Contra-rotating Vortices Research Articles

The nonlinear evolution of two surface quasi-geostrophic vortices

We investigate numerically the evolution of a baroclinic vortex in a two-level surface quasi-geostrophic model. The vortex is composed of two circular patches of uniform buoyancy, one located at each level. We vary the vortex radii, the magnitude of buoyancy, and the vertical distance between the two levels. We also study different radial profiles of buoyancy for each vortex. This article considers two main situations: firstly, initially columnar vortices with like-signed buoyancies. These vortices are contra-rotating, are linearly unstable and may break. Secondly, we consider initially tilted vortices with opposite-signed buoyancies, which may align vertically. Numerical experiments show that (1) identical contra-rotating vortices break into hetons when initially perturbed by low azimuthal modes; (2) unstable, vertically asymmetric, contra-rotating vortices can stabilise nonlinearly more often than vertically symmetric ones, and can form quasi-steady baroclinic tripoles; (3) co-rotating vortices can align when the two levels are close to each other vertically, and when the vortices are initially horizontally distant from each other by less than three radii; (4) for initially more distant vortices, two such vortices rotate around the plane center; and(5) in all cases, the vortex boundaries are disturbed by Rossby waves. These results compare favorably to earlier results with internal quasi-geostrophic vortices. Further modelling efforts may extend the present study to fully three-dimensional ocean dynamics.

Numerical investigation on flow characteristics of multi-stream supersonic nozzle with serpentine configuration

The serpentine configuration is utilized in the multi-stream supersonic nozzle of the Adaptive Cycle Engine to improve the stealth performance of next-generation fighters. Compared to single-stream conventional nozzles, the multi-stream supersonic nozzle involves a larger number of design parameters and exhibits a strong interaction between the multiple streams. The serpentine configuration introduces a complex influence on the flow characteristics of the multi-stream supersonic nozzle, which presents significant challenges in designing high-performance nozzles. It is crucial to understand and effectively manage the flow features to optimize the design of multi-stream supersonic nozzles with a serpentine configuration. As a result, the effects of the serpentine configuration and nozzle pressure ratio (NPR) on the flow characteristics of the multi-stream supersonic nozzle were investigated numerically in this paper. The numerical simulation employs the implicit density-based algorithm and Shear Stress Transport k-ω turbulent model of the commercial software ANSYS Fluent. The results show that the shock waves and contra-rotating vortices are generated by the nonuniform main stream and corner vortices from the serpentine configuration when the streams mix, which lead to the decline of the aerodynamic performance compared to the no serpentine case. The mass flow rate of the main stream and tertiary stream reduce by 1.4 % and 4.2 %, respectively, and the thrust performance reduces by 0.35 %. As the NPR rises, the tertiary streams are compressed by the main stream. The mass flow rate of the tertiary streams drop sharply with the contraction of the actual throat area. The flow condition of the tertiary streams convert from the over-expanded condition of convergent-divergent nozzle to the critical condition of convergent nozzle. Due to the enlargement of the actual exit area for the main stream, the maximal thrust performance is at NPR = 7.0 instead of the designed NPR.

Aerodynamic Interaction of Migratory Birds in Gliding Flight

(1) Background: Many studies suggest that migratory bird groups fly in a V-formation to improve their aerodynamic efficiency, the goal being to reduce their energy expenditure to fly longer distances. To further validate this hypothesis, we numerically simulated the aerodynamic interaction of two gliding migratory birds and evaluated the aerodynamic forces as a function of the bird spacing. (2) Methods: Computational Fluid Dynamics (CFD) was used to model the flow pattern in and around the wake of Canada geese flying at an altitude of 1000 m and a speed of 13.9 m/s. (3) Results: The post-processing of the 3D results revealed a complex flow structure composed of two contra-rotating vortices developing at the wing tip. (4) Conclusions: In a plane perpendicular to the main flow direction, we showed that the bird’s wake could be broken down into two distinct zones: the downwash zone and the upwash zone, the latter being used by birds flying in formation to reduce their energy expenditure. The results of our study suggested an optimal wingtip spacing of -26cm to maximize the lift/drag ratio that characterizes aerodynamic efficiency.

