Leading-edge Vortex Structure Research Articles

Influence of kinematics on the growth of secondary wake structures behind oscillating foils

The wake of an oscillating hydrofoil in combined heaving and pitching motion is numerically evaluated at a range of phase offset (90∘≤ϕ≤270∘) and chord based Strouhal number (0.32 ≤Stc≤ 0.64), while the main analysis is currently reported at Stc=0.32. The Reynolds number is fixed at Re=8000. At ϕ=90∘, a dominant spanwise elliptic instability feature is observed due to the presence of a paired primary and secondary leading edge vortex (LEV) roller over the foil boundary. Subsequent outflux of vorticity from the secondary LEV leads to the formation of streamwise hairpin structures that evolve downstream on account of straining in the braid vorticity region. The spanwise wavelength (λz) of the growing streamwise secondary hairpin legs is approximately 0.86c before the LEV structures shed from the foil. This is followed by a reduction in λz, which coincided with continuous ejection of vorticity from the secondary leading edge structures, and the rearrangement of growing hairpin legs. However, the dominant association between spanwise instability and growth of secondary streamwise structures at ϕ=90∘ reduces with increasing ϕ towards 270∘. At ϕ=180∘, there is a delay in the emergence of dominant secondary hairpin structures that evolve through deformed trailing edge vortex rather than the secondary leading edge structure. Further increase in ϕ to 225∘ and 270∘ depicts the absence of secondary hairpin structures, despite the apparent local instability features on the leading edge rollers.

Analysis of the Effect of the Leading-Edge Vortex Structure on Unsteady Secondary Flow at the Endwall of a High-Lift Low-Pressure Turbine

The horseshoe vortex system at the leading edge of a low-pressure turbine (LPT) has unsteady flow characteristics, and the flow field in the downstream cascade channel also has unsteady characteristics. In this study, CFD simulations are performed with the help of commercial software CFX, and experimental checks are performed using a fan-shaped cascade test bench to investigate the flow characteristics of the endwall of the PACKB blade type of a high-lift LPT under the same incoming Reynolds number and two incoming boundary layer thickness conditions. The obtained results show that there are different flow alteration characteristics and alteration frequencies of the horseshoe vortex system, among which the vortex system with a thicker boundary layer is larger in size and less spaced from each other, which is more likely to induce the fusion of vortex systems. The centrifugal instability causes the instability of the horseshoe vortex system, and the instability frequency is inversely proportional to the thickness of the boundary layer. With two inlet boundary layers, the instability frequencies of the vortex system are 125 Hz and 175 Hz, respectively, and the ratio of the frequency to the thickness of the boundary layer is reciprocal to each other. The stimulation effect of the unstable horseshoe vortex system on the downstream secondary flow intensity is greater than that of the steady state. The thin boundary layer case generates a greater unsteady loss in the cascade channel than the thick boundary layer case due to the poor stability of the vortex system.

The leading-edge vortex over a swift-like high-aspect-ratio wing with nonlinear swept-back geometry

The leading-edge vortex (LEV) is a common flow structure that forms over wings at high angles of attack. Over the years, LEVs were exploited for augmenting the lift of man-made slender delta wings aircraft. However, recent observations suggested that natural flyers with high-aspect-ratio (high-AR) wings, such as the common swift (Apus apus), can also generate LEVs while gliding. We hypothesize that the planform shape and nonlinear sweep (increasing towards the wingtip) enable the formation and control of such LEVs. In this paper, we investigate whether a stationary LEV can form over a nonlinear swept-back high-AR wing inspired by the swift’s wing shape and evaluate its characteristics and potential aerodynamic benefit. Particle image velocimetry (PIV) measurements were performed in a water flume on a high-AR swept-back wing inspired by the swift wing. Experiments were performed at four spanwise sections and a range of angles of attack for a chord-based Reynolds number of . Stationary LEV structures were identified across the wingspan by utilizing the proper orthogonal decomposition (POD) method for angles of attack of 5∘–15∘. The size and circulation of the stationary LEV were found to grow towards the wingtip in a nonlinear manner due to shear layer feeding and spanwise transport of mass and vorticity within the LEV, thus confirming that nonlinear high-AR swept-back wings can generate stationary LEVs. Our results suggest that the common swift can generate stationary LEVs over its swept-back wings to glide slower and at a higher rate of descent, with the LEVs potentially supporting up to 60% of its weight.

