Trapped Vortex Research Articles

Investigation of performance in a cylindrical trapped vortex combustor with swirler

Investigation of performance in a cylindrical trapped vortex combustor with swirler

Investigations on vortex evolution and wake dynamics of bio-inspired pitching hydrofoils

Recently, lots of oscillating targets inspired from motions of some insects and birds have been applied extensively to many engineering applications. The aim of this work is to reveal the performance and detailed flow structures over the pitching corrugated hydrofoils under various working conditions, using the SST [Formula: see text] transition model. First of all, the lift coefficients of a smooth oscillating airfoil at different reduced frequency and pitching angles show a good agreement with the experiments, characterized by the accurate prediction of the light and deep stall. For the pitching corrugated hydrofoils, it shows that the mean lift coefficient increases with the pitching magnitude, but it has an obvious drop at high reduced frequency for the case with large pitching amplitude, which is mainly induced by the pressure modification on the surface with smooth curvature, depending on the oscillation significantly. In addition, the mean drag coefficient also indicates that the drag turns into the thrust at high reduced frequency when the pitching amplitude exceeds to the value of 10°. Increasing the reduced frequency delays the flow structure and leads to the deflection of the wake vortical flow. The Reynolds number also has an impact on the hydrofoil performance and wake morphology. Furthermore, regarding the shape effect, it seems that hydrofoil A (consisting of two protrusions and hollows and the aft part with smooth curvature) achieves the higher lift than hydrofoil B (comprising several protrusions and hollows along the surface), specially at high reduced frequency. Although the frequency collected from two hydrofoils remains nearly the same near the leading edge and in the wake region, the high sub-frequency is evidently reduced for hydrofoil B in second and third hollows, due to the relatively stable trapped vortices. Then, the wake transition from the thrust-indicative to drag-indicative profile for hydrofoil B is also slower compared with hydrofoil A. Finally, it is observed that with the increase of the thickness, the lift/drag ratio decreases and the slow wake transition is detected for the thin hydrofoil, which is associated with the relatively low drag coefficient.

Schooling benefits from a system of active and passive hydrofoils

Fishes school to swim more efficiently while not much of fish schooling is understood from the perspective of fluid–structure interactions. To understand the benefits of fish schooling, we investigate thrust, efficiency, and wake structure of a two-hydrofoil system where the upstream hydrofoil is forced pitching (active) and the downstream hydrofoil is free pitching (passive). The streamwise and lateral separations between the two hydrofoils are 0.109c and 1.0D, respectively, where c is the chord length and D is the diameter of the leading edge. The active hydrofoil pitches with normalized amplitudes A∗=0.55 – 0.80 and normalized frequencies StD=0.23–0.33. It is found that the upstream active hydrofoil undergoes up to 38% thrust enhancement in the presence of the downstream passive hydrofoil while the downstream passive hydrofoil achieves the same thrust as the upstream active hydrofoil under certain conditions. The maximum combined thrust and efficiency achieved for the two-hydrofoil system are 95% and 180% higher than those for a single isolated hydrofoil. These are some reasons for fish swimming in school. Three distinct flow structures (vortex impingement flow, vortex trapping flow, and vortex splitting flow) are identified in the A∗– StD domain, where the vortex trapping flow provides the greatest thrust and efficiency enhancement.

Optimizing trapped field in superconductors with perforations

The use of artificial holes can improve the performance of high-temperature bulk superconductors. We report the results of Monte Carlo simulations of the trapped magnetic flux in superconducting samples with different configurations of perforated holes. Vortex trapping and emission at the hole boundary were modeled to account for flux pinning on the holes. It was found that the trapped flux can reach its maximum value, which does not depend on the number of holes. The dependence of the trapped flux on the diameter and number of holes is explored and described by the suggested relations that account for pinning on the holes and the effective hole area. Recommendations are provided to attain the maximum trapped flux for a fixed number or diameter of holes.