Modeling of Three-dimensional Unsteady Wake Past a Large Migratory Bird during Flapping Flight

This preliminary study aimed to model the aerodynamic behavior of a large migratory bird during a forward flapping flight. Computational Fluid Dynamics (CFD) was used to model the flow around and in the wake of a Canada Goose flying at an altitude of 1000m and a speed of 13.9m/sec. Flapping of the wings was modeled through dynamic meshing and subroutines implemented in a computational code using the Finite Volumes method. Monitoring of the flow quantities during the unsteady calculation revealed a close relationship between the wing-flapping dynamics and the cyclic variation of the forces acting on the bird. Post-processing of the 3D results revealed a complex flow pattern mainly composed of two contra-rotating vortices developing at the wingtip. In a perpendicular plane to the main flow direction, we demonstrated that the bird's wake can be divided into two distinct zones: the downwash zone and the upwash zone. The latter is used by birds flying in formation to reduce their energy expenditure. We have also shown that when the bird flaps its wings, the trail of upwash left by the wingtips moves up and down in a wave-like motion. Further studies, which will include several birds, will be necessary to understand all the aerodynamic implications related to the flight of migratory birds in formation.

Experimental investigation on convective heat transfer enhancement by EHD in wire to plate configuration

The purpose of this study was to experimentally investigate the enhancement of forced convection heat transfer by corona wind in a channel scaled to potential future drying systems. An experimental rig was designed to investigate the thermal and dynamic effects of electro-hydrodynamic (EHD) induced air flow on the grounded bottom wall. The primary air flow, blown into the channel at controlled temperature and velocity, was disturbed by EHD secondary flow generated by a wire electrode. Using IR imaging, the heat transfer exchange at the wall was calculated in several configurations (i.e. electrodes location, wire-electrode to plate distance, applied voltage, primary air flow velocity), allowing the selection of optimal configurations according to the objective of minimum energy consumption or homogeneous transfer. Particle Image Velocimetry (PIV) characterization conducted on some configurations highlighted the vortices generated by the combination of the secondary and the primary air flows. Analysis of the flow patterns allowed to explain the enhancement of the local and average heat transfer coefficients. Higher enhancement could be obtained in the cross-wire configuration but led to heterogeneous treatment. For a more homogeneous one, the longitudinal wire configuration should be preferred thanks to the development of contra-rotating vortices moving in the channel.

Peak Flow in Model Aorta Through Bi-Leaflet Mechanical Heart Valve with Varying Orientation.

This study aims to numerically investigate the effect of bi-leaflet mechanical heart valve (BMHV) orientation on flow pattern in a realistic human aorta model with branches. The aorta model geometry is based on anatomical shape and dimensions involving aortic arch with twist and branches. Unsteady numerical simulations have been carried out to investigate the peak systolic flow in aorta through a BMHV installed at three different orientations, marked as 0°, 45°, and 90°. Velocity, vorticity, and strain fields were obtained in various cross sectional planes for all the cases to examine the spatial flow evolution starting from the sinus along the aortic arch. The valve leaflets were seen to produce small-scale stream wise contra-rotating vortices. These vortices changed their positions around the axis of aorta while advecting from one cross-sectional plane to another; thereby, clearly indicating swirl in the aortic flow. The net viscous dissipation energy loss (EL), obtained from strain field, was found to rise because of the presence of BMHV. The increase in the EL varied depending on the valve orientation as it changed from 43% for 0° to a maximum of 53% for 90° compared with no valve case. Similarly, the wall shear stress registered an increase by up to 4 Pa in the ascending aorta because the presence of BMHV; however, no significant effect of the valve orientation was noticed.

Vortex bursting near a free surface

When the cross-section is reduced, a vortex displays spiralling and elasticity, bursting when the external velocity drops. How are these properties affected near a free surface? To answer, a visualization experiment in water is considered where an oscillating obstruction is pitched orthogonally to a rolling oscillation expending minimum energy. We show that short aerated vortices in adverse pressure gradient and shear remain stable during stretching, bursting into bubbles when relaxed after maximum stretching. A vertical aerated double helix (DH) root vortex of contra-rotating vortex tubes is produced near a Rossby number of 0.20. The vortex approaches an inclination of to the vertical, where stretching intensifies the vorticity to the maximum extent. Vortex inclinations up to are stable and unstable thereafter. Vortex bursting commences only when the inclination crosses by the slightest amount. When relaxed, oscillations are produced, breaking the vortex into arrays of bubbles, sometimes precisely at inclination. The bubble diameter, modelled by equating the effects of the centrifugal force (CF) on the vortex core pressure, to surface tension scales with the inverse of the square of the rotational velocity. When a high CF is withdrawn, the bursting DH aerated vortex cone is crushed due to surface tension. A root vortex and a coiled-up trailing edge vortex together form a compact Hill’s vortex when the highest CF is relieved, bursting into bubbles spectacularly scattered in the unstable sector. The DH vortex breakup is the connection point of vortices in proximity. At high CF, the DH has stacks of Taylor air tubes at to the external flow bursting when relaxed. Kelvin waves abound when bursting.

