Vortex Path Research Articles

Center vortex geometry at finite temperature

The geometry of center vortices is studied in SU(3) gauge theory at finite temperature to capture the key structural changes that occur through the deconfinement phase transition. Visualizations of the vortex structure in temporal and spatial slices of the lattice reveal a preference for the vortex sheet to align with the temporal dimension above the critical temperature. This is quantified through a correlation measure. A collection of vortex statistics, including vortex and branching point densities, and vortex path lengths between branching points, are analyzed to highlight internal shifts in vortex behavior arising from the loss of confinement. We find the zero-temperature inclination of branching points to cluster at short distances vanishes at high temperatures, embodying a rearrangement of branching points within the vortex structure. These findings establish the many aspects of center vortex geometry that characterize the phase transition in pure gauge theory. Published by the American Physical Society 2024

Aerodynamic and acoustic investigations of rotor-rotor wake interaction

The paper presents part of the experimental activities carried out in the GARTEUR Action Group RC/AG-26 to study the acoustic and aerodynamic characteristics of small rotor configurations, including the influence of the rotor-rotor interactions. Two rotors, equipped with two-bladed propellers of a diameter of D=330.2 mm, were operated at two rotating speeds of 8025 RPM and 10120 RPM and arranged in different configurations. Isolated rotor and two different coaxial configurations in hover conditions were assessed. The aerodynamic loads and the rotor slipstream characterisation, in terms of flow field velocity, tip vortex characterization as well as acoustics emissions, were performed using a six-component load cell, Particle Image Velocimetry and microphone measurements. The isolated pusher rotor presented better aerodynamic performance with respect to the puller one. The coaxial configurations result in a loss of thrust and an increase in torque compared to the isolated rotors. The upper rotor is barely affected by the presence of the lower one, while the lower rotor experiences a thrust loss ranging from 17 to 19%. The configuration with the larger rotor distance experiences greater performance loss. The mean velocity fields of the isolated rotors are measured and compared with the coaxial configurations for assessing the roto-rotor interaction, the results support the aerodynamic behaviour. The isolated rotor slipstream characterisation was carried out by phase-locked measurements covering the full rotor azimuth angle range from 0° to 350° with an increment of 30°. Tip vortex paths are followed up to a vortex age of 3.5 rotor revolutions. The 3D reconstruction of the tip vortices is conducted to gain insight into the slipstream behaviour and enhance the occurring vortex roll-up during the second revolution. The tip vortex jitter is investigated and the anisotropic distribution is confirmed along the slipstream. Furthermore, the noise emission intensifies in magnitude for the coaxial configuration with increasing relative gap between the rotors. The turbulent kinetic energy and the meandering of the blade tip vortices support the findings of the acoustics results.

Numerical study of cooling performance and flow characteristics of film hole-broken rib composite structure with squealer tip

The gas turbine blade tip might face substantial heat loads because of leakage flow between the blades and the casing. For blade tip cooling, a composite cooling structure with film holes and broken ribs is first used on GE-E3 blade in this work. The flow and cooling characteristics of the innovative structure are studied by numerical simulation under various blowing ratio (BR) conditions. Meanwhile, the impact of modifying both the rib angle and the rib height on the adiabatic film cooling effectiveness (AFCE) at the tip of the squealer is analyzed. According to the results, adding rib structures to the squealer tip can effectively regulate the paths of cavity vortices and kidney-shaped vortex pairs (KVP) at the tip. As a result, the averaged AFCE at the blade tip is improved. The notch pressure-side broken rib structure has good aerothermal performance, and the highest AFCE at BRs of 0.50, 1.00, and 1.50 basically occur under the “R60-100 %” condition (R60 refers to the rib structure of 60°, and 100 % is the ratio of rib height to notch depth), and the corresponding AFCE are 27.71 %, 26.00 %, and 32.47 % higher than those of the no-rib case, respectively. The corresponding AFCE increased by 27.71 %, 26.52 %, and 32.47 %, respectively, compared to the no-rib condition. The highest AFCE at a BR of 1.50 occurs at “R75-70 %“, which is a 38.20 % increase in AFCE compared to the no rib case. The improvement in AFCE is due to the difference in the flow of the cooling jets, which are subject to cavity vortices at different BRs. The analysis shows that the addition of ribs disrupts the formation of KVPs and weakens the influence of the cavity vortex, thus reducing the low AFCE region at the lower end of the tip groove and increasing the AFCE. However, due to the blocking effect of the ribs, the pressure loss at the blade tip is elevated. The proposed blade tip cooling structure is expected to provide new ideas for the next generation of advanced gas turbine cooling designs.

