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

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 Leading Edge Blowing to Mimic and Enhance the Serration Effects for Aerofoil

2021 ◽  
Vol 11 (6) ◽  
pp. 2593
Author(s):  
Yasir Al-Okbi ◽  
Tze Pei Chong ◽  
Oksana Stalnov
Keyword(s):  
Boundary Layer ◽  
Angle Of Attack ◽  
Leading Edge ◽  
Boundary Layer Separation ◽  
Shear Instability ◽  
Vortical Flow ◽  
High Angle ◽  
High Angle Of Attack ◽  
Layer Separation ◽  
Aerodynamic Performances

Leading edge serration is now a well-established and effective passive control device for the reduction of turbulence–leading edge interaction noise, and for the suppression of boundary layer separation at high angle of attack. It is envisaged that leading edge blowing could produce the same mechanisms as those produced by a serrated leading edge to enhance the aeroacoustics and aerodynamic performances of aerofoil. Aeroacoustically, injection of mass airflow from the leading edge (against the incoming turbulent flow) can be an effective mechanism to decrease the turbulence intensity, and/or alter the stagnation point. According to classical theory on the aerofoil leading edge noise, there is a potential for the leading edge blowing to reduce the level of turbulence–leading edge interaction noise radiation. Aerodynamically, after the mixing between the injected air and the incoming flow, a shear instability is likely to be triggered owing to the different flow directions. The resulting vortical flow will then propagate along the main flow direction across the aerofoil surface. These vortical flows generated indirectly owing to the leading edge blowing could also be effective to mitigate boundary layer separation at high angle of attack. The objectives of this paper are to validate these hypotheses, and combine the serration and blowing together on the leading edge to harvest further improvement on the aeroacoustics and aerodynamic performances. Results presented in this paper strongly indicate that leading edge blowing, which is an active flow control method, can indeed mimic and even enhance the bio-inspired leading edge serration effectively.

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.

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

2015 ◽  
Vol 12 (105) ◽  
pp. 20150051 ◽  
Author(s):  
Jan W. Kruyt ◽  
GertJan F. van Heijst ◽  
Douglas L. Altshuler ◽  
David Lentink
Keyword(s):  
Aspect Ratio ◽  
High Aspect Ratio ◽  
Angle Of Attack ◽  
Radial Distance ◽  
Leading Edge ◽  
High Angle ◽  
High Angle Of Attack ◽  
Low Aspect Ratio ◽  
Aspect Ratios ◽  
Low Aspect Ratio Wings

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.

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.

On the Vortex Breakdown Phenomenon in High Angle of Attack Flows Over Delta Wing Geometries

2014 ◽  
Author(s):  
Eric D. Robertson ◽  
Varun Chitta ◽  
D. Keith Walters ◽  
Shanti Bhushan
Keyword(s):  
Vortex Breakdown ◽  
Delta Wing ◽  
Angle Of Attack ◽  
Leading Edge ◽  
Navier Stokes ◽  
Detached Eddy Simulation ◽  
High Angle ◽  
High Angle Of Attack ◽  
Eddy Simulation ◽  
Wing Model

Using computational methods, an investigation was performed on the physical mechanisms leading to vortex breakdown in high angle of attack flows over delta wing geometries. For this purpose, the Second International Vortex Flow Experiment (VFE-2) 65° sweep delta wing model was studied at a root chord Reynolds number (Recr) of 6 × 106 at various angles of attack. The open-source computational fluid dynamics (CFD) solver OpenFOAM was used in parallel with the commercial CFD solver ANSYS® FLUENT. For breadth, a variety of classic closure models were applied, including unsteady Reynolds-averaged Navier-Stokes (URANS) and detached eddy simulation (DES). Results for all cases are analyzed and flow features are identified and discussed. The results show the inception of a pair of leading edge vortices originating at the apex for all models used, and a region of steady vortical structures downstream in the URANS results. However, DES results show regions of massively separated helical flow which manifests after vortex breakdown. Analysis of turbulence quantities in the breakdown region gives further insight into the mechanisms leading to such phenomena.

Sign in / Sign up

Export Citation Format

Share Document