Vortex Path Research Articles

Improving the Flux Pinning With Artificial BCO Nanodots and Correlated Dislocations in YBCO Films Grown on IBAD-MgO Based Template

To improve the performance of high-temperature superconductors in electrical power systems, a modified IBAD-MgO based template is used as a substrate for the growth of YBa $_{2}$ Cu $_{3}$ O $_{6+x}$ (YBCO) doped with various concentrations of BaCeO $_{3}$ (BCO). The highest critical current density ( $J_\mathrm{c}$ ) in a wide applied field and angular range is realized in the films doped with 2 $\%$ and 4 $\%$ BCO content. This is explained to arise from the optimized BCO dopant concentration, which helps to immobilize more vortices both in low and high field ranges. Besides the vortex pinning by randomly distributed BCO nanoparticles, the edge-type dislocations, mainly occurring due to the underlying IBAD-MgO based template, are also a main source of pinning the vortices as seen by the appearance of $c$ -axis peak of $J_\mathrm{c} (\theta)$ in undoped and 2 $\%$ doped films. The disappearance of c-peak in films doped with higher concentration can be qualitatively explained by the vortex path model, where the paths of the vortices arising because of high density of BCO particles disguise the columnar type pinning effect coming from the threading dislocations within the films. Therefore, the BCO nanodots can be used for improving the flux pinning besides the threading dislocations in YBCO films on metal substrates.

Analysis of critical current anisotropy in commercial coated conductors in terms of the maximum entropy approach

The anisotropy of flux pinning in three industrially produced coated conductor tapes has been investigated by analyzing the angular dependence of critical current (Ic) in an applied magnetic field. Commercial coated conductors from the SuperOx, SuNAM and Superpower companies were included in the study, which covered range of small applied magnetic fields (0.3–2 T) and temperatures close to liquid nitrogen boiling point (65–77 K). Measurements were performed in the full 360° angular range using samples of the full 4 mm nominal width. The results were analyzed in the framework of the maximum entropy approach-based vortex path model that provided analytical functions fitting remarkably well with the experimental data for all three conductors. Several angular-dependent pinning mechanisms detected in the analysis were tentatively linked with defect structures reported in the literature. The asymmetry of the angular Ic dependence is discussed as well as different Ic values in the two possible orientations of magnetic field applied parallel to the flat face of the coated conductor, observed mostly at lower applied fields.

Computational Characterization of a Rectangular Vortex Generator on a Flat Plate for Different Vane Heights and Angles

Vortex generators (VG) are passive flow control devices used for avoiding or delaying the separation of the boundary layer by bringing momentum from the higher layers of the fluid towards the surface. The Vortex generator usually has the same height as the local boundary layer thickness, and these Vortex generators can produce overload drag in some cases. The aim of the present study was to analyze the characteristics and path of the primary vortex produced by a single rectangular vortex generator on a flat plate for the incident angles of β = 10 ∘ , 15 ∘ , 18 ∘ and 20 ∘ . A parametric study of the induced vortex was performed for six VG heights using Reynolds average Navier–Stokes equations at Reynodls number R e = 27,000 based on the local boundary layer thickness, using computational fluid dynamics techniques with OpenFOAM open-source code. In order to determine the vortex size, the so-called half-life radius was computed and compared with experimental data. The results showed a similar trend for all the studied vortex generator heights and incident angles with small variations for the vertical and the lateral paths. Additionally, 0.4H and 0.6H VG heights at incident angles of β = 18 ∘ and β = 20 ∘ showed the best performance in terms of vortex strength and generation of wall shear stress.