CNN-based vane-type vortex generator modelling

The simplicity and accuracy of Computational Fluid Dynamics (CFD) tools have made them the most widely used method for solving fluid dynamics problems. However, these tools have some limitations, being the most significant the required computational resources. This fact, added to the growth of the Artificial Intelligence, has led to an increasing number of studies using data-driven methods to solve fluid dynamic problems. Flow control devices are a very popular research topic, since their implementation can significantly improve the behaviour of the flow. Among these devices, Vortex Generators (VGs) can be highlighted for their simplicity, efficiency and numerous applications. In this study, a Convolutional Neural Network (CNN) is proposed to predict the flow behaviour on the wake behind VGs. In order to obtain data for training the network, 20 different CFD simulations were conducted, considering the same inflow conditions but different vane heights and angles of attack of the VGs. The results show that the CNN is able to accurately predict the velocity and vorticity fields on the wake of the VG, being the most conflictive cases the ones with tall VGs, large angles of attack and close distances to the VGs. Additionally, the vortical structure, vortex path and velocity profiles on the vortex core of the main vortex are evaluated, showing good agreements with CFD results.

Analysis of Vortex Dominated Flow over Double Delta Wing

The maneuverability of fighter aircraft designed using double delta wing configuration at low subsonic speed in addition to under high angle of attacks are highly appealing and fascinating to comprehend. Flow pattern formed by double delta wing help to facilitate lifting force under high angle of attack and at such low subsonic speed. In comparison to delta wing, vortex formations by strake part of wing play an important role to create a persistent vortex core capable of delaying stalling. In this investigation, force coefficients of 75°/45° double delta wing configuration is extracted both from wing tunnel experiment and numerical calculation between 0° to 30° angles of attack at Reynolds number of 1.4 × 105 based on root chord length. It has also been attempted to investigate the flow pattern formation over wing leeward surface, interaction between vortices formed by strake/wing based on Q-criterion and pressure distribution along propagation path of vortices obtained from CFD (Computational Fluid Dynamics) calculations. Implications of interplay between vortices and pressure distribution in lift generation have also been analyzed. It has been found that around 15° angle of attack strake and wing vortices merge and gets more intensified. With higher angle of attack the lift generation significantly diminishes because of vortex bursting

Studying the Oscillating Flow around Unconfined Obstacle of Elliptical Cross-Section

The present paper focuses on the effect of geometric shape of a cylindrical body submerged in an oscillating flow by means of numerical investigations. The cylindrical body has an elliptic cross-section with a variation of its elliptic ratio (ratio between major and minor axes of the elliptic section). The flow direction is oriented along the major axis. Three regimes from (Tatstuno and Bearman) maps are studied namely, a symmetric regime A and two other asymmetric regimes D and F. The flow field structures, the Morison coefficients of longitudinal forces and the root mean square (r.m.s.) of transverse forces are computed with respect to the elliptic ratio variation. For the case of cylinders with slightly elliptic cross-section, vortices and pressure fields are very similar to those of a circular cylinder. For the case of regime A, the vortex shedding is always symmetric despite the unbreakable variation of the elliptic ratio. On the other hand, the reduction of the elliptic ratio weakens the asymmetry of the flow for regimes D and F. Moreover, the flow in each regime becomes completely symmetric at a given value of the elliptic ratio. In fact, the predicted longitudinal component of the force acting on the cylinder decreases with the reduction of this ratio. This results in the same manner on the behavior of Morison coefficients. With regard to the symmetric regime A, the transverse force does not manifest itself for all considered ratios. On the other hand, the transverse force in the case of asymmetric regimes decreases rapidly with increasing the ellipticity of the cylinder. The present study showed us that the inertia coefficient is sensitive to the vortex path; however, the drag coefficient is independent of the vortex path. Both coefficients depend simultaneously on Reynolds number and the geometric shape of the body.

Wind Resistance Mechanism of an Anole Lizard-Inspired Climbing Robot.

