First published online November 5, 2004
Journal of Experimental Biology 207, 4299-4323 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.01262
Dragonfly flight: free-flight and tethered flow visualizations reveal a diverse array of unsteady lift-generating mechanisms, controlled primarily via angle of attack
Adrian L. R. Thomas*,
Graham K. Taylor,
Robert B. Srygley
,
Robert L. Nudds
and
Richard J. Bomphrey
Department of Zoology, Oxford University, South Parks Road, Oxford,
OX1 3PS, UK
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Fig. 1. Sketches of three solutions to the Navier–Stokes and continuity
equations that lead to local flow separation patterns. These three types of
flow separation are commonly observed in experimental situations. (A) The open
negative bifurcation line consists of a negative bifurcation line from which a
separatrix emerges at the front of the separation. The negative bifurcation
always occurs in a pair with a positive bifurcation line. This kind of
separation is often found when a vortex approaches and impacts with a surface;
it is also involved in the separation over delta wings at moderate angle of
attack when two symmetric negative bifurcation lines form at the leading edges
and a single positive bifurcation line forms down the centreline of the delta.
The negative bifurcation contains no discrete critical points, but the
bifurcations – attachment and separation lines – are formed from a
critical point in a cross flow. (B) The Werlé–Legendre separation
has been studied since the 1960s, and occurs at the base of a dust-devil, or
over a delta wing at high angles of attack. The Werlé–Legendre
separation is a combination of a saddle point, from which a negative
bifurcation line emerges, and a focus. The separatrix arises from the saddle
point and negative bifurcation line. (C) The simple U-shaped separation occurs
in dynamic stall, or in the post-stall flow over a wing. It contains a
free-slip critical point (focus) above the line of symmetry, combined with a
node of attachment, and the separatrix emerges from a saddle-point and the
negative bifurcation (separation) lines that emerge from it at the front of
the separation.
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Fig. 12. Collection of flow visualization images selected to show the process of
identification of critical points. Bifurcations in smoke streaklines are
diagnostic of critical points. Blue arrows point to stagnation points either
where a smoke stream hits the wings or head, or where the flow over the LEV
touches down on the top surface of the wings, or on the body. Yellow arrows
point to the free-slip critical point above the body. Red arrows point to
smoke bifurcation at the saddle point in the wake caused by the shear layer
between the downwash behind the attached LEV and the upwash of the LEV that
was shed from the previous downstroke. (A–C) Flow over the wings at
about half wing length. (D–F) Flow over the midline and interaction with
the wake. (G–H) The free-slip critical point above the midline.
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Fig. 14. Characteristic smoke patterns associated with the forewing downstroke in
normal counterstroking flight. The video images show a tethered hawker
Aeshna grandis; the topological interpretation is the same for all
three species. The critical points in the 3D flow field are denoted by black
spots (N=node; F=focus; S=saddle); dotted lines represent hypothetical surface
streamlines. Visualizations are shown for 5 spanwise stations along the wing
(A–E), marked by colour-coded slices in the figure. The LEV is
continuous with the vortices trailing from the wingtips (A). The LEV diameter
is similar across the wing, and the flow is topologically similar at all three
stations inboard of the wingtip (B–D). The flow over the midline of the
insect clearly shows that the LEV is continuous across the midline (E),
indicating the existence of a free-slip focus above the thorax. The topology
is the same throughout the downstroke: we have chosen those images that show
the downstroke flow structures most clearly for each spanwise station.
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Fig. 2. Free-flight flow visualization of the wake of the dragonfly Sympetrum
sanguineum in counterstroking flight. (A–F) Composite figure of
sequential images extracted from a 250 Hz high speed video recording (video S1
in supplementary material). The dragonfly is moving from left to right through
the smoke plane, which is approximately at the near wing hinge in (A). Wake
structure is incoherent. There is no sign of a starting vortex, but some sort
of vortex structure (stopping vortex? Wingtip vortex in oblique view?) is
apparent in (C–E) (green arrows) and a wake element of sorts can be seen
between the green arrows in (D). This wake element rapidly loses its identity
after it is shed, being hard to detect after two frames (1/125th of a second).
The visualised wake is not consistent with a series of discrete vortex
elements such as, for example, vortex rings.
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Fig. 3. Free-flight flow visualization of the wake of the dragonfly Sympetrum
sanguineum in counterstroking flight with the smoke-plane close to the
right (far) wingtip. (A–J) Consecutive images from a 250 Hz high-speed
video recording. In contrast to the centreline flow shown in
Fig. 2, here the wingtip
vortices are clear (purple arrows), and form wake elements (green arrows) that
persist for several frames. However even here at the wingtips, where the wake
structure is at its most coherent, the wake elements lose their identity after
five frames (A–E, 1/50th of a second). The difference between the
apparent structure of the wake elements between the tip region and the
centreline region suggests that wake elements have a complex structure,
consistent with the lack of any defined starting vortex.