Aerodynamic impact of a streamwise vortex on a propeller

In some situations, a vortex is involved in the inflow of a propeller, e.g. a ground vortex or a vortex shed from the upstream parts of the airframe of the aircraft. In order to study the impact of the vortex on the propeller, the Batchelor vortex is utilised to model the impinging vortex, and its effect on the propeller is calculated by Computational Fluid Dynamics. The loading on the propeller increases due to the impingement of a contra-rotating vortex, and the opposite is true for a co-rotating vortex. However, the torque coefficient of the propeller changes consistently with the thrust coefficient which leads to the efficiency of the propeller being independent of the impinging vortex. With the vortex impinging on the propeller, a strong pressure fluctuation on the blade is observed, especially on the leading edge of the blade. This cyclic pressure also results in a periodic bending moment on the blade, which may have an impact on the structural fatigue of the blade.

Vortices' Characteristics to Explain the Flange Height Effects on the Aerodynamic Performances of a Diffuser Augmented Wind Turbine

Flange height is between the geometric features that contribute efficiently to improve the diffuser aerodynamic performances. Results obtained from wind tunnel experiments, particle image velocimetry (PIV) measurements, and numerical simulations reveal that at the diffuser inlet section, the wind velocity increases as the flange height increases. Nevertheless, there is an optimal ratio (flange height/inlet section diameter, Hopt/Da ≈ 0.15) beyond it, the flange height effect on the velocity increase diminishes. This behavior can be explained by both the positions of the two contra-rotating vortices generated downstream of the diffuser and the pressure coefficient at their centers. Indeed, it was found that, as the flange height increases, the two vortices move away from each other in the flow direction and since the flange height exceeds (Hopt/Da), they became too distant from each other and from the flange. While the pressure coefficients at the vortices' centers increase with (H/Da), attain a maximum when (Hopt/Da) is reached, and then decrease. This suggests that the wind velocity increase depends on the pressure coefficient at the vortices' centers. Therefore, it depends on the vortices' locations which are in turn controlled by the flange height. In practice, this means that the diffuser could be more efficient if equipped with a control system able to hold the vortices too near from the flange.

Magnetohydrodynamic flows and heat transfer in a twin-channel induction heating tundish

Induction heating (IH) electromagnetic field has a significant positive effect on the uniform temperature distribution in tundishes and decreases molten steel superheat to improve the quality of billets in continuous casting. To achieve the effect of electromagnetic force and Joule heating for molten steel in a channel-type IH tundish, a magnetohydrodynamic model is developed and validated by industrial experiment data. The molten steel in the tundish channel has a rotating velocity under the off-centre electromagnetic force. Its flow pattern consists of two contra-rotating vortices close to the outlet zone of the channels. The upper and lower vortices in the tundish channel change with time. The temperature distribution in the cross-sections is almost stable, and the maximum temperature increases by 30.1°C. Molten steel runs upward after flowing past the tundish channel, whose density decreases because of IH. The upward motion changes when the electromagnetic force acts on the molten steel. Results reveal that electromagnetic force has a significant effect on the flow of molten steel in the tundish, and that flow pattern varies in the tundish with or without electromagnetic force under Joule heating.

Propeller and inflow vortex interaction: vortex response and impact on the propeller performance

The aerodynamic operating conditions of a propeller can include complex situations where vorticity from sources upstream can enter the propeller plane. In general, when the vorticity enters in a concentrated form of a vortex, the interaction between the vortex and blade is referred to as blade–vortex interaction or BVI. The interaction may affect the propeller performance as well as its noise production. In the present paper, investigations of the interaction of a wing tip vortex generated by a lifting surface upstream of the rotor plane and an eight-bladed propeller are reported. Utilizing two ends of an upstream wing with non-symmetrical airfoil, the rotation of the incoming vortex could be made to co-rotate or to contra-rotate with the propeller. The ensuing velocity fields were quantified with the help of particle image velocimetry (PIV), and the propeller performance was evaluated with the help of a rotating shaft balance (RSB) mounted on the propeller shaft. The results describe the displacement of the vortex core, as it moves through the rotor plane as well as the positive effect on the thrust and torque of the contra-rotating vortex and the opposite of it in the case of the co-rotating vortex. The current research could be applied to analyse the influence of the incoming vortex on the propeller, e.g., ground vortex, tip vortex shed from a control surface, etc.