An experimental study of the unsteady vortex structures of a clapping-wing micro air vehicle

It is a challenge to explore the unsteady vortex structures of flexible flapping wings of X-shaped flapping-wing micro air vehicles (also known as clapping-wing micro air vehicles (CWMAVs)). The objective of this paper is to obtain the influence of wind speed, flapping frequency, and angle of attack (AoA) on the instantaneous and average force coefficients of CWMAVs, investigate flow visualization of the leading-edge vortex (LEV), trailing-edge vortex (TEV) and wake vortex (WV) structures and identify some novel flow mechanisms of flapping propulsion. This paper proposes a mechanics and particle image velocimetry (PIV) measuring platform in a low-speed tunnel. In addition, cross-correlation peak analysis and kriging image reconstruction are applied for postprocessing to enhance PIV image recognition. Combining the time evolution of the forces, the force coefficients, and the flow visualization, we find the evolution and effects of LEV, TEV and WV structures.

Effect of leading-edge curvature on the aerodynamics of insect wings

The aerodynamic performance of an insect wing is largely dependent on the leading-edge vortex (LEV) on the wing formed during the rotational translation phase of its flapping motion. The geometry of the wing can directly influence the formation and strength of the LEV. The wing geometry is broadly defined by the aspect ratio and the radii of the wing’s moment of inertia, which can be optimised to obtain the highest possible lift at a given Reynolds number. However, the shape of the leading edge can also affect the LEV and the tip vortex structure. A straight leading edge and a sharp wingtip can affect the spanwise vorticity flux responsible for a stable LEV, thereby affecting the lift and drag. To investigate this in the current study, the wing shape is initially approximated by the Beta function with a straight leading edge and is varied systematically by increasing the leading-edge curvature. The lift and power economy are observed to be enhanced with the leading-edge curvature. The reasons behind the lift enhancement are further investigated by analysing the LEV structures over the wings of various leading-edge curvatures. The curved leading edge is observed to enhance lift at both low as well as high Reynolds numbers relevant to insects.

On the stabilization of leading-edge vortices with different reduced frequencies over finite-aspect-ratio pitch-up wings

An implicit large-eddy simulation method is used to numerically simulate the flowfield around a finite-span rectangular wing in pitch motion. The effect of the reduced frequency (k) on the stability of the leading-edge vortex (LEV) is studied. The stability of LEVs is considered to include adhesion and structural stability. The wing is pitched at a position of around 1/4c (chord length), and the flow has a Reynolds number of 9053. Reduced frequencies of 0.2, 0.4, 0.8, and 1.6 are studied. For k = 0.2, the calculated results are in good agreement with experimental measurements, which demonstrates the reliability of the calculation method. The results show that the structural stability of LEVs can be significantly enhanced by increasing k. Larger values of k reduce the spatial scale of the LEVs, as well as prevent them from growing too fast and bursting. Moreover, a larger reduced frequency also delays the decrease in the LEV circulation, allowing the LEV structure to maintain stability at a larger angle of attack (α). In addition, a larger value of k also helps to enhance the adhesion stability of the LEVs. Numerical simulation results show that smaller values of k encourage the formation of secondary vortices. These accelerate the backflow between the LEVs and the wing, thus promoting the upward movement of the LEVs and reducing their adhesion stability. However, analysis of the convection terms in the vortex dynamic equations indicates that lower k is beneficial for the vorticity in the LEV to be transported outwards, but very small k will cause the reverse transport of vorticity, which is harmful to the stability of the LEV.

Effects of the Propeller Advance Ratio on Delta Wing UAV Leading Edge Vortex

Delta wing is a triangular-shaped platform that can be applied into the unmanned aerial vehicle (UAV) or drone applications. However, the flow above the delta wing is governed by complex leading-edge vortex structures which result in complicated aerodynamics behaviour. At higher angles of attack, the vortex burst can take place when the swirling flow is unable to sustain the adverse pressure gradient. More studies are needed to understand these vortex phenomena. This paper addresses an experimental study of active flow control called propeller on a generic 55° swept angle sharp-edged delta wing model. In this experiment, a propeller was placed at two different locations. The first location was at the apex of the wing while the second position was at the rear of the wing. The experiments were conducted in a 1.5 × 2.0 m2 closed-loop wind tunnel facility at Universiti Teknologi Malaysia. The freestream velocities were set at 20 m/s and 25 m/s. The research consisted of an intensive surface pressure measurement above the wing surface to investigate the effects of rotating propeller towards the leading-edge vortex. The experiments were divided into four configurations. The clean wing configuration was performed without the propeller and followed by pusher-propeller configuration using 10-inch 9-inch propellers. The final configuration was the tractor-propeller with a 10-inch propeller. The results emphasise the influences of the propeller size and its location corresponding to vortex properties above the delta-winged UAV model. The findings had indicated that the vortex peak is increased when the propeller is installed for both pusher and tractor configurations. The results also indicate that the pressure coefficient is increased when the propeller advance ratio increases.

