Low Vortex Research Articles

Evolution of Kosterlitz-Thouless-Berezinskii (BKT) Transition in Ultra-Thin NbN Films

We explore the Berezinskii-Kosterlitz-Thouless (BKT) transition in ultrathin NbN superconducting films. We have measured the super fluid density (ns∝1/λ2), normal carrier density and resistivity (ρ) of a set of NbN thin films. Our results show that while the ground state is well described by BCS theory, at elevated temperatures, ultra-thin films show sudden drop in superfluid density associated with the BKT transition close to superconducting transition temperature (Tc). The sudden drop starts at higher superfluid density than expected from 2D XY model, which can be understood by considering the low vortex core energy and slight inhomogeneity in the system. Resistivity data is explained using effective medium theory (EMT) including both Aslamazov-Larkin and KT fluctuations. This is also in good agreement with the corresponding superfluid density.

Vortex dynamics in ferromagnetic superconductors: Vortex clusters, domain walls, and enhanced viscosity

We demonstrate that there is a long-range vortex-vortex attraction in ferromagnetic superconductors due to polarization of the magnetic moments. Vortex clusters are then stabilized in the ground state for low vortex densities. The motion of vortex clusters driven by the Lorentz force excites magnons. This regime becomes unstable at a threshold velocity above which domain walls are generated for slow relaxation of the magnetic moments and the vortex configuration becomes modulated. This dynamics of vortices and magnetic moments can be probed by transport measurements.

Time resolved measurements of vortex-induced vibrations of a positively buoyant tethered sphere in uniform water flow

Vortex-induced vibrations (VIV) of a positively buoyant (light) tethered sphere in uniform flow as well as its wake characteristics were measured in a closed loop water channel. Experiments were performed at free stream velocities ranging between 0.048 and 0.22m/s, corresponding to sphere Reynolds numbers ranging from ReD=430 to 1925. The measurements were done using high-speed sphere tracking as well as time resolved particle image velocimetry in a horizontal plane located at the sphere’s center. Until the Hopf bifurcation, the sphere remained stationary and the wake was characterized by a train of hairpin vortices exhibiting near-symmetry in the vertical plane similar to stationary sphere visualization results. For our limited parameter range, the amplitude response of two different data sets (same sphere and free stream velocity but different water viscosity) collapsed better when plotted versus ReD than when plotted versus the reduced velocity, U⁎. The amplitude response beyond the first bifurcation displayed continuously increasing rms amplitudes in agreement with the sphere’s small mass parameter (<critical mass). Beyond the traditional ‘lock-in’ regime (U⁎=3–8), the sphere’s oscillation frequency strongly increased extending the ‘lock-in’ phenomenon to larger ReD. Vortex shedding characteristics were analyzed using the directional swirling strength in conjunction with the vorticity as the former enables vortex identification. Vortex shedding dynamics were similar over the whole ReD range. However, for low ReD clear counter-rotating vortex pairs were observed, at the highest ReD the spatio-temporal vortex shedding pattern in the far wake was characterized by high frequency, fragmented vortical structures organized in a low frequency ‘saw tooth’ pattern, the latter associated with the sphere’s oscillation frequency. Furthermore, phase averaged results showed a decreasing vortex pinch-off phase as ReD increased.

Prediction of Polaronlike Vortices and a Dissociation Depinning Transition in Magnetic Superconductors: The Example ofErNi2B2C

In borocarbide ErNi2B2C, the phase transition to the commensurate spin density wave at 2.3 K leaves 1/20 part of Ising-like Er spins practically free. Vortices polarize these spins nonuniformly and repolarize them when moving. At a low spin relaxation rate and at low bias currents, vortices carrying magnetic polarization clouds become polaronlike and their velocities are determined by the effective drag coefficient, which is significantly bigger than the Bardeen-Stephen (BS) one. As current increases, at a critical current J(c) vortices release polarization clouds and the velocity as well as the voltage in the I-V characteristics jump to values corresponding to the BS drag coefficient. The nonuniform components of the magnetic field and magnetization drop as velocity increases, resulting in weaker polarization and discontinuous dynamic dissociation depinning transition. As current decreases, on the way back, vortices are retrapped by polarization clouds at the current J(r)<J(c).