The stable operation of climbing robots exposed to high winds is of great significance for the health-monitoring of structures. This study proposes an anole lizard-like climbing robot inspired by its superior wind resistance. First, the stability mechanism of the anole lizard body in adhesion and desorption is investigated by developing adhesion and desorption models, respectively. Then, the hypothesis that the anole lizard improves its adhesion and stability performance through abdominal adjustment and trunk swing is tested by developing a simplified body model and kinematic model. After that, the structures of the toe, limb, and multi-stage flexible torso of the anole lizard-like climbing robot are designed. Subsequently, the aerodynamic behavior of the proposed robot under high-speed airflow are investigated using finite element analysis. The results show that when there is no obstacle, the climbing robot generates the normal force to enhance toepad friction and adhesion by tuning the abdomen’s shape to create an air pressure difference between the back and abdomen. When there is an obstacle, a component force is obtained through periodic oscillation of the spine and tail to resist the frontal winds resulting from the vortex paths generated by the airflow behind the obstacle. These results confirm that the proposed hypothesis is correct. Finally, the adhesion and wind resistance performance of the anole lizard-like climbing robot is tested through the developed experimental platform. It is found that the adhesion force is equal to 50 N when the pre-pressure is 20 N. Further, it is shown that the normal pressure of the proposed robot can reach 76.6% of its weight in a high wind of 14 m/s.

Effect of shape of frontbody and afterbody on flow past a stationary cylinder at Re = 100

We numerically study the effect of the shape of frontbody and afterbody on the flow past a cylinder at a Reynolds number of 100. Two-dimensional simulations have been carried out using an in-house sharp-interface immersed boundary method-based flow solver. The cylinder cross section is considered as a semi ellipse on both windward and leeward sides. The semi-minor axis on the windward side (frontbody parameter, LF) and the leeward side (afterbody parameter, LA) varies from 0 to 0.5 to render cylinders of different cross sections. The effect of LF and LA is quantified on the following variables: drag coefficient, lift coefficient, the Strouhal number, vortex formation length, vortex fluctuation energy, the flow separation point, and cylinder bluffness. While the drag linearly decreases with both LF and LA, the gradient with respect to LF is nearly twice larger than LA. The computed vortex formation length scales directly with drag in the LF-LA plane, while the vortex fluctuation energy scales inversely. The lift and the Strouhal number vary non-monotonically in the LF-LA plane, explained in terms of vortex formation length and the flow separation point, respectively. We briefly quantify wake signatures in the LF-LA plane. The downstream vortex paths are traced, and in general, two vortex shedding patterns, 2S and C(2S), are correlated with values of LF and LA. A dynamic mode decomposition analysis of the flow modes helps to explain the computed fluid-flow characteristics.

Interaction between breaking-induced vortices and near-bed structures. Part 1. Experimental and theoretical investigation

The present work describes the vortex–vortex interactions observed during laboratory experiments, where a single regular water wave is allowed to travel over a discontinuous rigid bed promoting the generation of both near-bed and surface vortices. While near-bed vortices are generated by the flow separation occurring at the bed discontinuity, surface vortices are induced by the wave breaking in conjunction with a breaking-induced jet. A ‘backward breaking’ (previously observed in the case of solitary waves) occurs at the air–water interface downstream of the discontinuity and generates a surface anticlockwise vortex that interacts with the near-bed clockwise vortex. With the vortex–vortex interaction influenced by many physical mechanisms, a point-vortex model, by which vortices evolve under both self-advection (in relation to both free surface and seabed) and mutual interaction, has been implemented to separately investigate the vortex- and wave-induced dynamics. The available data indicate that both self-advection and mutual interaction are the governing mechanisms for the downward motion of the surface vortex, with the effect of the breaking-induced jet being negligible. The same two mechanisms, combined with the mean flow, are responsible for the almost horizontal and oscillating path of the near-bed vortex. The investigation of the vortex paths allow us to group the performed tests into three distinct classes, each characterized by a specific range of wave nonlinearity. The time evolution of the main variables characterizing the vortices (e.g. circulation, kinetic energy, enstrophy, radius) and their maximum values increase with the wave nonlinearity, such dependences being described by synthetic best-fit formulas.

Study on the mud swimming motion of Paramisgurnus dabryanus

In the context of the rapid development of bionic technology, inspired by the swimming behavior of fish, a variety of robotic fish have been designed and applied to different underwater works and even military applications. However, in some operations, such as detection and salvage, vehicles need to travel under mud, a medium that is different from fluids. This complicating factor put higher requirements on robotic fish design. In this study, Paramisgurnus dabryanus, a fish species adept at swimming into the mud, was taken as a research object to investigate its profile and mud swimming behavior. First, a three-dimensional (3D) image scanner is used for profile scanning to acquire the point cloud data of the profile features of the loach. After modification, data coordinate points are extracted and used to fit the profile curve of loach and build geometric and mathematical models by means of Fourier function fitting. The next step includes the analysis of the motion of loach, determination of main parameters of the wave equation, and establishment of the fish body wave curve of a loach in the swimming using MATLAB software. Saturated mud having a water content of 37% is adopted as an environmental medium to numerically simulate the swimming behavior in mud, identifying the distribution of vortex path, and velocity field of loach’s motion. The rationality of simulation results is verified by the loach mud swimming test, and the simulating results agree well with the experimental data. This study lays a preliminary foundation for the outer contour design of the robotic fish operating under mud and aims to carry out the drag reduction and accelerating design of the robotic fish. The robotic loach may be applied in fishery breeding, shipwreck salvage operations, and so on.