The onset of dynamic stall at a high, transitional Reynolds number

Dynamic stall due to a ramp-type pitching motion is investigated on the NACA 0012 airfoil at chord Reynolds number of $Re_{c}=1.0\times 10^{6}$ through the use of wall-resolved large-eddy simulation. Emphasis is placed on the unsteady boundary-layer interactions that develop as the airfoil approaches stall. At this Reynolds number it is shown that turbulent separation moves upstream across much of the airfoil suction surface. When turbulent separation reaches the leading-edge separation bubble, a bursting event is initiated leading to a strong coherent leading-edge vortex structure. This vortex wraps up the turbulent shear layer to form a large dynamic stall vortex. The use of large-eddy simulation elucidates the roll of the laminar separation bubble in defining the onset of the dynamic stall process. Comparisons are made to identical simulations at lower Reynolds numbers of $Re_{c}=0.2\times 10^{6}$ and $0.5\times 10^{6}$. This comparison demonstrates trends in the boundary-layer mechanics that explain the sensitivity of the dynamic stall process to Reynolds number.

Volumetric measurement and vorticity dynamics of leading-edge vortex formation on a revolving wing

Leading-edge vortex (LEV) is a hallmark of insect flight that forms and remains stably attached on high angle-of-attack (AoA), low aspect ratio (AR) wings undergoing revolving or flapping motion. Despite the efforts on explaining the stability of LEV when it reaches steady state in revolving wings, its formation process remains largely underexplored. Here, we investigate the LEV formation on a revolving wing (AoA = 45°, AR = 4 and Re = 1500), starting with a constant acceleration in the first chord (c) length of travel and then rotating at a constant velocity. The ‘Shake-the-box’ (STB) Lagrangian particle tracking velocimetry (PTV) system together with a volumetric patching process were performed to reconstruct the entire time-resolved flow field. Results show that LEV reaches steady state after approximately 4c of travel, within which its formation can be separated into three stages. In the first stage, a conical LEV structure begins to form with a tangential downstream convection of (negative) radial shear vorticity. In the second stage, the radial vorticity within the LEV is further convected downwards by the developed downwash, which limits its growth, whereas vorticity stretching increases the LEV strength. In the third stage, vorticity tilting and spanwise convection reduce the LEV strength at its rear end next to the trailing edge and, therefore, preventing it from growing. Our results suggest that insect wings with AR ~ 4, Re ~ 103 and flapping amplitude between 2c and 4c may barely or even not reach the steady state of LEV, indicating an indispensable role of transient LEV dynamics in understanding insect flight.

Uncoupling the effects of aspect ratio, Reynolds number and Rossby number on a rotating insect-wing planform

The individual and combined influences of aspect ratio ($A$), Reynolds number ($Re$) and Rossby number ($Ro$) on the leading-edge vortex (LEV) of a rotating wing of insect-like planform are investigated numerically. A previous study from our group has determined the wingspan to be an appropriate length scale governing the large-scale LEV structure. In this study, the$A$range considered is further extended, to show that this scaling works well as$A$is varied by a factor of 4 ($1.8\leqslant A\leqslant 7.28$) and over a$Re$range of two orders of magnitude. The present study also extends this scaling for wings with an offset from the rotation axis, which is typically the case for actual insects and often for experiments. Remarkably, the optimum range of$A$based on the lift coefficients at different$Re$coincides with that observed in nature. The scaling based on the wingspan is extended to the acceleration terms of the Navier–Stokes equations, suggesting a modified scaling of$Ro$, which decouples the effects of$A$. A detailed investigation of the flow structures, by increasing$Ro$in a wide range, reveals the weakening of the LEV due to the reduced spanwise flow, resulting in a reduced lift. Overall, the use of span-based scaling of$Re$and$Ro$, together with$A$, may help reconcile apparent conflicting trends between observed variations in aerodynamic performance in different sets of experiments and simulations.

The leading-edge vortex on a rotating wing changes markedly beyond a certain central body size.