A simulation study of a heavy rainfall process over the Yangtze River valley using the two-way nesting approach

In this study, the major features of a heavy rainfall event in the Yangtze River region on 3–7 June 2011 and its event-related large-scale circulation and predictability were studied. Both observational analysis and model simulation were used, the latter being based on the Weather Research and Forecasting (WRF) model forced by NCEP Global Forecast System (GFS) datasets. It was found that, during 3–5 June, the western Pacific subtropical high apparently extended to the west and was much stronger, and the Indian summer monsoon trough was slightly weaker than in normal years. The east-west oriented shear line over the middle and lower reaches of the Yangtze River was favorable for the transportation and convergence of water vapor, and the precipitation band was located slightly to the south of the shear line. During 6–7 June, the western Pacific subtropical high retreated eastward, while the trough over the Okhotsk Sea deepened. The low vortex in Northeast China intensified, bringing much more cold air to the middle and lower reaches of the Yangtze River, and the shear line over this area moved slightly southward. The convection band moved southward and became weaker, so the rainfall during 6–7 June weakened and was located slightly to the south of the previous precipitation band. Many of the observed features, including background circulation and the distribution and amount of precipitation, were reproduced reasonably by the WRF, suggesting a feasibility of this model for forecasting extreme weather events in the Yangtze River region.

Scanning Hall probe microscopy of unconventional vortex patterns in the two-gap MgB2superconductor

The low magnetic field vortex patterns nucleation and evolution in a high-quality two-gap superconductor MgB${}_{2}$ single crystal have been investigated by low-temperature scanning Hall probe microscopy. Large areas have been imaged with single-vortex resolution while changing systematically the thermodynamic parameters for field and temperature. The obtained patterns have been studied and compared with those of a reference 2H-NbSe${}_{2}$ single crystal. We found that the observed vortex patterns in MgB${}_{2}$ (e.g., stripes, clusters) appear due to competing vortex-vortex interactions as suggested by the theory of type-1.5 superconductivity.

How oblique trailing edge of a hydrofoil reduces the vortex-induced vibration

The effect of hydrofoil trailing edge shape on the wake dynamic and flow induced vibration is investigated at high Reynolds number, Re=0.5 × 106–2.9 x 106. Two NACA 0009 hydrofoils with blunt and oblique trailing edges are tested. The velocity field is surveyed with the help of Laser Doppler Velocimetry (LDV), and Particle-Image-Velocimetry, (PIV). Proper-Orthogonal-Decomposition (POD) is used to extract coherent structures from PIV data. Besides, flow induced vibration measurements and high-speed visualization are also performed. A significant reduction of vortex induced vibration is obtained with the oblique trailing edge, in accordance with former reports. High speed videos clearly demonstrate that for both tested hydrofoils, the alternate vortices detach from upper and lower corners of the trailing edge. Due to the oblique truncation, the lower detachment location is shifted upstream with respect to the upper one. Therefore, as the upper vortex rolls up, it coincides with the passage of the lower vortex, leading to their collision. This strong interaction leads to a redistribution of the vorticity, which no more concentrates within the core of Karman vortices. The analysis of the phase locked average of velocity profiles reveals that the oblique truncation leads to a thickening of the core of upper and lower vortices as well as a disorganization of the alternate shedding in the near wake, recovers downstream. We strongly believe that the collision between upper and lower vortices and the resulting vorticity redistribution is the main reason of the vibration reduction obtained with oblique trailing edge. This result paves the way for further optimization of the trailing edge shape.

Particle removal properties of stormwater runoff with a lab-scale vortex separator

ABSTRACT A lab-scale vortex separator, based on a swirling motion to remove settleable particles, was used to remove suspended solids in urban stormwater runoff. However, the treatment is limited to the removal of large settleable particles. The synthesized stormwater runoff was made with tap water and the addition of road sediment. The vortex separator has internal components designed to enhance vortex separation by minimizing the turbulence, increasing the efficiency, and preventing captured pollutants from washout. As the flow continues to spiral down around the inlet baffle, a low energy vortex motion directs settleable particles into the protected sediment storage zone. Advanced vortex separation provided an extendable and stabilized flow path while protecting the captured pollutants for a wide range of flow rates. The range of the inflow rate was 30–115 l/m, and the size of the influent particles varied from 75 to 200 μm. Overall removal efficiencies of 51.8% for SS, 26.6% for COD, 70.5% for TP, and...