Source Term Modelling of Vane-Type Vortex Generators under Adverse Pressure Gradient in OpenFOAM

An analysis of the generation of vortices and their effects by vane-type vortex generators (VGs) positioned on a three-dimensional flat plate with a backward-facing ramp and adverse gradient pressure is carried out. The effects of a conventional vortex generator and a sub-boundary layer vortex generator are implemented by using a source term in the corresponding Navier-Stokes equations of momentum and energy according to the so-called jBAY Source Term Model. The influence of the vortex generator onto the computational domain flow is modelled through this source term in the Computational Fluid Dynamics (CFD) simulations using the open-source code OpenFOAM. The Source Term Model seems to simulate relatively well the streamwise pressure coefficient distributions all along the flat plate floor as well as certain parameters studied for vortex characterization such as vortex path, decay and size for the two vane-type vortex generators of different heights studied. Consequently, the implementation of the Source Term Model represents an advantage over a fully Mesh-Resolved Vortex Generator Model for certain applications as a result of a meaningful decrease in the cell number of the computational domain which implies saving computational time and resources.

CALCULATED RESEARCH OF INFLUENCE OF HELICOPTER MAIN ROTORS GEOMETRY ON THE EFFICIENCY IN HOVER MODE BASED ON THE NONLINEAR VORTEX MODEL

The efficiency of the helicopter main rotor in the hover mode is very important, because this mode essentially determines the performance characteristics of the helicopter. A feature of the helicopter rotor aerodynamics is a significant inductive blade influence that highly defines its aerodynamic characteristics. The problem of the influence of the blade twist and spatial geometric layout of the main rotor on its aerodynamic characteristics in the hover mode for a fixed value of the rotor solidity has been considered in this article. As a criterion of efficiency of the rotor in the hover mode relative efficiency (FoM – Figure of Merit) is used. The results are obtained by numerical simulation based on the nonlinear vortex blade model of the rotor, developed at the Helicopter Design Chair of the MAI. The model allows taking into account a complicated spatial shape of the free vortex path of the rotor blades that determines their inductive interaction. As the example of a four-blade main rotor with rectangular blades in plan, the influence of the value of the blades twist on the efficiency in the hover mode is studied. For different values of the rotor thrust, the values and ranges of the blade twist angles are determined, providing the maximum positive effect of the efficiency increase in hovering. For a fixed value of the blade twist, the rotor solidity, and the same operating conditions, the effect of various schemes and configurations of rotor on its efficiency in hover mode is studied. A single rotor with a different number of blades (from 2 to 6), an X-shaped rotor, coaxial rotor and rotor with crossed blades type "synchropter" are considered. The values of the efficiency increase in hovering depending on the rotor layout in comparison with the two-blade rotor are obtained. The comparative analysis of inductive velocities and streamlines for the "synchropter" rotor scheme, coaxial rotor scheme and its equivalent single rotor scheme is presented. The obtained results can be useful at the stage of preliminary design of vertically taking-off aircraft when selecting the parameters of their main rotor system.

Angular and field dependent flux pinning in artificially doped YBCO films on IBAD-MgO based template

The self-organized artificial pinning structure in superconducting thin films of YBa2Cu3O6+x (YBCO) is optimized on a new type of IBAD-MgO based template by doping YBCO with non-superconducting BaCeO3 (BCO) and BaZrO3 (BZO). In these films, the YBCO is well ordered, no large angle grain boundaries are seen and the isotropic BCO particles are randomly distributed while the BZO grows as unidirectionally splayed and shortened nanorods. Additionally, the low-angle grain boundaries formed during the growth process have an impact on the flux pinning. The flux pinning behaviour can be explained by the vortex path model, where the pinning paths are shorter in BZO doped than in BCO doped films. In BZO doped films, the vortices are pinned with greater pinning force and thus the critical current density Jc is higher than in BCO doped films, especially in high magnetic fields, where the wide peaks in Jc(θ) were seen along the YBCO c-direction. This direction dependent pinning can be explained by the nearly similar diameters of BZO nanorods with those of vortices, thus efficiently increasing the vortex pinning in the vicinity of YBCO c-axis.