Stable attachment of a leading-edge vortex (LEV) plays a key role in generating the high lift on rotating wings with a central body. The central body size can affect the LEV structure broadly in two ways. First, an overall change in the size changes the Reynolds number, which is known to have an influence on the LEV structure. Second, it may affect the Coriolis acceleration acting across the wing, depending on the wing-offset from the axis of rotation. To investigate this, the effects of Reynolds number and the wing-offset are independently studied for a rotating wing. The three-dimensional LEV structure is mapped using a scanning particle image velocimetry technique. The rapid acquisition of images and their correlation are carefully validated. The results presented in this paper show that the LEV structure changes mainly with the Reynolds number. The LEV-split is found to be only minimally affected by changing the central body radius in the range of small offsets, which interestingly includes the range for most insects. However, beyond this small offset range, the LEV-split is found to change dramatically.

Pitching effect on transonic wing stall of a blended flying wing with low aspect ratio

Numerical simulation of the pitching effect on transonic wing stall of a blended flying wing with low aspect ratio was performed using improved delayed detached eddy simulation (IDDES). To capture the discontinuity caused by shock wave, a second-order upwind scheme with Roe’s flux-difference splitting is introduced into the inviscid flux. The artificial dissipation is also turned off in the region where the upwind scheme is applied. To reveal the pitching effect, the implicit approximate-factorization method with sub-iterations and second-order temporal accuracy is employed to avoid the time integration of the unsteady Navier–Stokes equations solved by finite volume method at Arbitrary Lagrange–Euler (ALE) form. The leading edge vortex (LEV) development and LEV circulation of pitch-up wings at a free-stream Mach number M = 0.9 and a Reynolds number Re = [Formula: see text] is studied. The Q-criterion is used to capture the LEV structure from shear layer. The result shows that a shock wave/vortex interaction is responsible for the vortex breakdown which eventually causes the wing stall. The balance of the vortex strength and axial flow, and the shock strength, is examined to provide an explanation of the sensitivity of the breakdown location. Pitching motion has great influence on shock wave and shock wave/vortex interactions, which can significantly affect the vortex breakdown behavior and wing stall onset of low aspect ratio blended flying wing.

Effect of radius of gyration on a wing rotating at low Reynolds number: A computational study

A numerical study of flow structure and lift generation on a simplified rotating wing model caused by increasing the distance from the rotational axis is presented. Deterioration in characteristic leading-edge vortex structure and lift force reduction occur as this distance increases.

Leading Edge Vortex Circulation Development on Finite Aspect Ratio Pitch-Up Wings

We study the leading edge vortex (LEV) development and LEV circulation of pitch-up wings at a low Reynolds number of 500. Wings of different aspect ratios are linearly pitched up from 0 to 90 deg at two reduced pitch rates of 0.1 and 0.2. The flowfield is described by solving the unsteady three-dimensional incompressible Navier–Stokes equations on composite overlapping grids. The -criterion is used to isolate the LEV structure from shear layer. The simulation shows that forces increase with the aspect ratio and the pitch rate. The stall angle increases with the pitch rate but decreases with the aspect ratio. also It is shown that the LEV circulation depends on the wing aspect ratio: increasing wing aspect ratio increases the rate of LEV circulation generation during the pitch-up motion. It also shows that the reduced pitch rate could delay the LEV circulation development. Furthermore, the study shows that the LEV circulation distribution better matches the spanwise force distribution for higher aspect ratio wings.

Computational investigation of cicada aerodynamics in forward flight

Free forward flight of cicadas is investigated through high-speed photogrammetry, three-dimensional surface reconstruction and computational fluid dynamics simulations. We report two new vortices generated by the cicada's wide body. One is the thorax-generated vortex, which helps the downwash flow, indicating a new phenomenon of lift enhancement. Another is the cicada posterior body vortex, which entangles with the vortex ring composed of wing tip, trailing edge and wing root vortices. Some other vortex features include: independently developed left- and right-hand side leading edge vortex (LEV), dual-core LEV structure at the mid-wing region and near-wake two-vortex-ring structure. In the cicada forward flight, approximately 79% of the total lift is generated during the downstroke. Cicada wings experience drag in the downstroke, and generate thrust during the upstroke. Energetics study shows that the cicada in free forward flight consumes much more power in the downstroke than in the upstroke, to provide enough lift to support the weight and to overcome drag to move forward.