About the effects of an oscillating miniflap upon the wake on an airfoil, all immersed in turbulent flow

The present research analyzes the asymmetry in the rolling up shear layers behind the blunt trailing edge of an airfoil 4412 with a miniflap acting as active flow control device and its wake organization. Experimental investigations relating the asymmetry of the vortex flow in the near wake region, able to distort the flow increasing the downwash of an airfoil, have been performed. All of these in a free upstream turbulent flow (1.8% intensity). We examine the near wake region characteristics of a wing model with a 4412 airfoil without and with a rotating miniflap located on the lower surface, near the trailing edge. The flow in the near wake, for 3 x-positions (along chord line) and 20 vertical points in each x-position, was explored, for three different rotating frequencies, in order to identify signs of asymmetry of the initial counter rotating vortex structures. Experimental evidence is presented showing that for typical lifting conditions the shear layer rollup process within the near wake is different for the upper and lower vortices: the shear layer separating from the pressure side of the airfoil begins its rollup immediately behind the trailing edge, creating a stronger vortex while the shear layer from the suction side begins its rollup more downstream creating a weaker vortex. The experimental data were processed by classical statistics methods. Aspects of a mechanism connecting the different evolution and pattern of these initial vortex structures with lift changes and wake alleviating processes, due to these miniflaps, will be studied in future works.

Effective merging dynamics of two and three fluid vortices: Application to two-dimensional decaying turbulence

We present a kinetic theory of two-dimensional decaying turbulence in the context of two-body and three-body vortex merging processes. By introducing the equations of motion for two or three vortices in the effective noise due to all the other vortices, we demonstrate analytically that a two-body mechanism becomes inefficient at low vortex density n<<1. When the more efficient three-body vortex mergings are considered (involving vortices of different signs), we show that n~t(-ξ), with ξ=1. We generalize this argument to three-dimensional geostrophic turbulence, finding ξ=5/4, in excellent agreement with direct Navier-Stokes simulations [McWilliams et al., J. Fluid Mech. 401, 1 (1999)].

Kelvin wave hydraulic control induced by interactions between vortices and topography

AbstractThe interaction of a dipolar vortex with topography is examined using a combination of analytical solutions and idealized numerical models. It is shown that an anticyclonic vortex may generate along-topography flow with sufficient speeds to excite hydraulic control with respect to local Kelvin waves. A critical condition for Kelvin wave hydraulic control is found for the simplest case of a 1.5-layer shallow water model. It is proposed that in the continuously stratified case this mechanism may allow an interaction between low mode vortices and higher mode Kelvin waves, thereby generating rapidly converging isopycnals and hydraulic jumps. Thus, Kelvin wave hydraulic control may contribute to the flux of energy from mesoscale to smaller, unbalanced, scales of motion in the ocean.

Intermittent trapping of a liquid-like vortex state visualized by scanning Hall probemicroscopy

We have used scanning Hall probe microscopy to investigate vortex structures and vortex dynamicsin Bi2Sr2CaCu2O8 + δ thin films in very low perpendicular magnetic fields. After nominally zero field cooling inthe Earth’s field we find that the vortices appear to be in a stable glassy state in our highlydisordered samples. After applying a cancellation field of a few Oersted at low temperature,however, the system enters a new regime at very low magnetic induction when the onlyimage contrast is due to vortices that are intermittently trapped on strong pinningcentres. This state shares many of the signatures of the re-entrant vortex liquidphase that has been theoretically predicted in these highly anisotropic materialsat very low vortex densities. Analysing the trapping times for vortices in thefluctuating state we estimate that the pinning potential of typical strong pinningcentres is about 900 K under our experimental conditions. To our knowledge, this isthe first direct experimental evidence for the existence of a dynamic liquid-likevortex state in this highly anisotropic material at very low magnetic induction.

