Low Aspect Ratio Wings Research Articles

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.

Suction control of flow separation of a low-aspect-ratio wing at a low Reynolds number

Suction-based flow control is explored as one of means of enhancing the performance of a low aspect ratio (AR) wing. Large-eddy simulation (LES) was performed to investigate the flow past a three-dimensional (3D) NACA0012 profile wing tip at a chord-based Reynolds number of 35966, such configurations and flow conditions are typical of those utilized in a small unmanned air system (UAS). The flow separation appears at an angle of attack a = 15°, which is used as the example for our investigation. After validating of our numerical results, the suction distribution was designed and mounted near the wing root, and two different control parameters, suction coefficients and angles, were investigated to assess the benefits of flow control and to determine their effectiveness. Our results show that suction control can improve the lift, and the suction coefficient has larger effect on the aerodynamic performances of the wing than that of the suction angle. The lift enhancement is attributed to the standing vortex near the leading edge and the trajectory of wing tip vortex is also affected during the suction control.

On the lift-optimal aspect ratio of a revolving wing at low Reynolds number.

Lentink & Dickinson (2009 J. Exp. Biol.212, 2705-2719. (doi:10.1242/jeb.022269)) showed that rotational acceleration stabilized the leading-edge vortex on revolving, low aspect ratio (AR) wings and hypothesized that a Rossby number of around 3, which is achieved during each half-stroke for a variety of hovering insects, seeds and birds, represents a convergent high-lift solution across a range of scales in nature. Subsequent work has verified that, in particular, the Coriolis acceleration plays a key role in LEV stabilization. Implicit in these results is that there exists an optimal AR for wings revolving about their root, because it is otherwise unclear why, apart from possible morphological reasons, the convergent solution would not occur for an even lower Rossby number. We perform direct numerical simulations of the flow past revolving wings where we vary the AR and Rossby numbers independently by displacing the wing root from the axis of rotation. We show that the optimal lift coefficient represents a compromise between competing trends with competing time scales where the coefficient of lift increases monotonically with AR, holding Rossby number constant, but decreases monotonically with Rossby number, when holding AR constant. For wings revolving about their root, this favours wings of AR between 3 and 4.

Lift-Generation and Moving-Wall Flow Control Over a Low Aspect Ratio Airfoil

Despite the big interest in both, micro-air vehicles (MAV) and flow-control strategies, only few studies have investigated the flow-control possibilities over low aspect ratio (LAR) wings flying at low Reynolds numbers (Re). The present study verified the LAR thick airfoils' conformity with the nonlinear lift approximation equation. Then, a moving-wall flow control method was designed and tested over an LAR thick airfoil (0.57 aspect ration (AR), NACA0015 shaped) performing at a chord-based Re of 4 × 104. The moving belt control postponed the stall onset by 25 deg and produced a 103% gain in lift without any saturation signs at a control speed ratio of Ub/U = 6. Particle image velocimetry (PIV) measurements confirmed the effectiveness of the moving-wall control strategy on the upper surface flow reattachment. Moreover, other quantities such as the, vortices, and the swirling strength are investigated.

Asymmetric separated flow about a Nikolsky wing with a protrusion

Abstract The flow of an inviscid incompressible fluid about a wing of low aspect ratio with a parabolic planform (a Nikolsky wing) is investigated; a protrusion, whose height grows according to a parabolic law, is mounted on the leeward side of the wing in the symmetry plane. It is shown that for symmetric boundary conditions, along with a symmetric solution, an asymmetric solution also exists. The dependence of the asymmetric solution on the wing geometry is examined. It is shown that critical values of the wing curvature and the height of the protrusion exist, for which the asymmetric solution continuously transitions to the symmetric one. In addition, it is shown that a limit asymmetric solution exists which corresponds to an infinitely large protrusion. The stability of the solutions found is discussed.

