Different Forms of the Leading Edge Vortex
The leading edge vortex over the insect wings
Insects are capable fliers. However, insect wings are traditionally thought of as too small to provide enough lift to support their flight. According to recent research, the leading edge vortex is said to be stably attached on the wing upper surface from the leading edge to about 3/4 wing chord length [1]. And this leading edge vortex can contribute a lot to the enhancement of the lift and agility [2].
In the left figure, the cross-section of the insect’s wing is coloured with black and the dot represents the leading edge. The airflow (blue line) separates at the leading edge of the wing, circulates and then reattaches to the upper surface. In this process, the leading edge vortex (blue area) is enclosed by the flow streamline over the wing surface. The pressure in this region decreases due to the high circulation speed, which creates a suction force perpendicular to the wing. This leading edge vortex feature can contribute up to 2/3 of the required lift generation for insects [3].
The left figure illustrates the leading edge vortex over the insect wing in 3D. The grey area represents the insect wing and the blue curves
attached at the wing surface are the tracks of the leading edge vortex. The
orange curve and the black rotating arrows are used to indicate the velocity to
which the flow is subjected. The flow in vortex is going wing span wise and at
the same time, rotating against the vortex axis. The
leading edge vortex is stably attached on the wing at high angle of attack and
does not show any unstable signs like shedding [1]. Also, the lift force on the
wing is nearly constant according to Dickinson et al [4], who prove that the leading edge vortex is stable even when the
wings are flapping.
The leading edge vortex over the delta wings
Delta wing is named as its wing planform is similar to the Greek letter delta. A typical example of the delta wings is shown in the left figure.The leading edge vortex is found on such an highly swept delta wing. This leading edge vortex has the similar appearance and characteristic as that on the insect wing. But the difference is that the delta wing is in a fixed state while the insect’s wings are in flapping motion.
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The structure of the leading edge vortex over the delta wing is shown in the left figure. It can be seen that the leading edge vortex over the delta wing is made up of three parts which are vortex sheet, vortex core and secondary vortex [5]. The flow, whose direction is denoted with the symbol V, separates at the leading edge of the delta wing. Then it rolls up to form the primary vortex above the upper side of the wing surface. The second vortex, which is generated from the primary vortex, has much smaller size and is a minor feature. For the primary vortex, the velocity of the flow can be decomposed into axial velocity and spiral velocity. The axial velocity means the flow goes along the vortex core and the spiral velocity means that the flow is rotating around the vortex axis.
By employing the highly swept delta wings, the aircraft can enhance the lift force and delay the angle of stall which allows the improvement of the flying performance [6]. |
The leading edge vortex over the bird wings
The reveal of the leading edge vortex on insect’s wings has changed the understanding of insects’ flight. At the same time, the understanding of birds flight might also be revised as the leading edge vortex is also found on swifts wings. The wing of the swift consists of the arm wing and the hand wing as indicated in the left figure.
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The lift force is generated on both the arm wing and the hand wing. For the arm wing, the lift force is generated in the same way as the conventional aerofoil. For the hand wing, it is thin and has a highly swept feature which is similar to the delta wing. And the leading edge vortex is generated over the leading edge as shown in the left figure [7].With the leading edge vortex, the lift force becomes quite high even at high angle of attack. However, at the high angle of attack, the drag force on the wing is also large and it is believed that the swifts can take advantage of this drag force to improve its agility [8].
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Reference
[1] C.P.Ellington, C.Berg, A.P.Willmott and A.L.R.Thomas. (1996). Leading-edge vortices in insect flight. Nature. 384, p626-630. Online Available: <http://www.nature.com/nature/journal/v384/n6610/pdf/384626a0.pdf>. Last accessed 6th May 2014.
[2] P.S.Sanjay.(2003). The aerodynamics of insect flight. Journal Expenrimental Biology 206, p4191-4208.
[3] C.VandenBerg and C.P.Ellington (1997). The three-dimensional leading edge vortex of a ‘hovering’ model hawkmoth. Phil. Trans. R. Soc. Lond. B352, p329-340.
[4] M.H.Dickinson, F.O.Lehmann and S.P.Sane (1999). Wing rotation and the aerodynamic basis of insect flight. Science 284, p1954-1960.
[5] P.B.Earnshaw. (1962). an Experimental Investigation of the Structure of a Leading-Edge Vortex. Online Available: <http://naca.central.cranfield.ac.uk/reports/arc/rm/3281.pdf?origin=publication_detail>. Last accessed 6th May 2014.
[6] H.Chen, C.Pan and J.Wang. (2013). Effects of sinusoidal leading edge on delta wing performance and mechanism. Science China Technological Sciences. 56 (3), p772-779. Online Available: <http://link.springer.com/article/10.1007%2Fs11431-013-5143-3#page-1>. Last accessed 6th May 2014.
[7] Eastern Kentucky University. (2014). Ornithology Lecture Notes 2-Bird Flight I. Available: <http://people.eku.edu/ritchisong/554notes2.html>. Last accessed 6th May 2014.
[8] J.Videler, E.Stamhuis and G.Povel. (2004). Leading-Edge Vortex Lifts Swifts. Science 306 , p1960-1962. Online Available: <http://www.shahomework.com/mathclass/archives/fall2005/downloads/PrimarySourceArticles/29.pdf>. Last accessed 6th May 2014.
[1] C.P.Ellington, C.Berg, A.P.Willmott and A.L.R.Thomas. (1996). Leading-edge vortices in insect flight. Nature. 384, p626-630. Online Available: <http://www.nature.com/nature/journal/v384/n6610/pdf/384626a0.pdf>. Last accessed 6th May 2014.
[2] P.S.Sanjay.(2003). The aerodynamics of insect flight. Journal Expenrimental Biology 206, p4191-4208.
[3] C.VandenBerg and C.P.Ellington (1997). The three-dimensional leading edge vortex of a ‘hovering’ model hawkmoth. Phil. Trans. R. Soc. Lond. B352, p329-340.
[4] M.H.Dickinson, F.O.Lehmann and S.P.Sane (1999). Wing rotation and the aerodynamic basis of insect flight. Science 284, p1954-1960.
[5] P.B.Earnshaw. (1962). an Experimental Investigation of the Structure of a Leading-Edge Vortex. Online Available: <http://naca.central.cranfield.ac.uk/reports/arc/rm/3281.pdf?origin=publication_detail>. Last accessed 6th May 2014.
[6] H.Chen, C.Pan and J.Wang. (2013). Effects of sinusoidal leading edge on delta wing performance and mechanism. Science China Technological Sciences. 56 (3), p772-779. Online Available: <http://link.springer.com/article/10.1007%2Fs11431-013-5143-3#page-1>. Last accessed 6th May 2014.
[7] Eastern Kentucky University. (2014). Ornithology Lecture Notes 2-Bird Flight I. Available: <http://people.eku.edu/ritchisong/554notes2.html>. Last accessed 6th May 2014.
[8] J.Videler, E.Stamhuis and G.Povel. (2004). Leading-Edge Vortex Lifts Swifts. Science 306 , p1960-1962. Online Available: <http://www.shahomework.com/mathclass/archives/fall2005/downloads/PrimarySourceArticles/29.pdf>. Last accessed 6th May 2014.