Three-Dimensional Simulation of Exhaust Particle Dispersion and Concentration Fields in the Near-Wake Region of the Studied Ground Vehicle

In the present study, the interaction effects of different vehicle speeds and exhaust tailpipe exit velocity and temperature conditions on the three-dimensional flow structure, exhaust particle dynamic behavior, formation and evolution processes (i.e., nucleation, coagulation, condensation, and dispersion), number and volume concentration, and nucleation rate fields in the near-wake region behind the studied ground vehicle in urban road microenvironment were comprehensively simulated using large-eddy simulation (LES) with the aerosol dynamics and dispersion model based on our recently established direct quadrature method of moments (DQMOM) approach. The numerical results show that the particles are drawn up into the recirculation region and then moved toward the upper vortex by its lower vortex accordingly. The particle nucleation rate is much more associated with the distribution of the root mean square (RMS) temperature than with that of the temperature itself. Some of the sulfuric acid concentration em...

Turbulent boundary layer separation control and loss evaluation of low profile vortex generators

The present paper analyses the results of a detailed experimental study on low profile vortex generators used to control the turbulent boundary layer separation on a large-scale flat plate with a prescribed adverse pressure gradient, typical of aggressive turbine intermediate ducts. This activity is part of a joint European research program on Aggressive Intermediate Duct Aerodynamics (AIDA). Laser Doppler Velocimetry and a Kiel total pressure probe have been employed to perform measurements in the test section symmetry plane and in cross-stream planes to investigate the turbulent boundary layer, with and without control device application. Velocity fields, Reynolds stresses, and total pressure distributions are analysed and compared for the controlled and non controlled flow conditions to characterize the mean flow behaviour. The detail and the accuracy of the measurements allow the evaluation of the deformation works of the mean motion in the test section symmetry plane. Normal and shear contributions of viscous and turbulent deformation works have been analysed and employed to explain the distribution of the total pressure loss. For the controlled flow the discussion of the flow field is extended considering the effects of the vortex generated in the cross-stream planes. The experimental data allow the evaluation of the global amount of losses, considering a balance of total pressure fluxes in the different measuring planes.

Aerosol Behavior in the near Wake Region of the Vehicle

In the present study, the interaction effects of different vehicle speeds and vehicular exhaust tailpipe exit velocity conditions on the exhaust particle dynamic behaviors in the near-wake region behind the studied vehicle were numerically investigated. The results showed that the particles were drawn up into the recirculation region and then moved toward into the upper vortex by its lower vortex. A lower particle number concentration behind the studied vehicle is observed for a higher vehicle speed because of the rapidly increasing dilution ratio and the nucleation rate. The particle volume concentration is the highest in the recirculation region behind the studied vehicle and is diluted gradually beyond the recirculation region in the downstream.

Experimental Evidence of Rotating Stall in a Pump-Turbine at Off-Design Conditions in Generating Mode

An experimental investigation of the rotating stall in reduced scale model of a low specific speed radial pump-turbine at runaway and turbine brake conditions in generating mode is achieved. Measurements of wall pressure in the stator are performed along with high-speed flow visualizations in the vaneless gap with the help of air bubbles injection. When starting from the best efficiency point (BEP) and increasing the impeller speed, a significant increase of the pressure fluctuations is observed mainly in the wicket gates channels. The spectral analysis shows a rise of a low frequency component (about 70% of the impeller rotational frequency) at runaway, which further increases as the zero discharge condition is approached. Analysis of the instantaneous pressure peripheral distribution in the vaneless gap reveals one stall cell rotating with the impeller at sub-synchronous speed. High-speed movies reveal a quite uniform flow pattern in the guide vanes channels at the normal operating range, whereas at runaway the flow is highly disturbed by the rotating stall passage. The situation is even more critical at very low positive discharge, where backflow and vortices in the guide vanes channels develop during the stall cell passage. A specific image processing technique is applied to reconstruct the rotating stall evolution in the entire guide vanes circumference for a low positive discharge operating point. The findings of this study suggest that one stall cell rotates with the impeller at sub-synchronous velocity in the vaneless gap between the impeller and the guide vanes. It is the result of rotating flow separations developed in several consecutive impeller channels which lead to their blockage.