Flutter analysis of hybrid metal-composite low aspect ratio trapezoidal wings in supersonic flow

An effective 3D supersonic Mach box approach in combination with non-classical hybrid metal-composite plate theory has been used to investigate flutter boundaries of trapezoidal low aspect ratio wings. The wing structure is composed of two main components including aluminum material (in-board section) and laminated composite material (out-board section). A global Ritz method is used with simple polynomials being employed as the trial functions. The most important objective of the present research is to study the effect of composite to metal proportion of hybrid wing structure on flutter boundaries in low supersonic regime. In addition, the effect of some important geometrical parameters such as sweep angle, taper ratio and aspect ratio on flutter boundaries were studied. The results obtained by present approach for special cases like pure metallic wings and results for high supersonic regime based on piston theory show a good agreement with those obtained by other investigators.

Effects of wing shape, aspect ratio and deviation angle on aerodynamic performance of flapping wings in hover

This numerical study is focused on assessing the effect on the aerodynamic hovering performance of wing shapes defined by the radius of the first moment of the wing area (r1¯) and aspect ratio (AR). In addition, the effect of introducing a deviation angle in the kinematics is examined. The performance of r1¯=0.43, 0.53, and 0.63 wings with AR of 1.5, 2.96, 4.5, and 6.0 is investigated at Reynolds numbers (Re) = 12, 400, and 13 500. The performance trends of the wing shapes have been observed to be independent of Re for both 2-angle and 3-angle kinematics. This is because high suction pressures associated with the leading-edge vortex are predominantly spread in the distal (away from the wing root) and leeward regions (towards the trailing-edge) of high flapping velocities for all the cases. While the deviation angle is detrimental to the production of lift and power economy (PE, defined as the ratio of the mean lift coefficient to the mean aerodynamic power coefficient) at Re = 12 due to strong viscous effects, it improves PE at Re = 400 and 13 500. A high instantaneous angle of attack at the stroke reversal results in high lift peak for 3-angle kinematics but its effect at Re = 400 and 13 500 is attenuated by strong vortical structures on the underside of the wing. Maximum PE is achieved at AR = 2.96, as a low AR wing does not produce enough lift and high AR wings consume more aerodynamic power. Although the lift is maximized using high r1¯ and AR wings, our results show that low r1¯ and high AR wings are best for maximizing PE for a given lift in insects.

Self-adaptive flaps on low aspect ratio wings at low Reynolds numbers

Self-adaptive flap is a bio-inspired passive flow control device which emulates the covert feathers of birds in a most rudimentary form. An exploratory study to ascertain the effectiveness of these self-adaptive flaps in delaying flow separation on low aspect ratio wings of three different planforms were carried out in a low speed wind tunnel at a Reynolds number of 105 based on their root chord. Flap widths between 0.08c and 0.15c; placement location between 0.4x/c and 0.8x/c were investigated. The flapped configurations exhibit good stall margin and higher CLmax in most cases. The chordwise placement location of adaptive flap for better lift enhancement varies with planform. Adaptive flap placed at the maximum span location on varying span planform exhibits better lift characteristics than flap placed at other chordwise location. However, for a constant span planform like rectangle, chordwise location close to trailing edge seems to be optimal for flap placement. No detrimental effects are observed in the drag characteristics of the single flapped configurations of rectangular and inverse Zimmerman planform. However, significant drag increase is observed for the flapped configurations of Zimmerman model and double flapped configurations. The temporal behavior of the adaptive flap is aperiodic. Spectral analysis of the flap oscillation shows multiple dominant frequencies between 10 Hz and 50 Hz for different angles of attack for both rectangular and inverse Zimmerman planform. The peaks occur at same frequencies for most angles of attack irrespective of planform. Skin friction pattern obtained from oil flow visualization experiments shows that the separation bubble which extends the entire chord for clean model at post-stall angles of attack, is contained upstream of the flap which leads to better post-stall lift characteristics.