Measurement of fluid velocity development behind a circular cylinder using particle image velocimetry (PIV)

In this paper we present a non-intrusive experimental approach for obtaining a two-dimensional velocity distribution around a 22 mm diameter circular cylinder mounted in a water tunnel. Measurements were performed for a constant Reynolds number of 7670 using a commercial standard particle image velocimetry (PIV) system. Different flow patterns generated behind the circular cylinder are discussed. Both instantaneous and time-averaged velocity distributions with corresponding streamlines are obtained. Key concepts in fluid mechanics, such as contra-rotating vortices, von Kármán vortex street, and laminar-turbulent flow, are discussed. In addition, brief historical information pertaining to the development of flow measurement techniques—in particular, PIV—is described.

Dynamics of secondary flow behind backward-facing step in a narrow channel

Dynamics of the flow-field behind a backward-facing step in a narrow channel is studied experimentally using time resolved PIV technique. Secondary flow represented by vortical structures is studied using Oscillation Pattern decomposition technique. The low-frequency quasi-periodical structures appearing in the region just behind the step close to the channel bottom are studied. Typical dynamics of vortex structures involving contra-rotating vortex pair train, vortices coalescence and splitting have been observed.

Interaction of dipole eddies with the western continental slope of the Mozambique Channel

Sea Level Anomaly (SLA) data were used to track a southward propagating eddy dipole along the western slope of the Mozambique Channel over some 6 months. In April 2005, this dipole (with the cyclone to the south) was close to the continental slope off southern Mozambique. The contact zone between the contra-rotating vortices and the slope was surveyed by ship using onboard (S-)ADCP and CTD lines. The data showed strong (>1.4ms−1) southward (geostrophic) currents over the slope adjacent to the anticyclone with horizontal divergence over the shelf edge. Significant slope upwelling between the dipole and the shelf was evident, concomitant with enhanced nutrient and chlorophyll levels enriching shelf near-surface waters. Satellite observations depicted a 300km long surface chlorophyll filament extending offshore in the frontal zone between the contra-rotating vortices. A satellite-tracked drifter deployed at the coastal base of this filament confirmed the offshore advection of chlorophyll-enriched shelf water, which ultimately wrapped around the cyclone and filling its centre. The slope upwelling was also clearly evident in hourly temperature data collected by a recorder deployed on a nearby reef (Zambia Reef) in a depth of 18m. According to the SLA data, the dipole took several weeks to pass Zambia Reef causing prolonged bouts of upwelling that finally ceased when it left the continental slope and moved southwards into the open ocean. Further analysis showed that lone anticyclones and cyclones against the Mozambique continental shelf also induce slope upwelling as a result of horizontal divergence created by the radial circulation of the vortex. In the case of cyclones, the divergence occurs north of the contact zone. Overall, this case study confirms that eddies moving southwards along the western side of the Mozambique Channel are the main mechanism for pumping nutrients into the otherwise oligotrophic surface waters, and moreover, provide a vigorous mechanism for shelf–open ocean exchange.

Numerical investigation on the primary breakup of an inelastic non-Newtonian liquid jet with inflow turbulence

Direct Numerical Simulations of the primary breakup of an inelastic non-Newtonian liquid jet with inflow turbulence are presented in this paper. The jet's structure, surface behavior, non-Newtonian characteristics as well as its specific breakup mechanism are investigated and discussed. The shear thinning viscosity of the liquid phase plays an important role during jet injection resulting in circumferential rotation of interfacial waves. Streamwise contra-rotating vortex pairs as well as triple vortex structures are observed in the liquid phase. The local Ohnesorge number, which has a branch-structure distribution in the liquid phase before disintegration, is found to be 30% smaller in regions near the nozzle exit and in the shear layer than in the jet tip, suggesting a clear non-Newtonian influence. A cavity breakup mechanism for this type of non-Newtonian jet is identified and explained, giving a new perspective for jet disintegration analysis.

Three-dimensional flow effects on forced convection heat transfer in a channel with stepwise-varying width

A numerical investigation based on the finite volume methodology of the three-dimensional laminar flow and conjugate heat transfer inside a plate–fin heat sink with stepwise-varying channel width is presented. Results are obtained for the three-dimensional flow developed in the vicinity of the flow contraction as manifested by the existence of two pairs of contra-rotating longitudinal vortices, one at the corners of the fin leading edge and the other at the flow separated regions. The topology of the flow is thoroughly examined and the contributions of the endwalls and the front surface of the flow-contracting fin to the mechanisms that cause the emergence of the three-dimensionality are determined. It is established that the corner longitudinal vortices are in fact horseshoe vortices, which emerge due to the deceleration effect of the fin front surface on the oncoming fluid. Furthermore, the interaction between the corner longitudinal vortices and the downstream recirculation bubble formed over the fin tip is also elucidated. The analysis covers a wide range of flow conditions within the laminar region in order to monitor the effect of the Reynolds number on the flow topology and to verify that the flow remains symmetrical in this region. From the numerical results, it is deduced that the recirculation downstream of the flow contraction has a beneficial impact on the heat sink thermal performance, a fact quantified through the local Nusselt number distributions.

