-vector symmetry, the local ${T}_{1}^{\ensuremath{-}1}$ and ${T}_{2}^{\ensuremath{-}1}$ in the vortex core region show different behaviors. When the NMR relaxation direction is parallel to the $d$-vector component, the local NMR relaxation rate is anomalously suppressed by the negative coherence effect due to the spin dependence of the odd-frequency $s$-wave spin-triplet Cooper pairs. The difference between the local ${T}_{1}^{\ensuremath{-}1}$ and ${T}_{2}^{\ensuremath{-}1}$ in the site-selective NMR measurement is expected to be a method to examine the $d$-vector symmetry of candidate materials for spin-triplet superconductors.

Modeling Inspiratory Flow in a Porcine Lung Airway.

Inspiratory flow in a multigeneration pig lung airways was numerically studied at a steady inlet flow rate of 3.2 × 10-4 m3/s corresponding to a Reynolds number of 1150 in the trachea. The model was validated by comparing velocity distributions with previous measurements and simulations in simplified airway geometries. Simulation results provided detailed maps of the axial and secondary flow patterns at different cross sections of the airway tree. The vortex core regions in the airways were visualized using absolute helicity values and suggested the presence of secondary flow vortices where two counter-rotating vortices were observed at the main bifurcation and in many other bifurcations. Both laminar and turbulent flows were considered. Results showed that axial and secondary flows were comparable in the laminar and turbulent cases. Turbulent kinetic energy (TKE) vanished in the more distal airways, which indicates that the flow in these airways approaches laminar flow conditions. The simulation results suggested viscous pressure drop values comparable to earlier studies. The monopodial asymmetric nature of airway branching in pigs resulted in airflow patterns that are different from the less asymmetric human airways. The major daughters of the pig airways tended to have high airflow ratios, which may lead to different particle distribution and sound generation patterns. These differences need to be taken into consideration when interpreting the results of animal studies involving pigs before generalizing these results to humans.

Experimental study on impingement spray and near-field spray characteristics under high-pressure cross-flow conditions

The fuel spray injected into a direct injection (DI) engine is substantially affected by both the in-cylinder air flow and the piston cavity wall impingement. The combined effect of the air flow and the wall impingement plays an important role on the spray development, mixture formation, and subsequent combustion. In this study, the effects of cross-flow and flat wall impingement on the spray development and dispersion were investigated. The spray was injected by a valve covered orifice (VCO) nozzle under various cross-flow velocities and ambient pressures. Impingement spray images in a vertical plane and several horizontal planes were obtained by a high speed video camera and a continuous wave laser sheet. A high speed video camera connected with a long-distance microscope was employed to obtain the near-field spray images. The results show that cross-flow favors spray dispersion while the high ambient pressure tends to compress the spray profiles. Additionally, under an approximate liquid-to-air momentum flux ratio q, when the ambient pressure and cross-flow velocity were varied, at 2 ms ASOI the outlines of the spray in the windward side agree well, whereas the spray extended further in the leeward side at a lower ambient pressure. At the plane of y = 25 mm, a complex vortex movement was observed that resulted in a non-uniform distribution of droplets in the upper part of the spray in the leeward side. In addition, at the plane of y = 45 mm, an empty belt area occurred in the vortex core region revealing that the density of the droplets in this region was quite low. The quantitative analysis shows that with increasing cross-flow velocity, the spray tip penetration decreases slightly before impingement while the spray tip penetrates further on the wall surface after impingement. The high cross-flow velocity favors the spray breakup and dispersion leading to a larger wall-jet vortex while the high ambient pressure restrains the spray dispersion leading to a smaller spray tip penetration and vortex height. For near-field spray, the spray image at higher ambient pressure shows fewer ligaments. With increasing cross-flow velocity, the whole spray shifted downstream. The spray outline was wider at the initial stage (0.05 ms ASOI) than that at steady stage (2 ms ASOI) of spray evolution.

Integral and statistical characteristics of the turbulent swirl flow in a straight conical diffuser

The results of the experimental investigations of the turbulent swirl flow in a straight conical diffuser with inlet diameter 0.4m and total divergence angle 8.6? are presented in this paper. The incompressible swirl flow field is generated by the axial fan with outer diameter 0.397m. The measurements were performed in one measuring section downstream the axial fan impeller in the conical diffuser in position (z/R0 = 1) with original classical probes and an one-component laser Doppler anemometry (LDA) system, for four flow regimes. The comparative measurements of axial and circumferential velocities are presented. The Reynolds number, calculated on the basis of the average velocity, ranges from 149857 to 216916. Integral parameters, such as volume flow rate, average circulation and swirl number, are determined. Statistical characteristics, such as level of turbulence, skewness and flatness factors, are calculated. The highest levels of turbulence for axial velocity are reached in region 0.4 < r/R < 0.6, where D = 2R. The highest levels of turbulence for circumferential velocity are reached for the regimes with lower circulation in r/R ? 0.4, i.e., in the vortex core region for the cases with higher circulation.

A phase-slip moment method for condensing flows

The classic method of moment is extended to account the slips of velocity and temperature between the liquid phase and the gas phase in condensing flows. The moments of droplet size distribution are defined on the liquid phase, instead of on the mixture of liquid and gas, so that the gas phase and the liquid phase can be treated separately. The interphase interaction is realized through source terms which include the mass, momentum and energy exchanges. As a demonstration, this new method of moments is integrated into a numerical method and is applied to a vortex shedding problem with condensation. Due to the very strong velocity slip, a dry zone is found in the vortex core region. This phenomenon was experimentally observed but not discovered by the classic moment method.