Experimental and numerical studies of low aspect ratio wing at critical Reynolds number

In this study, the three-dimensional flow behaviors and aerodynamics characteristics of a NACA0003 wing with an aspect ratio (AR) of 1 at Reynolds numbers of 1.0×105 has been investigated both experimentally and numerically. The force measurement and flow visualization (oil flow visualization) through the wind tunnel experiment were applied to verify the reliability of the simulation. The investigation with different angles of attack reveals that flow structure contained three dimensional laminar separation bubble (LSB) and wing-tip vortex. As increasing angle of attack, the area occupied by LSB expands both streamwise and spanwise. The wing-tip vortex has strong impact on the formation of LSB. In addition, the spanwise load distribution has shown that both three dimensional LSB and wing-tip vortex provide additional vortex force.

Lift enhancement through flexibility of plunging wings at low Reynolds numbers

In this paper the combined effect of two mechanisms for lift enhancement at low Reynolds numbers are considered, wing oscillations and wing flexibility. The force, deformation and flow fields of rigid and flexible low aspect ratio (AR=3) and high aspect ratio (AR=6) wings oscillating at a fixed post-stall angle of attack of 15° and amplitude of 15% of chord are measured. The force measurements show that flexibility can increase the time-averaged lift coefficient significantly. For low aspect ratio wings the maximum lift coefficient across all Strouhal numbers was Cl=1.38 for the rigid wing as opposed to Cl=2.77 for the flexible wing. Very similar trends were observed for the high aspect ratio wings. This increase is associated with significant deformation of the wing. The root is sinusoidally plunged with small amplitude but this motion is amplified along the span resulting in a larger tip motion but with a phase lag. The amount it is amplified strongly depends on Strouhal number. A Strouhal number of Src=1.5 was selected for detailed flow field measurements due to it being central to the high-lift region of the flexible wings, producing approximately double the lift of the rigid wing. For this Strouhal number the rigid wings exhibit a Leading Edge Vortex (LEV) ring. This is where the clockwise upper-surface LEV pairs with the counter-clockwise lower-surface LEV to form a vortex ring that self-advects upstream and away from the wing's upper surface. Conversely the deformation of the flexible wings inhibits the formation of the LEV ring. Instead a strong upper-surface LEV forms during the downward motion and convects close to the airfoil upper surface thus explaining the significantly higher lift. These measurements demonstrate the significant gains that can be achieved through the combination of unsteady aerodynamics with flexible structures.

Span morphing using the GNATSpar wing

Rigid wings usually fly at sub-optimal conditions generating unnecessary aerodynamic loses represented in flight time, fuel consumption, and unfavourable operational characteristics. High aspect ratio wings have good range and fuel efficiency, but lack manoeuvrability. On the other hand, low aspect ratio wings fly faster and are more manoeuvrable, but have poor aerodynamic performance. Span morphing technology allows integrating both features in a single wing design and allows continuously adjusting the wingspan to match the instantaneous flight conditions and mission objectives. This paper develops, a novel span morphing concept, the Gear driveN Autonomous Twin Spar (GNATSpar) for a mini-UAV. The GNATSpar can be used to achieve span extensions up to 100% but for demonstration purposes it is used here to achieve span extensions up to 20% to reduce induced drag and increase flight endurance. The GNATSpar is superior to conventional telescopic and articulated structures as it uses the space available in the opposite sides of the wing instead of relying on overlapping structures and bearings. In addition, it has a self-locking actuation mechanism due to the low lead angle of the driving worm gear. Following the preliminary aero-structural sizing of the concept, a physical prototype is developed and tested in the 7′×5′ wind-tunnel at the University of Southampton. Finally, benefits and drawbacks of the design are highlighted and analysed.

Optimum design of a flexible wing for portable aerial vehicle

In this paper, the aerodynamic characteristics of a flexible wing of low aspect ratio designed for a portable aerial vehicle (PAV) at low Reynolds number are analysed. In the analysis, the aeroelastic effect due to the elastic deformation of the flexible wing on the lift-drag ratio has been taken into account by using a bidirectional fluid-structure loose coupling method. Three types of structural layout for the flexible wing with the primary structure made of carbon/epoxy composite covered by nylon fabric skin are designed and compared. The numerical analysis results show that the flexible wing has improved lift-drag ratio and stall characteristics comparing with a rigid wing. A prototype of the optimised wing model has been manufactured for testing purpose. Based on the work, the wing design and manufacture methods for PAV are discussed according to specific design requirements.

