Leading Edge Vortices in Insect Flight
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Transcript of Leading Edge Vortices in Insect Flight
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LEADING EDGE VORTICES
IN INSECT FLIGHT
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Introduction
Vortices that are formed on the leading edge of a
flapping insect wing.
Structure, evolution, stability and decay of such
vortices.
Why do we need to study them? Of what
consequence is insect flight to us?
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Flight goes small, really small
Micro Air Vehicles
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Unconventional methods
We have now a need for Micro Air Vehicles
(MAVs), that have a wing span of less than 15cm
and weigh less than 80g.
Conventional fixed wing configurations are just not
efficient enough at the low Re that these MAVs
operate at.
We need to look elsewhere.
Flapping Wings that we see in creatures of all
sizes!
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The best biological analogs for MAVs- Insects
Biological Inspirations
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Insect Flight-An Overview
• Insects are capable of unique flight patterns.
• Sustained hovering, slow flight and precise maneuvering.
• Complicated unsteady, three dimensional flow patterns.
• Difficult to analyze with the conventional laws of aerodynamics.
• They don‟t have streamlined wings.
• They have very high angles of attack, much higher than threshold values for stall.
Source: [2]
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What lets them fly?
The source of lift in insects has now been identified as
the Leading Edge Vortex(LEV).
The high angles of attack causes flow separation at the
leading edge of the wing and generates a vortex.
This vortex creates the low pressure at the top of the
wing that generates the lift.
How stable is this vortex? How does it evolve? What is
the mechanism that supports it?
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The Three Dimensional Leading Edge Vortex of
a „Hovering‟ model Hawkmoth
Coen Van Den Berg And Charles.
P. Ellington (1997)
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Objectives
A 3-D flow visualization experiment using a scaled
up model of the hawkmoth wing- “The Flapper”.
Examined the LEV, its generation, evolution and
decay.
Examined the presence of an axial flow of the
vortex, that caused it to be spiral in shape with an
intrinsic helix angle.
Source: [1]
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The Flapper
•Scaling factor – 9.6
•Forewing and hind wing that
can twist independently.
•Four degrees of freedom-
•Positional angle ϕ,
•Elevation angle θ,
•Angle of attack of the leading
section αle, and angle of attack
of the trailing section αt.
Image source: The three-dimensional leading-edge vortex of
a `hovering' model hawkmoth, Coen Van Den Berg and
Charles P. Ellington, Phil. Trans. R. Soc. Lond. B (1997) 352,
329±340.
Source: [1]
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The experiment
Smoke (vaporized oil) is released from a smoke
rake built into the leading edge of the wing.
Flow cross sections were recorded at four
positional angles ϕ= (50, 30, 0 and -36) o.
For each angle, five span wise positions along the
wing were monitored: 0.25R, 0.50R, 0.63R, 0.75R
and 0.87R.
Source: [1]
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Pictorial Representation of Flapper Action
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Vortex size and position parameters
Image source: The three-dimensional leading-edge vortex of a `hovering' model hawkmoth, Coen
Van Den Berg and Charles P. Ellington, Phil. Trans. R. Soc. Lond. B (1997) 352, 329±340.
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At different positional angles and different
spanwise positions along the wing
Results
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General Observations
A clear LEV is seen with a strong axial flow
component.
During the first half of the stroke, ie., between
ϕ=510 and ϕ=00, the LEV was quite stable over the
major portion of the wingspan.
The LEV grew unstable near the tip of the wing
where it meets a large tip vortex.
A similar smaller LEV is present during the
upstroke as well.
No LEV observed at φ=50o
Source: [1]
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Image source: The three-dimensional leading-edge vortex of a `hovering' model hawkmoth, Coen
Van Den Berg and Charles P. Ellington, Phil. Trans. R. Soc. Lond. B (1997) 352, 329±340.
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At φ=30o
• Clear LEV present with axial flow component from base to tip.
• LEV moved away from the wing surface and chord wise back.
• Height of vortex increased from 1 cm at 0.5R to 3.5cm at 0.63R.
• Vortex was oval at 0.25R (w/h=1.4) and became circular (w/h=1) as it
moved towards the tip.
Image source: The three-dimensional leading-edge vortex of a `hovering' model hawkmoth, Coen Van Den Berg and Charles
P. Ellington, Phil. Trans. R. Soc. Lond. B (1997) 352, 329±340.
Source: [1]
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At φ=0o
•The LEV had grown considerably by this time.
•At 0.25R its size was 1.5cm and at 0.75R it had grown to 7 cm.
•The shape of the LEV again showed the same progression from oval to circular.
•Between 0.25R and 0.63R, the LEV remained close to the surface.
•Separation started at that point and at 0.75R and 0.87R, the vortex moved away
rapidly.