On the dependence of an empty flanged diffuser performance on flange height: Numerical simulations and PIV visualizations

Flanged diffuser shrouding small wind turbine, is among the most tested devices for increasing wind energy. The height of the flange is between the geometric futures of the diffuser that contributes efficiently in improving diffuser performances. Results obtained from numerical simulations and PIV visualizations show that when a flange is mounted at the outlet area of the diffuser, two contra-rotating vortices were created at this location. These two vortices move away from each other in the flow direction as the flange height increases and they seem to lengthen in the streamwise direction and to extend in the two directions when the flange height becomes taller. A critical ratio (Flange height/Inlet section diffuser diameter = 0.1) has been found. Beyond this value, due to the remoteness of vortices from the flange, the flange height seems to be without significant effect on increasing wind velocity.

Piezoelectric and electromagnetic solutions aimed at realizing an active Gurney flap

Gurney flaps are small devices whose dimensions are usually percentage units of the airfoil chord. A Gurney flap can change the flow field in the region of the trailing edge by introducing two contra-rotating vortices behind the flap, therein altering Kutta’s condition and circulation. This helps avoiding flow separation and moves stall occurrence to higher angles of attack. Active rotor technologies would, for instance, enable a helicopter to operate at reduced rotor tip speed while preserving flight performance capabilities. This article shows some results carried out on different, original, and feasible solutions, aimed at realizing “smart” Gurney flaps for rotorcraft blades. In particular, piezo patches, piezo stacks, and electromagnet-based solutions were exploited finding that only the last two solutions are able to satisfy specs. The last one (electromagnets) performs better from the viewpoints of implementation, cost (one order of magnitude), weight (less than 100 g with respect to 150 g of piezo stack), and power consumption (5 W with respect to 50 W).

Large Eddy Simulation of a Controlled Diffusion Compressor Cascade

In this research a Controlled Diffusion (CD) compressor cascade stator blade is simulated at a Reynolds number of ∼700,000, based on inflow velocity and chord length, using Large Eddy Simulation (LES). A wide range of flow inlet angles are computed, including conditions near the design angle, and at high negative and positive incidence. At all inlet angles the surface pressure distributions are well-predicted by the LES. Near the design angle the computed suction side boundary layer thickness agrees well with experimental data, whilst the pressure side boundary layer is poorly predicted due to the inability of LES to capture natural boundary layer transition on the present grid. A good estimation of the loss is computed near the design angle, whilst at both high positive and negative incidences the loss is less well predicted owing to discrepancies between the computed and experimental boundary layer thickness. At incidences above the design angle a laminar separation bubble forms near the leading edge of the suction surface, which undergoes a transition to turbulence. Similar behaviour is noted on the pressure surface at negative incidence. At high negative incidence contra-rotating vortex pairs are found to form around the leading edge in response to an unsteady stagnation line across the span of the blade. Such structures are not apparent in time-averaged statistical data due to their highly-transient nature.

Development of an enhanced MHD micromixer based on axial flow modulation

A magneto-hydrodynamic (MHD) stirrer was analytically modeled, designed and experimentally tested. A novel modulation technique is presented which allows enhancing the mixing quality in a short amount of time. The stirrer was realized with two PCB layers and a glass cover; the channel presents electrodes posed on the bottom wall and on the sidewalls. All the electrodes are AC fed in order to avoid electrolysis and bubble formation during the stirring process. A fully programmable circuit allows creating vortices inside the mixing channel and to move the fluids with an oscillating motion from inlet to outlet; the electrodes on the bottom wall provide contra-rotating vortices and are fed with AC zero mean value square waves in-phase and in opposition of phase with respect to a magnetic field generated by an electromagnet. The sidewalls are fed by a modulated signal whose carrier is in phase with the magnetic field, while the modulant is a low frequency square wave with programmable frequency, amplitude, DC offset and duty-cycle; as an effect it is possible to make oscillate the fluid from inlet to outlet and enhance the stirring process by interaction of this axial oscillation with the contra-rotating vortices. Experimental efficiency of 90% can be reached in an amount of time of 24 s.