A CFD Study on Leading Edge Wing Surface Modification of a Low Aspect Ratio Flying Wing to Improve Lift Performance

Low Aspect Ratio (LAR, aspect ratio < 2) wings exhibit highly nonlinear lift curves. Controlling such LAR Mini Aerial Vehicles (MAVs) at high Angle of Attacks (AOA) and in cross-wind conditions is difficult due to the nonlinear lift. Also, maneuvering these MAVs at high AOA is highly constrained by elevator throw. However, high AOA is a requisite for MAVs to enable slow flight in constrained spaces. Conventional aircraft use devices called vortex generators (or turbulators) that delay the separation of boundary layer by artificially inducing vortices. A similar phenomenon is observed in golf balls, where the surface dimples mimic the above effect. It is also observed that the local vortices aid in additional lift generation. This paper presents a Computational Fluid Dynamics (CFD) study on the effect of dimples over the aerodynamic performance of a LAR flying wing with aspect ratio 1.45 & SELIG4083 cross section. A Modified Inverse Zimmerman planform wing has been considered similar to the MAV (Black Kite) developed by the National Aerospace Laboratories Bangalore (NAL). The 3D model was generated in SolidWorks™ and CFD analysis was performed in SolidWorksTM Flow Simulation. The baseline smooth surface wing was validated with the aerodynamic data of SELIG4083 airfoil from literature. Following it, several dimpled wing configurations on the leading edge were created and analyzed. It was observed that dimples create local vortices that aid in lift generation, increasing the effective flight envelope. The associated drag due to local vortices should be overcome by generating additional thrust at high AOA.

Strong geographical variation in wing aspect ratio of a damselfly, Calopteryx maculata (Odonata: Zygoptera).

Geographical patterns in body size have been described across a wide range of species, leading to the development of a series of fundamental biological rules. However, shape variables are less well-described despite having substantial consequences for organism performance. Wing aspect ratio (AR) has been proposed as a key shape parameter that determines function in flying animals, with high AR corresponding to longer, thinner wings that promote high manoeuvrability, low speed flight, and low AR corresponding to shorter, broader wings that promote high efficiency long distance flight. From this principle it might be predicted that populations living in cooler areas would exhibit low AR wings to compensate for reduced muscle efficiency at lower temperatures. I test this hypothesis using the riverine damselfly, Calopteryx maculata, sampled from 34 sites across its range margin in North America. Nine hundred and seven male specimens were captured from across the 34 sites (mean = 26.7 ± 2.9 SE per site), dissected and measured to quantify the area and length of all four wings. Geometric morphometrics were employed to investigate geographical variation in wing shape. The majority of variation in wing shape involved changes in wing aspect ratio, confirmed independently by geometric morphometrics and wing measurements. There was a strong negative relationship between wing aspect ratio and the maximum temperature of the warmest month which varies from west-east in North America, creating a positive relationship with longitude. This pattern suggests that higher aspect ratio may be associated with areas in which greater flight efficiency is required: regions of lower temperatures during the flight season. I discuss my findings in light of research of the functional ecology of wing shape across vertebrate and invertebrate taxa.

Comparative analysis points to functionally significant variation in wing feather structure among a large and diverse sample of modern birds

Comparative analysis points to functionally significant variation in wing feather structure among a large and diverse sample of modern birds

Shape Shifting Target

This article is a study of morphing aircrafts, which has attracted many research groups around the globe. The unmanned aerial vehicles (UAV) have some attractive features for aeronautic research. An important factor in morphing systems is the scale of the air vehicle on which they will be incorporated. With the development of more accurate analysis tools and advanced smart materials, researchers are once again investigating compliant morphing aircraft to improve aircraft performance. Such an aircraft would have the potential to adapt and optimize their shape to improve flight performance or to achieve multi-objective mission roles. Low aspect ratio wings provide more manoeuvrability and allow for high flight speeds, but at the cost of efficiency. An aircraft can be fast or efficient, but not both. Compliant control surfaces also lack the discontinuities found in hinged mechanisms, and thus have the potential to reduce drag and noise significantly. Compliant structures are promising solutions because of their low weight and maintenance costs. Large-scale morphing on commercial aircraft may not be practical in the near term. But the application of morphing to secondary structures, such as a compliant control surface, is a realistic goal.