A New Class of Edge Baroclinic Waves and the Mechanism of Their Generation

Edge baroclinic waves are generated in a geostrophic flow with a vertical shear near a solid surface. The study investigates a new class of baroclinic waves in flows with horizontal and vertical shears and a linear distribution of potential vorticity. It is shown that taking account of the horizontal shear leads to the appearance of new features of wave dynamics. These include the nonmodal growth of energy in the initial stage of development, the time dependence of the vertical wave scale, and the possibility of generation of stationary or blocked waves. The horizontal shear makes the mechanism of generation of baroclinic waves by initial vortex perturbations more efficient. One important feature is associated with vortex paths, which are formed by the superposition of a baroclinic wave on the flow with horizontal shear.

Influence of Propulsion Type on the Stratified Near Wake of an Axisymmetric Self-Propelled Body

To better understand the influence of swirl on the thermally-stratified near wake of a self-propelled axisymmetric vehicle, three propulsor schemes were considered: a single propeller, contra-rotating propellers (CRP), and a zero-swirl, uniform-velocity jet. The propellers were modeled using an Actuator-Line model in an unsteady Reynolds-Averaged Navier–Stokes simulation, where the Reynolds number is R e L = 3.1 × 10 8 using the freestream velocity and body length. The authors previously showed good comparison to experimental data with this approach. Visualization of vortical structures shows the helical paths of blade-tip vortices from the single propeller as well as the complicated vortical interaction between contra-rotating blades. Comparison of instantaneous and time-averaged fields shows that temporally stationary fields emerge by half of a body length downstream. Circumferentially-averaged axial velocity profiles show similarities between the single propeller and CRP in contrast to the jet configuration. Swirl velocity of the CRP, however, was attenuated in comparison to that of the single propeller case. Mixed-patch contour maps illustrate the unique temperature distribution of each configuration as a consequence of their respective swirl profiles. Finally, kinetic and potential energy is integrated along downstream axial planes to reveal key differences between the configurations. The CRP configuration creates less potential energy by reducing swirl that would otherwise persist in the near wake of a single-propeller wake.

Experimental study on the vortex structure and path instability of freely falling annular disks

Bodies freely falling in steady water or air are common scenes encountered in various scientific and engineering fields, including the flapping flight of birds and the reentry of a space shuttle. In this work, the freely falling annular thin disks with small dimensionless moments of inertia I* and Reynolds number Re are investigated experimentally in a water tank. We use stereoscopic vision to record the position and orientation of the disks. The flow structure behind the disks is studied by applying fluorescent dye visualization and PIV method. Varying the geometry dimensionless parameter (the inner to outer diameter ratio η and I*) of the disks reveals two new falling patterns. When ηcrit1=0.6 0.8, the circular vortex loops shed frequently from the disk, which causes a lengthways vibration superimposed onto straight vertical motion. We also observe another two falling patterns reported previously: hula-hoop and helical motion. By comparing the wake structure of the two motions, we find that the vortex layer twists more violently in the hula-hoop motion, which is the reason for the different trajectory between them. Further research on flow field reveals that the torque on the disk that causes the vibration is due to the formation, elongation and shedding of the vortex.

Unsteady Pylon Loading Caused by Propeller-Slipstream Impingement for Tip-Mounted Propellers

An experimental analysis was performed of the unsteady aerodynamic loading caused by the impingement of a propeller slipstream on a downstream lifting surface. When installed on an aircraft, this unsteady loading results in vibrations that are transmitted to the fuselage and are perceived inside the cabin as structure-borne noise. A pylon-mounted tractor–propeller configuration was installed in a low-speed wind tunnel at Delft University of Technology. Surface-microphone and particle-image-velocimetry measurements were taken to quantify the pressure fluctuations on the pylon and visualize the impingement phenomena. It was confirmed that the propeller tip vortex is the dominant source of pressure fluctuations on the pylon. Along the path of the tip vortex on the pylon, a periodic pressure response occurs with strong harmonics. The amplitude of the pressure fluctuations increases with increasing thrust setting, whereas the unsteady lift coefficient displays a nonmonotonic dependency on the propeller thrust. The lowest integral unsteady loads were obtained for cases with approximately integer ratios between the pylon chord and the wavelength of the perturbation associated with the propeller tip vortices. This implies that structure-borne-noise reductions might be obtained by matching the pylon chord with an integer multiple of the axial separation between the propeller tip vortices.

