On the distribution of leading-edge vortex circulation in samara-like flight

2015 ◽  
Vol 776 ◽  
pp. 316-333 ◽  
Author(s):  
Eric Limacher ◽  
David E. Rival
Keyword(s):  
Shear Layer ◽  
Angle Of Attack ◽  
Leading Edge ◽  
Speed Ratio ◽  
Leading Edge Vortex ◽  
Tip Speed Ratio ◽  
Edge Vortex ◽  
Effective Angle ◽  
Rotating Plates ◽  
Vorticity Flux

As an abstraction of natural samara flight, steadily rotating plates in a free-stream flow have been studied. Particle image velocimetry on span-normal planes has been conducted to show that increasing rotation, as captured by the dimensionless parameter of tip speed ratio, causes a transition of the mean wake topology from that of a bluff body to that of a stable leading-edge vortex. Despite its notable effect on topology, a change in tip speed ratio has negligible effect on leading-edge circulation at a given spanwise position, local effective angle of attack and local effective velocity. The effective angle-of-attack distribution was held constant at different tip speed ratios by comparing rotating plates with different twist profiles. The shear-layer velocity profile at the leading edge was also resolved, allowing quantification of the vorticity flux passing through the leading-edge shear layer. Interestingly, the observed equilibrium values of circulation are not sensitive to changes in shear-layer vorticity flux.

scholarly journals Genetic Algorithm Based Optimization of Wing Rotation in Hover

Fluids ◽  
2018 ◽  
Vol 3 (3) ◽  
pp. 59 ◽  
Author(s):  
Alexander Gehrke ◽  
Guillaume Guyon-Crozier ◽  
Karen Mulleners
Keyword(s):  
Genetic Algorithm ◽  
Lift Coefficient ◽  
Angle Of Attack ◽  
Leading Edge ◽  
Airfoil Surface ◽  
Pitching Motion ◽  
Leading Edge Vortex ◽  
Final Population ◽  
Edge Vortex ◽  
Effective Angle

The pitching kinematics of an experimental hovering flapping wing setup are optimized by means of a genetic algorithm. The pitching kinematics of the setup are parameterized with seven degrees of freedom to allow for complex non-linear and non-harmonic pitching motions. Two optimization objectives are considered. The first objective is maximum stroke average efficiency, and the second objective is maximum stroke average lift. The solutions for both optimization scenarios converge within less than 30 generations based on the evaluation of their fitness. The pitching kinematics of the best individual of the initial and final population closely resemble each other for both optimization scenarios, but the optimal kinematics differ substantially between the two scenarios. The most efficient pitching motion is smoother and closer to a sinusoidal pitching motion, whereas the highest lift-generating pitching motion has sharper edges and is closer to a trapezoidal motion. In both solutions, the rotation or pitching motion is advanced with respect to the sinusoidal stroke motion. Velocity field measurements at selected phases during the flapping motions highlight why the obtained solutions are optimal for the two different optimization objectives. The most efficient pitching motion is characterized by a nearly constant and relatively low effective angle of attack at the start of the half stroke, which supports the formation of a leading edge vortex close to the airfoil surface, which remains bound for most of the half stroke. The highest lift-generating pitching motion has a larger effective angle of attack, which leads to the generation of a stronger leading edge vortex and higher lift coefficient than in the efficiency optimized scenario.

Unsteady force generation and vortex dynamics of pitching and plunging aerofoils

2012 ◽  
Vol 709 ◽  
pp. 37-68 ◽  
Author(s):  
Yeon Sik Baik ◽  
Luis P. Bernal ◽  
Kenneth Granlund ◽  
Michael V. Ol
Keyword(s):  
Strouhal Number ◽  
Vortex Dynamics ◽  
Angle Of Attack ◽  
Leading Edge ◽  
Leading Edge Vortex ◽  
Reduced Frequency ◽  
Unsteady Force ◽  
Edge Vortex ◽  
Effective Angle ◽  
Flow Evolution