Power reduction and the radial limit of stall delay in revolving wings of different aspect ratio.

Airplanes and helicopters use high aspect ratio wings to reduce the power required to fly, but must operate at low angle of attack to prevent flow separation and stall. Animals capable of slow sustained flight, such as hummingbirds, have low aspect ratio wings and flap their wings at high angle of attack without stalling. Instead, they generate an attached vortex along the leading edge of the wing that elevates lift. Previous studies have demonstrated that this vortex and high lift can be reproduced by revolving the animal wing at the same angle of attack. How do flapping and revolving animal wings delay stall and reduce power? It has been hypothesized that stall delay derives from having a short radial distance between the shoulder joint and wing tip, measured in chord lengths. This non-dimensional measure of wing length represents the relative magnitude of inertial forces versus rotational accelerations operating in the boundary layer of revolving and flapping wings. Here we show for a suite of aspect ratios, which represent both animal and aircraft wings, that the attachment of the leading edge vortex on a revolving wing is determined by wing aspect ratio, defined with respect to the centre of revolution. At high angle of attack, the vortex remains attached when the local radius is shorter than four chord lengths and separates outboard on higher aspect ratio wings. This radial stall limit explains why revolving high aspect ratio wings (of helicopters) require less power compared with low aspect ratio wings (of hummingbirds) at low angle of attack and vice versa at high angle of attack.

Illustration of Wing Deformation Effects in Three-Dimensional Flapping Flight

This study numerically investigates the aerodynamic effects of deformation on three-dimensional low aspect ratio wings engaged in hover kinematics. The immersed-boundary finite-volume method is used to solve the Navier–Stokes equations. Deformations are actively prescribed and are represented as deflections about axes that transect the rectangular planform. Two distinct deformation modes associated with tip and root deflection are explored, in addition to a rigid wing counterpart. Variation of the phase between deflection and flapping enables mimicry of a range of wing behavior observed in nature. It is found the introduction of root or tip deflection can increase efficiency by as much as 19.5% or 19.0% respectively. The development of vortical structures about the wing is investigated. It is shown that while the rotational axis of the leading-edge vortex is insensitive to deformation, the shape and orientation of cores of the vortex is modified.

A simplified concept for recovering a UUV to a submarine

The use of unmanned underwater vehicles (UUVs) to enhance the military capabilities of submarines and to reduce their operational risk is being considered by navies around the world. However, a major difficulty with the operations of UUVs is their recovery back to the submarine at the end of the mission. Various schemes have been proposed by a number of organisations working in this field; however, they all have major drawbacks. A simple concept is proposed based on a warp from the submarine to a low aspect ratio wing, similar to an otter board used by fishing trawlers. This approach allows the submarine to recover the UUV by slowly overtaking it. This is done at a sufficiently large transverse distance between the vessels where hydrodynamic interaction between the two is minimal, with the submarine travelling at a speed at which it can be safely controlled.

English

In this paper, we present a new numerical treatment of the lifting surface integral equation (LSIE) in the case of rectangular wing planform. The kernel of this equation possesses a strong singularity in Hadamard sense. First the equation is transformed into one containing weaker singularities, of Cauchy-type, and then the 2D singular integral is discretized by the aid of the Gauss- type quadrature formulae. Thus the problem is reduced to a finite system of linear algebraic equations. The numerical simulation reveals a very good agreement, in terms of jump pressure over the wing and aerodynamic coefficients, with the exact solution in the case of the low aspect ratio wing and also with other numerical solutions.