Comparison of the Effectiveness of Drag Reduction Devices on a Simplified Truck Model through Numerical Simulation

The aerodynamic efficiency of trucks is very low because of their non-streamlined box shape, which is subject to practical constraints, leaving little room for improvement in terms of aerodynamic efficiency. Hence, other means of improving the aerodynamic efficiency of trucks are needed, and one practical yet relatively simple method to reduce aerodynamic drag is deploying drag reduction devices on trucks. This paper describes a numerical study of flow over a simplified truck with drag reduction devices. The numerical approach employed was Reynolds-averaged Navier–Stokes (RANS). Four test cases with different drag reduction devices deployed around the tractor–trailer gap region were studied. The effectiveness of those drag reduction devices was assessed, and it was demonstrated that in all four cases, the aerodynamic drag was reduced compared with the baseline case without any drag reduction devices. The most effective device was case 4 (about 24% reduction), with a roof deflector, side extenders, and five cross-flow vortex trap devices (CVTDs). Flow field analysis was performed to shed light on drag reduction mechanisms, which confirmed our previous findings that the main reason for the drag reduction was the reduced pressure on the front face of the trailer, while the reduction in the turbulence level in the tractor–trailer gap region contributed much less to the overall drag reduction.

Formation of small-scale vortices in the core of a large merged vortex

The merger of two surface quasi-geostrophic vortices is examined in detail. As the two vortices collapse towards each other in the merging process, they trap their external fronts between them; these fronts are inserted into the final merged vortex, where they form a central, nearly parallel, sheared velocity strip, sensitive to barotropic instability. As a result, this strip breaks up into an alley of small vortices. Subsequently, these small vortices may undergo merger and grow in size in the core of the large merged vortex. The number of small trapped vortices decreases correspondingly. Finally, a single or two small vortices remain. These processes are analysed using a numerical model of the surface quasi-geostrophic equations. The sensitivity of this process to the initial vortex characteristics is explored. A parallel is drawn between this problem and the instability of a rectilinear strip of temperature with a central gap. The application of this problem to the Ocean is discussed.

Very-Large-Eddy Simulation of Nonreactive Turbulent Flow for Annular Trapped Vortex Combustor

A hybrid Reynolds-averaged Navier-Stokes (RANS) large eddy simulation (LES) method is applied in this work. It called very-large-eddy simulation (VLES) turbulence closure model. The aim of this present study is firstly to validate the accuracy of this method for a specific engineering application (a trapped vortex combustor) and secondly to describe its flow characteristics. The trapped vortex combustor is a new concept that utilizes a large recirculation vortex to stabilize the flame. An accurate prediction of the turbulent flow is meaningful for the trapped vortex combustor. The time-averaged velocity, root-mean-square (rms) velocity, and flow pattern are compared with the experimental data. And the LES model, RANS BSL k-ω model, and RANS k-ɛ model are also applied for the simulation with different mesh resolutions. The results show that the VLES BSL k-ω model provides improved accuracy for velocity prediction. The classical large vortex structure for the trapped vortex combustor is captured qualitatively by the VLES BSL k-ω model also. In addition, the vortex breakdown and processing vortex cone are visualized using the Q-criterion. Furthermore, the VLES BSL k-ω model is not sensitive to the gird resolution. The VLES method is able to predict the turbulent flow of trapped vortex combustor relatively well‎.

Power augmentation of Darrieus wind turbine blades using trapped vortex cavity

Enhancing the performance of Darrieus Vertical Axis Wind Turbines (VAWTs) is the key to boost their possible commercialisation in the yet highly competitive wind energy market. This study evaluates an active boundary layer control technique to increase the aerodynamic efficiency of Darrieus VAWTs. A cavity is created on the airfoil's suction side, where suction is applied to create a trapped vortex. High-fidelity Computational Fluid Dynamics (CFD) simulations are used after validation against experimental results. The effect of suction momentum ratio Cμ on the performance of the turbine is investigated. The turbine with suction cavity airfoils (SC) shows a higher power coefficient than the one with baseline airfoil (BL) blades, especially at low tip-speed ratios. For the selected test case, the net maximum predicted power coefficient is 0.435 for the SC turbine at TSR = 2.3, 0.375 for the baseline at TSR = 2.6. At the lower tip-speed ratios, i.e. TSR = 2, the power coefficient of the SC turbine is almost double that of the BL turbine. The feasibility of the active control technique is finally discussed, demonstrating its potential as an effective solution for the performance augmentation of the Darrieus turbine.