Numerical analysis of the effect of the change in the ride height on the aerodynamic front wing–wheel interactions of a racing car

An investigation into the influence of the ground clearance on the aerodynamic interactions between the inverted front wing and the wheel of a racing car was conducted using computational fluid dynamics. Height-to-chord ratios h/ c from 0.075 to 0.27 were assessed for a single-element wing with a fixed angle of 4° and for two wing spans, one of which completely overlapped the wheel and the other which had its endplate coincident with the inside face of the wheel. With a narrower span, a lower peak downforce was achieved at a higher ground clearance owing to changes in the lower endplate vortex strength whereas, with a wider span, no downforce loss was observed, with decreasing clearance for those tested. This contrasted distinctly with the performance of the wing in isolation. The wheel lift was scarcely affected with decreasing wing ground clearance for the narrower span but decreased significantly for the wide-span wing at low ground clearances. The vortex paths changed considerably with the ground clearance, with a strong coupling to the wing span; a state in which the main vortex was destroyed in the contact patch of the wheel was identified.

Effects of Double-Leakage Tip Clearance Flow on the Performance of a Compressor Stage With a Large Rotor Tip Gap

Effects of a large rotor tip gap on the performance of a one and a half stage axial compressor are investigated in detail with a numerical simulation based on large eddy simulation (LES) and available particle image velocimetry (PIV) data. This paper studies the main flow physics, including why and how the loss generation is increased with the large rotor tip gap. The present study reveals that when the tip gap becomes large, tip clearance fluid goes over the tip clearance core vortex and enters into the next blade's tip gap, which is called double-leakage tip clearance flow. As the tip clearance flow enters into the adjacent blade's tip gap, a vortex rope with a lower pressure core is generated. This vortex rope breaks up the tip clearance core vortex of the adjacent blade, resulting in a large additional mixing. This double-leakage tip clearance flow occurs at all the operating conditions, from design flow to near stall condition, with the large tip gap for the current compressor stage. The double-leakage tip clearance flow, its interaction with the tip clearance core vortex of the adjacent blade, and the resulting large mixing loss are the main flow mechanism of the large rotor tip gap in the compressor. When the tip clearance is smaller, flow near the end wall follows more closely with the main passage flow and this double-leakage tip clearance flow does not happen near the design flow condition for the current compressor stage. When the compressor with a large tip gap operates at near stall operation, a strong vortex rope is generated near the leading edge due to the double-leakage flow. Part of this vortex separates from the path of the tip clearance core vortex and travels from the suction side of the blade toward the pressure side of the blade. This vortex is generated periodically at near stall operation with a large tip gap. As the vortex travels from the suction side to the pressure side of the blade, a large fluctuation of local pressure forces blade vibration. Nonsynchronous blade vibration occurs due to this vortex as the frequency of this vortex generation is not the same as the rotor. The present investigation confirms that this vortex is a part of separated tip clearance vortex, which is caused by the double-leakage tip clearance flow.

A semi-analytical model for the acoustic impedance of finite length circular holes with mean flow

The acoustic response of a circular hole with mean flow passing through it is highly relevant to Helmholtz resonators, fuel injectors, perforated plates, screens, liners and many other engineering applications. A widely used analytical model [M.S. Howe. “Onthe theory of unsteady high Reynolds number flow through a circular aperture”, Proc. of the Royal Soc. A. 366, 1725 (1979), 205‐223] which assumes an infinitesimally short hole was recently shown to be insufficient for predicting the impedance of holes with a finite length. In the present work, an analytical model based on Green׳s function method is developed to take the hole length into consideration for “short” holes. The importance of capturing the modified vortex noise accurately is shown. The vortices shed at the hole inlet edge are convected to the hole outlet and further downstream to form a vortex sheet. This couples with the acoustic waves and this coupling has the potential to generate as well as absorb acoustic energy in the low frequency region. The impedance predicted by this model shows the importance of capturing the path of the shed vortex. When the vortex path is captured accurately, the impedance predictions agree well with previous experimental and CFD results, for example predicting the potential for generation of acoustic energy at higher frequencies. For “long” holes, a simplified model which combines Howe׳s model with plane acoustic waves within the hole is developed. It is shown that the most important effect in this case is the acoustic non-compactness of the hole.