AbstractExperimental studies of the flow topology, leading-edge vortex dynamics and unsteady force produced by pitching and plunging flat-plate aerofoils in forward flight at Reynolds numbers in the range 5000–20 000 are described. We consider the effects of varying frequency and plunge amplitude for the same effective angle-of-attack time history. The effective angle-of-attack history is a sinusoidal oscillation in the range $\ensuremath{-} 6$ to $2{2}^{\ensuremath{\circ} } $ with mean of ${8}^{\ensuremath{\circ} } $ and amplitude of $1{4}^{\ensuremath{\circ} } $. The reduced frequency is varied in the range 0.314–1.0 and the Strouhal number range is 0.10–0.48. Results show that for constant effective angle of attack, the flow evolution is independent of Strouhal number, and as the reduced frequency is increased the leading-edge vortex (LEV) separates later in phase during the downstroke. The LEV trajectory, circulation and area are reported. It is shown that the effective angle of attack and reduced frequency determine the flow evolution, and the Strouhal number is the main parameter determining the aerodynamic force acting on the aerofoil. At low Strouhal numbers, the lift coefficient is proportional to the effective angle of attack, indicating the validity of the quasi-steady approximation. Large values of force coefficients (${\ensuremath{\sim} }6$) are measured at high Strouhal number. The measurement results are compared with linear potential flow theory and found to be in reasonable agreement. During the downstroke, when the LEV is present, better agreement is found when the wake effect is ignored for both the lift and drag coefficients.

Instantaneous topology of the unsteady leading-edge vortex at high angle of attack

AIAA Journal ◽  
1993 ◽  
Vol 31 (8) ◽  
pp. 1384-1391 ◽  
Author(s):  
C. Magness ◽  
O. Robinson ◽  
D. Rockwell
Keyword(s):  
Angle Of Attack ◽  
Leading Edge ◽  
High Angle ◽  
High Angle Of Attack ◽  
Leading Edge Vortex ◽  
Edge Vortex

The leading-edge vortex and aerodynamics of insect-based flapping-wing micro air vehicles

2009 ◽  
Vol 113 (1142) ◽  
pp. 253-262 ◽  
Author(s):  
P. C. Wilkins ◽  
K. Knowles
Keyword(s):  
Entire Range ◽  
Angle Of Attack ◽  
Leading Edge ◽  
Micro Air Vehicles ◽  
Reynolds Numbers ◽  
Flapping Wing ◽  
Computational Method ◽  
Leading Edge Vortex ◽  
Air Vehicle ◽  
Edge Vortex

AbstractThe aerodynamics of insect-like flapping are dominated by the production of a large, stable, and lift-enhancing leading-edge vortex (LEV) above the wing. In this paper the phenomenology behind the LEV is explored, the reasons for its stability are investigated, and the effects on the LEV of changing Reynolds number or angle-of-attack are studied. A predominantly-computational method has been used, validated against both existing and new experimental data. It is concluded that the LEV is stable over the entire range of Reynolds numbers investigated here and that changes in angle-of-attack do not affect the LEV’s stability. The primary motivation of the current work is to ascertain whether insect-like flapping can be successfully ‘scaled up’ to produce a flapping-wing micro air vehicle (FMAV) and the results presented here suggest that this should be the case.

Unsteady Lift for Impulsively Started Transonic/Supersonic Flow

2015 ◽  
Author(s):  
Chen-Yuan Bai ◽  
Juan Li ◽  
Zi-Niu Wu
Keyword(s):  
Mach Number ◽  
Small Angle ◽  
Angle Of Attack ◽  
Phase Variation ◽  
Leading Edge ◽  
High Angle ◽  
Leading Edge Vortex ◽  
Three Phase ◽  
Edge Vortex ◽  
Starting Flow

The unsteady lift for incompressible starting flow of a flat plate at high angle of attack involves a repeatable three-phase variation: (a) initial lift drop, (b) a Wagner type lift increase enhanced by leading edge vortex and (c) a lift drop due to a lift-decreasing trailing edge vortex spiral induced by the leading edge vortex convected to the trailing edge. For compressible starting flow at small angle of attack, it is well known that the lift experiences an initial drop due to piston effect and then a Wagner type lift increase enhanced by compressibility. The third phase has not been reported in the past. In this paper we consider subsonic, transonic and supersonic starting flow at high angle of attack. Numerical computation using computational fluid dynamics is used to compute the flow and lift behavior is explained using existing theories. It is found that, when the angle of attack is 20 degrees, we still observe the three-phase lift variation for Mach number below 0.8. The second conclusion is that the lift during the Wagner type increase phase is a decreasing function of the Mach number, in contrast to what we know from piston and indicial function method for small angle of attack. Another important conclusion is, when the Mach number is high enough, say above 0.9, only two-phase variation is observed: (a) initial lift drop and (b) Wagner type lift increase. For supersonic starting flow the Wagner type lift increase is replaced by a linear increase.

scholarly journals Visualization of vortical flows around a rapidly pitching wing and propeller