Anomalous magnetic interference of cross-type Josephson junctions exposed to oblique magnetic fields

Gauge-invariant phase difference and critical currents of cross-type Josephson junctions with thin and narrow superconducting strips exposed to three-dimensional magnetic fields are theoretically investigated. When a sandwich-type Josephson junction in the xy plane is exposed to parallel magnetic fields H x and H y , the phase difference linearly depends on the spatial coordinates, x and y, and the critical currents exhibit the standard Fraunhofer-type magnetic interference. The perpendicular field H z , on the other hand, nonlinearly modulates the distribution of the phase difference and the critical currents as the functions of the oblique field exhibit anomalous magnetic interference. We obtain simple analytical expressions for critical currents of small cross-type junctions by neglecting the effects of self-field and trapped vortices. The resulting dc critical currents show anomalous and diverse interference patterns depending on the parallel and perpendicular magnetic fields.

Numerical Study of Airborne Particle Dynamics in Vortices Subject to Electric Field

Capture, selective collection and flight manipulation of airborne particulate are three important functional requirements in various actively growing aerosol technology applications. Aerodynamic drag, particle inertia and dielectrophoretic (DEP) force due to externally applied electrostatic forces influence the behavior of micron sized particles significantly, in such situations. In this work, we numerically study how a combination of these forces uniquely influences the behavior of uncharged or mildly charged airborne particles, distinct from that with their individual influences. Uncharged particle movements in a numerically fabricated well structured steady vortical flow between two curved electrode surfaces are analyzed. Vortical air circulation towards and away from the electrode tip enhances and deteriorates electrostatic particulate capture on the electrodes, termed as co and counter directions with respect to the electrostatic force on particles respectively. Particles in counter vortices under an electric field reveals a rich variety of unique behaviors due to the interplay of drag, inertia and DEP forces. Distortion of the vortex structure due to convexity of electrode surfaces results in an inverse inertial limit cycle trajectory trapping of particles; with the airborne particles spatially segregated, trapping larger particles further inside the vortex than the smaller particles.Value of a dimensionless number $\xi_v$, the ratio of DEP force and particle inertia, represents the combination of the operating flow and electric field strengths. Inertial cut-off of particle capture in this configuration abruptly shift to DEP capture at a critical value of $\xi_v\approx 0.2$, as its value increases. Selective deposition of particles within a closed range of size and density, emerges due to the interplay of vortex trapping, inertial expulsion and electrostatic capture.

Performance assessment of an innovative Gurney flap for straight-bladed vertical axis wind turbine

Based on the vortex trapping capability of the cavity and the lift improvement mechanism for the Gurney Flap (GF), their combined impact on aerodynamic efficiency of the straight-bladed vertical axis wind turbine (SB-VAWT) was investigated. In order to eliminate the flow resistance of the blade caused by GF in the windward zone, an active flow control technique, a pivoting GF could change its position as the blades rotates in a cycle, is proposed in this paper. A grid sensitivity analysis by Grid Convergence Index (GCI) is first carried out to examine the convergence of the numerical model. Considering different GF control strategies, Computational fluid dynamics (CFD) method was used to evaluate the potential of the movable cavity-GF (c-GF) for power augmentation of SB-VAWT with traditional fixed cavity-GF. The simulation results show that power coefficient for movable c-GF is increased by more than 20% in comparison with the clean one. An appropriate active control solution not only has excellent power extraction in the downwind revolution, but also can significantly eliminate the resistance effect in the upwind zone cause by fixed c-GF. In addition, the benefits of the movable c-GF were prominent especially at the smaller tip speed ratios which indicates that it can be used for both newly designed turbines and retrofitting solutions for existing rotors.

Performance Enhancement Analysis of a Horizontal Axis Wind Turbine by Vortex Trapping Cavity

Abstract A wind turbine blade has the particularity of containing twisted and tapered thick airfoils. The challenge with this configuration is the highly separated flow in the region of high twist. This research presents a numerical investigation of the effectiveness of a vortex trapping cavity (VTC) on the aerodynamics of the National Renewable Energy Laboratory phase VI wind turbine. First, simulations are conducted on the S809 profile to study the fluid flow compared to the airfoil with the redesigned VTC. Second, the blade is simulated with and without VTC to assess its effect on the torque and the flow patterns. The results show that for high angles of incidence at Rec = 106, the lift coefficient increases by 10% and the wake region appears smaller for the case with VTC. For wind speeds larger than 10 m/s, the VTC improves the torque by 3.9%. This is due to the separation that takes place in the vicinity of the VTC and leads to trapping early separation eddies inside the cell. These eddies roll up forming a coherent laminar vortex structure, which in turn sheds periodically out of the cell. This phenomenon favorably reshapes excessive flow separation, reenergizes the boundary layer, and globally improves blade torque.