Fluid flow characteristics of two types rotary engines

Hydrogen fueled rotary engines are potential alternatives to be green engines in vehicles. In this study, a numerical approach is used to investigate and compare the internal flow field characteristics of two types of rotary engines: triangular rotary engine (TRE) and liquid piston engine (LPE) also known as elliptical rotary engine. First, the three-dimensional fluid analysis models are developed for the two engines based on geometric designs of the rotors. The fluid analysis is carried out using the computational fluid dynamics (CFD) tool by assuming two working fluids: air and H2, and the dynamic mesh and turbulent flow models are integrated and simulated with ANSYS Fluent solver. For simulation convenience, combustion and thermal effects are neglected. The results show that the flow rate fluctuation coefficient and gas moment fluctuation coefficient are higher in LPE as compared to TRE, indicating LPE has relatively lower stability than TRE. The vortex number is found to be higher in LPE, signifying higher combustion efficiency of LPE as compared to TRE. However, LPE may have more energy dissipations, reduced exhaust and suction efficiency and high risk of leakages. The TRE has more stable flow, larger exhaust, lesser vortex, lower leakages, and simpler flow path as compared to LPE. Comparison of the two fluids i.e. air and H₂ shows that in TRE, air has high magnitude of average gas moment, high mass flow rate, high pressure and low flow velocity as compared to H₂, implying that air is more stable than H₂ in TRE. Conversely, H₂ is more stable than air in the case of LPE. The vortex number is relatively lower in both LPE and TRE using air indicating that air has low leakages than H₂ in both rotary engine types. This study can be used as a reference for a better rotary engine design in the future.

Performance Improvement of a Centrifugal Compressor for the Fuel Cell Vehicle by Tip Leakage Vortex Control

Heightened interests have been laid at the preliminary design and optimization of the centrifugal compressor for the fuel cell vehicle. The centrifugal compressor for fuel cell vehicle is driven by a high-speed motor; however, the limit of the motor speed makes the flow passage of the impeller long and narrow, which leads to a serious tip leakage loss. Serious tip leakage loss deteriorates the compressor performance. In this paper, 3-D numerical simulations were carried out with the aim of investigating the tip leakage loss in a prototype centrifugal compressor for a 100 kW fuel cell stack. The results revealed that the mixing loss caused by the interaction between the tip leakage vortex and the downstream tip leakage flow contributed to the major part of the tip leakage loss. The path of the tip leakage vortex almost followed the streamwise direction, while the downstream tip leakage flow exhibited strong circumferential momentum, which referred to the fact that they were nearly orthogonal. Therefore, a flow control approach, which was realized by enhancing the blade loading around the leading edge of blade tips in this paper, was proposed to decrease the interaction angle between the tip leakage vortex and the downstream tip leakage flow and then mitigate mixing loss by changing the flow direction of the tip leakage vortex. The results showed a smaller interaction angle was achieved in the optimized impeller compared with the baseline one. Meanwhile, the efficiency was also improved by 1.30% at design condition and the maximum efficiency improvement could be up to 10% at large mass flow condition of 92 000 r/min. Being manufactured and tested, the optimized compressor was proved to achieve an isentropic efficiency of 75.84% at design condition.

Experimental investigation of the influence on compressor cascade characteristics at high subsonic speed with pressure surface tip winglets

To investigate the influence of tip winglets on the tip leakage flow in a compressor cascade with different incidences, the experimental measurement combined with numerical simulation are used to study the conventional cascade and cascades with three different pressure surface tip winglets at five incidences of −6°, −3°, 0°, +3° and +6°. The results indicate that three different tip winglets at five incidences all restrain the occurrence of leakage flow, delay the mixing of leakage flow and the mainstream, change the formation path of leakage vortex and weaken its intensity, reduce the flow loss and improve the uniformity of flow field. The sensitivity of the flow field to variable incidences is reduced. The optimization degree of the flow field is proportional to the width of the blade tip winglet. The improvements are more obvious at positive angles. When the incidence reaches +6°, the flow loss is reduced by 12.4%.