Image source: The three-dimensional leading-edge vortex of a `hovering' model hawkmoth, Coen Van Den Berg and Charles
P. Ellington, Phil. Trans. R. Soc. Lond. B (1997) 352, 329±340.
Source: [1]
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At φ=-36o
•Compared to the LEV at ϕ=0o, the vortex height had decreased by 10-20%
between 0.25R and 0.63R.
•The same change in shape from oval to circular was also visible.
•At 0.75R, the LEV had separated and moved towards the trailing edge and a new
vortex had formed close to the leading edge.
Image source: The three-dimensional leading-edge vortex of a `hovering' model hawkmoth, Coen Van Den Berg and Charles
P. Ellington, Phil. Trans. R. Soc. Lond. B (1997) 352, 329±340.
Source: [1]
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Flow of the LEV from wing base to wing tip
provides stability
Axial Flow Velocity
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Axial flow velocity(Va) variation
Mean value of Va= 31.8 cms-1
Large span-wise variation.
Increased evenly from 0.25R (21.9 cms-1) to 0.50R
(44.2 cms-1).
Pretty much the same between 0.50R and R with a
slight decrease related to vortex breakdown and
separation.
Source: [1]
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Significance of Axial Flow
Axial flow adds an extra flow characteristic that
stabilizes the LEV.
Delays the separation of the LEV due to the
influence of the flow surrounding the wing.
First time that the axial velocity was detected as
this was the first 3D experiment.
Compare to Maxworthy(1979), Spedding and
Maxworthy(1986).
Source: [1]
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How much lift does the LEV produce?
Circulation and Lift
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Circulation of the LEV(C)
C=π.d.Vθ
„d‟ is the average of the vortex height and width.
As helix angle is 45o, we have Va=|Vθ|.
Hence Circulation is calculated for different span-
wise positions.
Source: [1]
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Variation of Circulation
Quadrupled between 0.25R and 0.50R
Remained constant or decreased further down,
owing to vortex separation.
Increased at all span wise positions from φ=30o to
0o.
At φ=-36o, the circulation collapsed at 0.75R owing
to vortex separation.
Source: [1]
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Circulation Vs. Spanwise Position
Image source: The three-dimensional leading-edge vortex of a `hovering' model hawkmoth, Coen Van Den Berg and Charles
P. Ellington, Phil. Trans. R. Soc. Lond. B (1997) 352, 329±340.
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Lift
Sectional lift per unit span calculated using the
value for circulation of LEV, „C‟, gives the lift
contribution of the LEV.
Lift increased by a factor of eight from 0.25R to
0.50R and then stabilized.
Lift similar from φ=0o to φ=30o.
Lift much reduced at φ=-36o.
Source: [1]
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Span-wise variation of Lift
Image source: The three-dimensional leading-edge vortex of a `hovering' model hawkmoth, Coen Van Den Berg and Charles
P. Ellington, Phil. Trans. R. Soc. Lond. B (1997) 352, 329±340.
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Contribution of LEV lift
Oriented perpendicular to the wing surface.
Provides two-thirds of the lift required to lift the
hawkmoth.
This is only the lower limit of the lift!
Added to this, there will be boundary layer
circulation that provides the rest of the lift required
to make the insect hover.
Source: [1]
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Rotational lift mechanisms or Dynamic Stall?
Mechanism of LEV Production
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Mechanism of LEV Production
LEV generated during pronation and recaptured at the start of the downstroke.
Circulation of vortex will be pre established at the start of the downstroke.
Circulation at any spanwise position is proportional to the square of local chord at that point.
LEV generated during downstroke itself.
Circulation will start to grow at the beginning of the downstroke.
Circulation is proportional to the product of the distance from the wing base and the local chord at the point.
Rotational Lift Mechanism Dynamic Stall
Source: [1]
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Conclusion
LEV provides two thirds of the required lift for the hawkmoth.
LEV stability is considerably improved by the axial flow component.
Even a marginal increase in LEV stability should greatly augment the lift coefficient.
An effective strategy for man made MAVs can be to increase LEV stability by increasing this axial flow component by utilizing span-wise blowing or suction.
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References
1. The three-dimensional leading-edge vortex of a `hovering' model hawkmoth, Coen Van Den Berg and
Charles P. Ellington, Phil. Trans. R. Soc. Lond. B (1997) 352, 329±340.
2. Fixed and flapping wing aerodynamics for micro air vehicle applications, Progress in Astronautics and
Aeronautics, Volume 195, Edited by Thomas J.Mueller
3. Title slide image source: http://www.moorhen.me.uk/iodsubject/moths_02.htm:
20080616_d30_20010624_0943_696 elephant hawk-moth in flight with honeysuckle (web
crop)(r+mbid@576).jpg
4. Slide 9 image source: www.wikipedia.org-Manduca sexta adult female taken by Shawn Hanrahan at the
Texas A&M University Insect Collection in College Station, Texas.
5. Slide 10 image source: http://www.redorbit.com/images/pic/29827/hawk-moth-manduca-sexta-image-1/