Wind Impact on Single Vortices and Counterrotating Vortex Pairs in Ground Proximity

Wall-resolved large eddy simulations are employed to investigate the behaviour of wake vortices and single vortices in ground proximity at a variety of wind conditions. The six considered strengths of wind, ranging between 0.5 and 4 times the initial wake vortex descent speed, w 0, include practically and theoretically significant wind speeds. A crosswind of 0.5 w 0 may lead to windward stall posing a potential hazard to subsequently landing aircraft, whereas theoretical considerations predict that at 4 w 0 the rebound of the luff vortex is completely suppressed. The same range of wind speeds is also used to investigate the effects of headwind and diagonal wind in order to discriminate between effects of environmental turbulence increasing with wind speed and the direction of the wind shear. The study has been complemented by a number of single vortex computations in order to differentiate between effects related to the mutual interaction of the vortex pair and the individual vortices with the turbulent boundary layer flow. It is shown that vortex ascent, descent, rebound and decay characteristics are controlled by (i) the interaction of the vortices with secondary vorticity detaching from the ground, (ii) the redistribution of vorticity of the boundary layer which is altering the path of the primary vortices by mutual velocity induction, and (iii) the interaction of the vortices with the environmental turbulence.

Investigation on bathtub vortex flow field by Particle Image Velocimetry

An experimental investigation on bathtub vortices has been performed by using Particle Image Velocimetry (PIV). Velocity fields associated to the free surface vortex were obtained at three horizontal planes and four Reynolds numbers, i.e. between 2400 and 11,000 (calculated with reference to the exit hole diameter and the mean exit velocity). Due to the unsteady behavior of the flow field, the vortex center positions have been identified and the vortex paths were reconstructed for all experiments. Average velocity fields have been calculated by aligning the vortex centers at each frame, in order to derive radial and tangential velocity profiles, to be compared at different Reynolds numbers and measurement planes. The results show that the radial motion assumes a potential behavior when it is near the exit hole, scaling quite well with the average exit velocity (thus with the corresponding Reynolds number). On the other hand, the tangential component is well approximated by the Rankine’s flow potential solution only near the free surface, the tangential velocity peak increment not being linearly proportional to the outlet velocity. Vorticity fields and circulation profiles have been derived from the measured velocity fields and discussed. Turbulence fluctuations statistical analysis gives also evidence of a clear dependence on Reynolds number and distance from the exit hole.

Electrochemical machining flow field simulation and experimental verification for irregular vortex paths of a closed integer impeller

Electrochemical machining (ECM) is an economical and effective method for machining hard-to-cut metal materials into complex shapes in aerospace and aeronautics fields, which are difficult to machine with conventional methods. As we all know, electrolyte flow field is one of the important factors in ECM irregular vortex paths of the closed integer impeller. To improve the stability of the whole processing, the flow field mathematical model was developed. The 3-D gap flow field simulation models of the reversed flow and forward flow patterns were also established, respectively. From the streamline, velocity, and pressure cloud picture of the electrolyte flow field simulation, the results showed that under the reversed pattern, the electrolyte flow velocity in the front gap and the side gap was not only higher but also more uniform than the forward pattern. Finally, the experimental verification was carried out and the experimental results were consistent with the simulation results. The whole process is stable and has no spark and no short circuit phenomenon with the reverse flow pattern. We successfully obtained 0.8-micron surface roughness of the machined workpiece. On the contrary, the irregular vortex paths cannot be successfully processed by forward flow pattern. The results indicate that reverse flow pattern is an effective and feasible method to machining irregular vortex paths of the closed integer impeller.