Vortex Trapping Cavity on Airfoil: High-Order Penalized Vortex Method Numerical Simulation and Water Tunnel Experimental Investigation

This paper presents a two-dimensional implementation of the high-order penalized vortex in cell method applied to solve the flow past an airfoil with a vortex trapping cavity operating under moderate Reynolds number. The purpose of this article is to investigate the fundamentals of the vortex trapping cavity. The first part of the paper treats with the numerical implementation of the method and high-order schemes incorporated into the algorithm. Poisson, stream-velocity, advection, and diffusion equations were solved. The derivation, finite difference formulation, Lagrangian particle remeshing procedure, and accuracy tests were shown. Flow past complex geometries was possible through the penalization method. A procedure description for preparing geometry data was included. The entire methodology was tested with flow past impulsively started cylinder for three Reynolds numbers: 550, 3000, 9500. Drag coefficient, streamlines, and vorticity contours were checked against results obtained by other authors. Afterwards, simulations and experimental results are presented for a standard airfoil and those equipped with a trapping vortex cavity. Airfoil with an optimized cavity shape was tested under three angles of attack: 3°, 6°, 9°. The Reynolds number is equal to Re = 2 × 104. Apart from performing flow analysis, drag and lift coefficients for different shapes were measured to assess the effect of vortex trapping cavity on aerodynamic performance. Flow patterns were compared against ultraviolet dye visualizations obtained from the water tunnel experiment.

Comparison of phase optimization algorithms for particle manipulation via acoustic tweezer

Acoustic-based manipulation is a safe and non-damaging way to handle objects without direct physical contact, and has many potential applications in the manipulation of biological cells and micro-robots. We present a calculation model to optimize the phases of such arrays to generate different shapes of acoustic tweezers. The optimized phase combinations of transducers were determined by both genetic and Broyden–Fletcher–Goldfarb–Shanno (BFGS), and it was found that the BFGS algorithm was suited to small arrays while the genetic algorithm was faster when the array number was large. Two tweezer shapes, twin trap and vortex trap, were realized by tuning the weight parameters in the optimization model. Using the platform of a field-programmable gate array (FPGA) controller, a 1500-μm-diameter polyvinyl chloride (PVCS) bead was levitated and moved along a defined path, which verified the feasibility of the optimization model. Moreover, the control performance of the particle was quantified by image detection, showing that the moving accuracy was controlled within 583.62 µm.

Overview of critical currents and vortex trapping phenomena in massive and nanometric high-TC superconductors

Overview of critical currents and vortex trapping phenomena in massive and nanometric high-TC superconductors

Mapping of laminar separation bubble and bubble-induced vibrations over a turbine blade at low Reynolds numbers

A detailed experimental investigation on the mapping of the LSB formation over a turbine blade with different airfoils, and their vibration effects were considered first in this study. Wind tunnel experiments were performed on FX 84-W-150, SD7062, Clark-Y and WASP airfoils at Re = 3.5 × 104, Re = 7 × 104 and various angles of attack. Experimental arrangements consisted of detailed instantaneous and time-dependent flow visualizations, quasi-wall shear stress measurements by means of a hot-film sensor, aerodynamic force measurements with an external force balance system. It was observed that there was a strong correlation between voltage signals obtained from the hot-film sensor and the smoke-wire results. Fluctuation amplitude started to increase within the LSB especially at the aft portion due to the transition induced trapped roll-up vortices. Then, fluctuations peaked upstream of the reattachment point. It was observed that the airfoil with different thickness and camber caused the LSB (either short or long) to form, resulting in variation of aerodynamic forces with the time. This could trigger the stochastic vibrations such as flap-wise and edge-wise vibrations at the blade, causing the stochastic fatigue loads and limiting the lifespan of turbine blades.