Oblique and streamwise vortex paths in a plane Couette flow using a RNL system

This paper revisits oblique wave and streamwise vortex scenarios in a plane Couette flow using restricted nonlinear simulations, where only a single Fourier mode for perturbation is retained. It is shown that this restriction of full dynamics gives a good approximation of the two subcritical paths. In particular, critical energy thresholds and edge states compare favorably with results obtained using direct numerical simulations by Duguet et al. (Phys. Rev. E 82 (2010), 026316).

Accuracy of the Cell-Set Model on a Single Vane-Type Vortex Generator in Negligible Streamwise Pressure Gradient Flow with RANS and LES

Passive flow control devices are included in the design of wind turbine blades in order to obtain better performance and reduce loads without consuming any external energy. Vortex Generators are one of the most popular flow control devices, whose main objective is to delay the flow separation and increase the maximum lift coefficient. Computational Fluid Dynamics (CFD) simulations of a Vortex Generator (VG) on a flat plate in negligible streamwise pressure gradient conditions with the fully-resolved mesh model and the cell-set model using Large Eddy Simulation (LES) and Reynolds-Averaged Navier-Stokes (RANS) were carried out, with the objective of evaluating the accuracy of the cell-set model taking the fully-resolved mesh model as benchmark. The implementation of the cell-set model entailed a considerable reduction of the number of cells, which entailed saving simulation time and resources. The coherent structures, vortex path, wall shear stress and size, strength and velocity profiles of the primary vortex have been analyzed. The results show good agreements between the fully-resolved mesh model and the cell-set mode with RANS in all the analyzed parameters. With LES, acceptable results were obtained in terms of coherent structures, vortex path and wall shear stress, but slight differences between models are visible in the size, strength and velocity profiles of the primary vortex. As this is considered the first application of the cell-set model on VGs, further research is proposed, since the implementation of the cell-set model can represent an advantage over the fully-resolved mesh model.

Numerical investigation on the effects of leakage flow from Guide vane-clearance gaps in low specific speed Francis turbines

Clearance gap in guide vanes of Francis turbine induces the leakage vortex. This vortex flow interacts with the main flow and leads to the instability in the fluid flow pattern and eventually deteriorates the performance of the turbine. In this study, the detailed numerical examination of the unsteady flow due to leakage vortex and influence of this in the performance of turbine is carried out using time dependent numerical analysis. The numerical simulation is carried out using SST turbulence model with numerical validation. The development of leakage vortex is studied within the clearance gap region. Time dependent numerical simulation is carried out to investigate the growth and vortex propagation considering five revolutions of the runner. Results shows that the leakage vortex travelling form the guide vane clearance gaps influence the performance of the runner. On analysing the leakage vortex path, it is seen that the vortex travels from the pressure side of runner blade to the suction side of adjacent blade opposite to the runner rotation. Furthermore, this leakage vortex is carried down to the draft tube where the vortex rope seems to grow up to 50% geometric progression of draft tube cone and gradually decreases before reaching the elbow. Upon investigation of resulting torque and head during runner rotation, the periodic variation of torque and head can be seen, however with different phase. This is inferred to be the influence of leakage vortex that travels along with the main flow.

Bifurcation of relative equilibria generated by a circular vortex path in a circular domain

We study the passive particle transport generated by a circular vortex path in a 2D ideal flow confined in a circular domain. Taking the strength and angular velocity of the vortex path as main parameters, the bifurcation scheme of relative equilibria is identified. For a perturbed path, an infinite number of orbits around the centers are persistent, giving rise to periodic solutions with zero winding number.

Sound from aeroelastic vortex-fibre interactions.

The motion of a line vortex moving past a one-dimensional flexible fibre is examined theoretically. A Schwarz-Christoffel conformal mapping enables the analytical solution of the potential flow field and its hydrodynamic moment on the flexible fibre, which is composed of a rigid segment constrained to angular motions on a wedge. The hydroelastic coupling of the vortex path and fibre motion affects the noise signature, which is evaluated for the special case of acoustically compact fibres embedded in a half plane. Results from this analysis attempt to address how the coupled interactions between vortical sources and flexible barbules on the upper surface of owl wings may contribute to their acoustic stealth. The analytical formulation is also amenable to application to vortex sound prediction from flexible trailing edges provided that an appropriate acoustic Green's function can be determined. This article is part of the theme issue 'Frontiers of aeroacoustics research: theory, computation and experiment'.

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.