Critical Current Anisotropy of Zr Doped MOCVD Coated Conductor in Magnetic Fields up to 8T

The critical current angular distributions in the magnetic field range from 0 up to 8 T have been measured for Zr doped MOCVD (metalorganic chemical vapor deposition) tape produced by Super Power Inc. The vortex path model was used to fit these distributions to specify a tilted peak near c-axis that corresponds to the presence of BaZrO3 columnar defects in the tape. The orientation of such defects was found to be 10.3o with respect to the c-axis direction. Peak in the ab direction was observed in the whole field range and it could be interpreted as pinning on spacer layers between CuO planes. The appearance of gauss distribution was found in high fields that confirms chosen model.

The Vortex Path Model Analysis of the Field Angle Dependence of the Critical Current Density in Nanocomposite YBa2Cu3 O 7−x – BaZrO3 Films Obtained by Low Fluorine Chemical Solution Deposition

In the present paper, we analyze the role of in situ grown BaZrO3 (BZO) inclusions in YBa2Cu3O7−x (YBCO) thin films prepared by chemical solution deposition using a low fluorine coating solution, on the field angle dependence of the critical current density, J c (𝜃), data using the vortex path model. In order to form a coherent picture on the BZO doping influence on the pinning properties of the YBCO matrix, detailed structural analyses performed by X-ray diffraction techniques and microstructural evaluation by transmission electron microscopy are also presented. The evaluation of different contributions to the overall, J c , permitted us to prove the effectiveness of the BZO inclusions acting as isotropic pinning centers, reflected in a uniform component of high relative value with respect to other components. For the studied 10 mol % BZO doping concentration, a threefold increase in the critical current density, J c , of the YBCO host is measured, in self-field at 77 K, corresponding to a value of J c =2.9MA/cm2, whereas a factor 10 is measured at 1 T (J c =0.35 MA/cm2).

Computational study of the vortex path variation with the VG height

An extensive range of conventional, vane-type, passive vortex generators (VGs) are in use for successful applications of flow separation control. In most cases, the VG height is designed with the same thickness as the local boundary layer at the VG position. However, in some applications, these conventional VGs may produce excess residual drag. The so-called low-profile VGs can reduce the parasitic drag associated to this kind of passive control devices. As suggested by many authors, low-profile VGs can provide enough momentum transfer over a region several times their own height for effective flow-separation control with much lower drag. The main objective of this work is to study the variation of the path and the development of the primary vortex generated by a rectangular VG mounted on a flat plate with five different device heights h = δ, h1 = 0.8δ, h2 = 0.6δ, h3 = 0.4δ and h4 = 0.25m, where 5 is the local boundary layer thickness. For this purpose, computational simulations have been carried out at Reynolds number Re = 1350 based on the height of the conventional VG h = 0.25m with the angle of attack of the vane to the oncoming flow β = 18.5°. The results show that the VG scaling significantly affects the vortex trajectory and the peak vorticity generated by the primary vortex.

Three ranges of the angular dependence of critical current of BaZrO3 doped YBa2Cu3O7 − δ thin films grown at different temperatures

The growth of BaZrO3 (BZO) in pulsed laser deposited YBa2Cu3O7−δ (YBCO) thin films was studied by varying the deposition temperature. It was found that there are three deposition temperature ranges based on the properties of Jc(θ), the angular dependence of critical current density. Samples made at a relatively low temperature (low-T samples) do not show a c-axis peak in Jc(θ) whereas mid-T samples exhibit a peak as B║c-axis of YBCO. In high-T samples the c-peak disappears again. In the low-T samples BZO rods are too splayed and short for a c-axis peak whereas in the high-T samples vortices move along correlated, but shortened rods as well as along stacking faults, which causes the c-axis peak to disappear. The superconducting properties of the films were studied with both magnetic and transport measurements and the structural properties were characterized using X-ray diffraction and transmission electron microscopy. Correlations between the structural and superconducting properties were analyzed using the vortex path model.