Passive flow control of shock-induced dynamic stall via surface-based trapped vortex generators

A novel passive approach for controlling the flow in a 2D dynamic stall at variabl freestream is investigated. 2D computational fluid dynamics simulations of an SC1095 airfoil with surface-based trapped vortex generator (STVG) type passive flow control were conducted. The airfoil was exposed to a fluctuating freestream of Mach 0.537 ± 0.205 and Re = 6.1 × 106 (based on the mean Mach number) and experienced a 10° ± 10° pitch oscillation with a frequency of 4.25 Hz. These conditions were selected as an approximation to the flow experienced by a UH-60A helicopter rotor airfoil section in an actual fast forward flight test case. The baseline simulations were cautiously validated with experimental data for both transonic flow and dynamic stall under the variable freestream. Then, 20 different local STVGs type geometry modifications were investigated as a means of passive flow control. Modifications were examined on both the airfoil’s upper and lower surfaces. Results showed that the STVGs were able to mitigate the negative effects of shock-induced dynamic stall. The best geometries could reduce the peak negative pitching moment by as much as 9–23% during the transonic phase of a cycle and by as much as 19–71% during the dynamic stall phase. Also, they were able to reduce peak drag by 8–20% in the transonic phase and by 15–44% in the dynamic stall phase. On the other hand, the lift-to-drag ratio was significantly increased by 3–28% per one rotor cycle. All the above advantages came at virtually no penalty in the lift.

Vortex trapping recaptures energy in flying fruit flies

Flapping flight is one of the most costly forms of locomotion in animals. To limit energetic expenditures, flying insects thus developed multiple strategies. An effective mechanism to reduce flight power expenditures is the harvesting of kinetic energy from motion of the surrounding air. We here show an unusual mechanism of energy harvesting in an insect that recaptures the rotational energy of air vortices. The mechanism requires pronounced chordwise wing bending during which the wing surface momentary traps the vortex and transfers its kinetic energy to the wing within less than a millisecond. Numerical and robotic controls show that the decrease in vortex strength is minimal without the nearby wing surface. The measured energy recycling might slightly reduce the power requirements needed for body weight support in flight, lowering the flight costs in animals flying at elevated power demands. An increase in flight efficiency improves flight during aversive manoeuvring in response to predation and long-distance migration, and thus factors that determine the worldwide abundance and distribution of insect populations.

Manipulation of microparticles using combined optical traps

Optothermal traps including vortex ones, have been implemented to capture and manipulate transparent and absorbing micro-objects. The current study deals with latex microspheres of various diameters and silver microparticles. The temperature gradient and corresponding convection fluxes of the liquid were formed by using a light filter absorbing at a wavelength of 0.53 μm as the bottom of the cell with the samples. Special phase masks were designed to form an optothermal trap of the Archimedes spiral shape. We have demonstrated the motion of microparticles along the Archimedes spiral in combined vortex trap caused by the transfer of an angular momentum.

Transport of oil droplets from a jet in crossflow: Dispersion coefficients and Vortex trapping

Understanding the trajectory of oil droplets in crossflow jets is important to estimate the pathways of hydrocarbons and to plan countermeasures. We report experimental results of an oil jet with release velocity around 1.5 m/s in a crossflow of 0.3 m/s. The hydrodynamics of the jet obtained with the Large Eddy Simulation (LES) were used to predict the migration of the oil droplets. Two Lagrangian techniques were explored, one with the inertia of the droplet is considered and the other that treats the droplets as massless particles with rising velocities corresponding to their size. We did not note a large difference between the two approaches. The droplets showed stronger segregation in the vertical direction, which renders the usage of a Gaussian distribution approximation in the vertical inapplicable. The dispersion coefficient at each direction was computed for different-sized droplets. The eddy diffusivity computed based on Boussinesq gradient approximation using the LES data was compared with the dispersion coefficients obtained based on Lagrangian tracking. We also found that droplets 500 μm and larger escape the vortex while smaller ones get trapped within the vortex. A similar outcome was observed using a vortex trapping function based on inward–outward force balancing at the elevation of the vortex core. The counter-rotating vortex pair (CVP) altered the distribution of droplets of 1 mm and smaller significantly, and bimodal concentration distributions with peaks near the CVP vortex cores and minimum concentration near the center plane were obtained in the lateral–horizontal​ direction. Therefore, measurements of the oil droplet size distribution (DSD) in the center plane of crossflow jets could underestimate the number of small droplets in the whole plume.