Effects of the Blade Shape on the Trailing Vortices in Liquid Flow Generated by Disc Turbines

Particle image velocimetry technique was used to analyze the trailing vortices and elucidate their relationship with turbulence properties in a stirred tank of 0.48 m diameter, agitated by four different disc turbines, including Rushton turbine, concaved blade disk turbine, half elliptical blade disk turbine, and parabolic blade disk turbine. Phase-averaged and phase-resolved flow fields near the impeller blades were measured and the structure of trailing vortices was studied in detail. The location, size and strength of vortices were determined by the simplified λ 2-criterion and the results showed that the blade shape had great effect on the trailing vortex characteristics. The larger curvature resulted in longer residence time of the vortex at the impeller tip, bigger distance between the upper and lower vortices and longer vortex life, also leads to smaller and stronger vortices. In addition, the turbulent kinetic energy and turbulent energy dissipation in the discharge flow were determined and discussed. High turbulent kinetic energy and turbulent energy dissipation regions were located between the upper and lower vortices and moved along with them. Although restricted to single phase flow, the presented results are essential for reliable design and scale-up of stirred tank with disc turbines.

Characteristics of Embedded-Shock-Free Compressible Vortex Rings: A Detailed Study Using PIV

The present study focus on evolution of compressible vortex ring generated at the open end of a shock tube through accurate measurement of velocity field using Particle Image Velocimetry (PIV). To investigate the unsteady characteristics of embedded shock-free, low Mach number vortex rings, two cases (shock Mach numbers, M=1.27 and M=1.37) are considered for PIV measurements. Time-dependent variations of circulation, core and ring diameters, and ring velocity are calculated from the measured velocity field. Pinching-off process is investigated in detail for both cases. Formation time and the time of complete detachment of the vortex ring from the trailing jet are identified from the velocity and vorticity field. The ring formation is complete at about t *(= t U b/ D)=1.75 and 1.65 for M=1.27 and 1.37, respectively, where t is time, U b is fluid velocity behind the shock at exit, and D is tube diameter. Complete detachment of the vortex ring from the trailing jet is observed at [Formula: see text] and 2.9 for M=1.27 and 1.37, respectively.

Prediction of Intake Vortex Risk by Nearest Neighbors Modeling

Vortex formation at intakes can cause damage, clogging, reduced flow efficiency, and even loss of life. For practical prediction of vortex risk, engineers often compare expected design parameters with published data by using parameter proximity to evaluate the relative risk of vortex formation. Unfortunately, this procedure is ill-defined, and the resulting risk estimates are highly subjective. In response, a formal equivalent of the data proximity procedure was developed by implementing the nearest neighbors algorithm on available experimental and field data. This database was partitioned and the machine learning parameters adjusted to obtain a stochastic model with maximum predictive accuracy. Unlike the flow parameters and submergence, the approach geometry was not found to be a significant factor in the model, although this may be attributable to data noise and range of tested values. The final model, which excluded the channel approach geometry, fit all vertical intake vortex formation data to within 0.1% error and perfectly fit the horizontal intake data. Probability charts generated from the model show regions of vortex formation and problems more numerous and larger on average than regions of low vortex probability, thus validating consideration of potential vortex formation risk for conservative intake design.

Simulation of Shell Thickness Distribution and Flow Field in Slab Continuous Casting Mold

The shell distribution in the slab continuous casting mold has been simulated coupling a 3-D flow, temperature and volume fraction equations of the molten steel in FLUENT. The simulated results show that the flow velocity around the upper vortex center is decrease and the location of lower vortex center move down as the nozzle port angle increases. The simulated shell thickness in the center on the narrow face become thicker at meniscus and the shell thickness in the center on wide face decreases but the basic distributions of the shell tend to consistency as the nozzle port angle increases. The simulated results also show that the effects of solidified shell on flow field in mold is slight but the velocity of molten steel near the solidified shell. There are remelting near the impact regoins implicit our attentions in order to avoid breaking out.