2017 ◽  
Vol 9 (1) ◽  
pp. 25-43
Author(s):  
Erlong Su ◽  
Ryan Randall ◽  
Lee Wilson ◽  
Sergey Shkarayev
Keyword(s):  
Boundary Layer ◽  
Leading Edge ◽  
Micro Air Vehicles ◽  
Dynamic Stall ◽  
Leading Edge Vortex ◽  
Edge Vortex ◽  
Effective Angle ◽  
Lift And Drag ◽  
Five Axis ◽  
Leading Edge Vortices

This study was conducted to visually investigate flows related to fixed-wing vertical-takeoff-and-landing micro air vehicles, using the smoke-wire technique. In particular, the study examines transition between forward flight and near-hover. The experimental model consists of a rigid Zimmerman wing and a propulsion system with contra-rotating propellers arranged in a tractor configuration. The model was pitched about the wing’s aerodynamic center at approximately constant rates using a five-axis robotic arm. Constant-rate pitching angles spanned 20° to 70°. No-pitching and four pitching-rates were used, along with three propulsive settings. Several observations were made during no-pitching tests. Turbulent wakes behind blades and laminar flow between them produces pulsations in the boundary layer. These pulsations alter the boundary layer from a laminar to turbulent state and back. An increase in lift and drag in the presence of a slipstream is a result of competing effects of the propulsive slipstream: (a) suppression of flow separation and increased velocity over the wing and (b) decrease of the effective angle of attack. Higher nose-up pitching-rates generally lead to greater trailing-edge vortex-shedding frequency. Nose-up pitching without a slipstream can lead to the development of a traditional dynamic-stall leading-edge vortex, delaying stall and increasing wing lift. During nose-up pitching, a slipstream can drive periodically shed leading-edge vortices into a larger vortical-structure that circulates over the upper-surface of a wing in a fashion similar to that of a traditional dynamic-stall leading-edge vortex. At lower nose-up pitching-rates, leading-edge vortices form at lower angles of attacks. As a slipstream strengthens, a few things occur: separation wakes diminish, separation occurs at a higher angle of attacks, and downward flow-deflection increases. Similar effects are observed for nose-up pitching, while nose-down pitching produces the opposite effects.

Three-dimensional vortex structure on a rotating wing

2012 ◽  
Vol 707 ◽  
pp. 541-550 ◽  
Author(s):  
Cem A. Ozen ◽  
D. Rockwell
Keyword(s):  
Three Dimensional ◽  
Leading Edge ◽  
Dimensional Structure ◽  
Radius Of Gyration ◽  
Leading Edge Vortex ◽  
Image Velocimetry ◽  
Edge Vortex ◽  
Vorticity Flux ◽  
Rotating Wing ◽  
Spanwise Flow

AbstractThe three-dimensional structure of the leading-edge vortex on a rotating wing is addressed using a technique of particle image velocimetry. Organized patterns of chordwise-oriented vorticity, which exist within the vortex, arise from the spanwise flow along the surface of the wing, which can attain a velocity the same order as the velocity of the wing at its radius of gyration. These patterns are related to the strength (circulation) and coherence of the tip and root vortices. The associated distributions of spanwise-oriented vorticity along the leading-edge vortex are characterized in relation to the vorticity flux and downwash along the wing.

scholarly journals Leading edge vortex in a slow-flying passerine

2012 ◽  
Vol 8 (4) ◽  
pp. 554-557 ◽  
Author(s):  
Florian T. Muijres ◽  
L. Christoffer Johansson ◽  
Anders Hedenström
Keyword(s):  
Angle Of Attack ◽  
Leading Edge ◽  
Pied Flycatcher ◽  
Passerine Birds ◽  
Leading Edge Vortex ◽  
Weight Support ◽  
Edge Vortex ◽  
Wing Tip ◽  
Leading Edge Vortices

Most hovering animals, such as insects and hummingbirds, enhance lift by producing leading edge vortices (LEVs) and by using both the downstroke and upstroke for lift production. By contrast, most hovering passerine birds primarily use the downstroke to generate lift. To compensate for the nearly inactive upstroke, weight support during the downstroke needs to be relatively higher in passerines when compared with, e.g. hummingbirds. Here we show, by capturing the airflow around the wing of a freely flying pied flycatcher, that passerines may use LEVs during the downstroke to increase lift. The LEV contributes up to 49 per cent to weight support, which is three times higher than in hummingbirds, suggesting that avian hoverers compensate for the nearly inactive upstroke by generating stronger LEVs. Contrary to other animals, the LEV strength in the flycatcher is lowest near the wing tip, instead of highest. This is correlated with a spanwise reduction of the wing's angle-of-attack, partly owing to upward bending of primary feathers. We suggest that this helps to delay bursting and shedding of the particularly strong LEV in passerines.

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