Multiple leading edge vortices of unexpected strength in freely flying hawkmoth

The Leading Edge Vortex (LEV) is a universal mechanism enhancing lift in flying organisms. LEVs, generally illustrated as a single vortex attached to the wing throughout the downstroke, have not been studied quantitatively in freely flying insects. Previous findings are either qualitative or from flappers and tethered insects. We measure the flow above the wing of freely flying hawkmoths and find multiple simultaneous LEVs of varying strength and structure along the wingspan. At the inner wing there is a single, attached LEV, while at mid wing there are multiple LEVs, and towards the wingtip flow separates. At mid wing the LEV circulation is ~40% higher than in the wake, implying that the circulation unrelated to the LEV may reduce lift. The strong and complex LEV suggests relatively high flight power in hawmoths. The variable LEV structure may result in variable force production, influencing flight control in the animals.

Relationship between aerodynamic forces, flow structures and wing camber for rotating insect wing planforms

AbstractWing deformation is observed during the flight of some insect species; however, the effect of these distorted wing shapes on the leading edge vortex (LEV) is not well understood. In this study, we investigate the effect of one of these deformation parameters, (rigid) wing camber, on the flow structures and aerodynamic forces for insect-like wings, using a numerical model of an altered fruit fly wing revolving at a constant angular velocity. Both positive and negative camber was investigated at Reynolds numbers of 120 and 1500, along with the chordwise location of maximum camber. It was found that negatively cambered wings produce very similar LEV structures to non-cambered wings at both Reynolds numbers, but high positive camber resulted in the formation of multiple streamwise vortices at the higher Reynolds number, which disrupt the development of the main LEV. Despite this, positively cambered wings were found to produce higher lift to drag ratios than flat or negatively cambered wings. It was determined that a region of low pressure near the wing’s leading edge, combined with the curvature of the wing’s upper surface in this region, resulted in a vertical tilting of the net force vector for positively cambered wings, which explains how insects can benefit from wing camber.

Effects of sinusoidal leading edge on delta wing performance and mechanism

Lift and drag characteristics of delta wings with low swept angle and various sinusoidal leading edges (SLE) were investigated in a wind tunnel. Three amplitudes and three wavelengths of SLE were tested. It is revealed that, in comparison with the baseline case, when the leading-edge amplitude A ≤5% C (root chord length of a delta wing), the stall of the delta wing can be delayed without penalty on the maximum lift coefficient; meanwhile, the lift-to-drag ratio was kept nearly unchanged. These are beneficial to aircraft maneuverability and agility. Surface oil and hydrogen-bubble flow visualization experiments were further conducted to provide a general view of the underlying flow mechanism of SLE on delta wings. It was found that, for the flow over delta wing with SLE, vortices were generated from every crest of SLE, in contrast to the dual leading-edge vortex structure generated from the apex of the base wing. At high angle of attack, the breakdown of those vortices originating from the crests of SLE may provide additional turbulent kinetic energy to the flow, resulting in the increase of the flow reattachment region on the leeward side, therefore the stall can be delayed.

Dual leading-edge vortex structure for flow over a simplified butterfly model

The dye visualization experiments show that a dual leading-edge vortex (LEV) structure exists on the suction side of a simplified butterfly model of Papilio ulysses at α = 8°−12°. Furthermore, the results of particle image velocimetry (PIV) measurement indicate that the axial velocity of the primary (outer) vortex core reaches the lower extreme value while a transition from a “wake-like” to a “jet-like” axial velocity profile occurs. The work reveals for the first time the existence of dual LEV structure on the butterfly-like forward-sweep wing configuration.

Effect of Frontal Gusts on Forward Flapping Flight

The response of a rigid flapping wing in forward flight, at Re = 10,000, subjected to frontal gusts has been investigated. The phasing and duration of the gusts and their impact on the various unsteady mechanisms are analyzed within a single flapping cycle. The gust is characterized by a step function with integral length scale much larger than that of the physical dimension of the micro air vehicle and with time scale much smaller than the flapping time period. The instantaneous lift and thrust profiles were observed to be influenced by a combination of the effective angle of attack, wing rotation, and the leading-edge vortex structures existing in the flow at any given time, with the leading-edge vortices themselves being influenced by the duration and magnitude of the change in effective angle of attack. Frontal gusts applied during the downstroke accelerated the development of the flow, resulting in the formation and detachment of multiple leading-edge vortices on the wing surface that increased the lift and thrust, illustrating the importance of the leading-edge vortex dynamics to force production. The effect of the gust is observed to be diminished when it occurs during rapid supination of the wing. The lift and thrust profiles are found to react in a similar fashion for gusts applied during the downstroke, whereas they experienced opposite effects during the upstroke. During the upstroke, force characteristics are shown to primarily react to effective angle-of-attack changes more than to changes in flow structures.