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Fig. 4. Free-flight smoke visualization of the flow around the wings of
Sympetrum sanguineum. There is a leading edge vortex (yellow arrows)
on the fore-wing counterstroking flight. (A–H) Consecutive images from a
250 Hz high-speed video recording. Perpendicular views from the b and c
cameras show that the dragonfly has taken off and cleared the perch and is
holding station, flying sideways in (A) as the forewing completes the
upstroke. The downstroke begins in (B), and the LEV is already present when
the wing cuts the smoke in (C). The structure of the LEV is consistent as the
intersection of the smoke and the wing moves towards the midline (D,E), and
the internal flows within the LEV are clear in (F). The LEV is shed at the
start of the upstroke in (G). There is no evidence of spanwise flow.
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Fig. 8. Free-flight smoke visualization of Aeshna mixta in counterstroking
flight flying with increasing left roll and yaw and consequent side-slip.
There is a leading edge vortex near the midline (above the wing hinge), which
exhibits spanwise flow running from the wingtip towards the centreline.
(A–J) Consecutive images from a 250 Hz high-speed video recording. (A)
shows the end of the upstroke, the dragonfly is aligned with the flow, with
little roll or yaw, and the smoke streams form a vertical plane. In (B) the
dragonfly begins the downstroke and a LEV is formed (yellow arrow), the
dragonfly has also begun to roll and yaw to the left. In (C) the LEV grows,
and the vertical plane of the smoke streams is distorted so that the centre of
the LEV bulges towards the midline at the yellow arrow indicating a spanwise
flow towards the midline. In (E–H) as the yaw increases and the LEV
grows during the downstroke the bulge in the smokestreams caused by spanwise
flow towards the midline also increases. The LEV is still present at the end
of the downstroke in (I) and at the beginning of the upstroke in (J).
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Fig. 9. Free-flight smoke visualization of the flow over the wings of Aeshna
grandis flapping in-phase in level flight, but with a slight yaw to the
left. The process of leading edge vortex formation is visualised, and the LEV
has spanwise flow from the centreline towards the wingtip. (A–J)
Consecutive images from a 250 Hz high-speed video recording. The leading edge
vortex forms over the forewing in image sequence (A–C) at the start of
the downstroke. Although the fore-wing moves upwards between images B and C,
the wing rotates in a nose-down sense about an axis of rotation close to the
mid-chord. This must cause a local increase in angle of attack at the leading
edge, and the separation bubble that develops into the LEV forms during this
phase of motion (yellow arrows). The smoke streams at the centre of the LEV
are distorted in (D), bulging out towards the wingtip, which shows that there
is a spanwise flow from centreline towards the wingtip – the opposite
direction to that seen in Fig.
6. The bulge in the leading edge vortex is still present in (E),
but decreases in (F) and is no longer apparent in (G–J), indicating that
there is no longer a spanwise flow within the leading edge vortex as the wings
approach the end of the downstroke and the LEV expands to cover both fore- and
hindwings. The shear layer (secondary vortices?) within the leading edge
vortex is apparent in (H–J), and the LEV has lifted off from the leading
edge of the forewing in (J) as indicated by the presence of a smoke
bifurcation at the point of the yellow arrow.
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Fig. 5. Free-flight smoke visualization of the flow around the wings of Aeshna
mixta executing a pitch-down manouver. (A–F) Consecutive images
from a 250 Hz high-speed video recording. Rotation of the hindwing at the end
of the downstroke causes a rapid increase in angle of attack, and initially
attached flow over the hindwing separates to form a large leading edge vortex.
In (A) the flow is still attached over the hindwing (yellow arrow), but in
(B), as the wing rotates, increasing angle of attack, the flow separates
(yellow arrow) forming a small separation bubble. This increases in size in
(C), and in (D) the stagnation point where the separatrix touches down on the
top surface of the hindwing is visualised (blue arrow). The LEV continues to
grow as angle of attack increases in (E), and still has not been shed in (F),
at the beginning of the upstroke. There is no evidence of any spanwise
flow.
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Fig. 6. Free-flight smoke visualization of the flow around the wings of Aeshna
mixta executing a roll to the right in counterstroking flight. The flow
field matches that which would be expected with conventional attached-flow
aerodynamics. (A–H) Consecutive images from a 250 Hz high speed video
recording. In (A–C) the wing is completing the upstroke and can be seen
(blue arrows) to have sliced through the smoke streams like a knife –
causing no vertical displacement. This suggests that the sections of the wing
intercepting the smoke plane are generating little or no lift. The wing
rotates in (C) and (D) at the beginning of the downstroke, and the flow
exhibits a downwards deflection indicating lift-generation, but the smoke
streams pass smoothly over the wing with no evidence of flow separation. The
flow remains attached until the end of the downstroke in (H), as the dragonfly
executes a roll to the right.
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Fig. 7. Free-flight smoke visualization of the flow around the wings of
Sympetrum sanguineum accelerating vertically with the wings stroking
in-phase. A leading edge vortex (yellow arrows) forms and grows to extend over
both sets of wings. (A–H) Consecutive images from a 250 Hz high-speed
video recording. The dragonfly is moving from left to right through the smoke
plane and the smoke is approximately 1/4 wing-length in (A) and coincident
with the wing hinge in (H). (A) The end of the upstroke. (B) During the
forewing rotation prior to the downstroke, there is some evidence of the start
of LEV formation. In (C) the LEV is already clearly formed (yellow arrow). In
(D–F) the LEV rapidly grows, the smoke streams within the LEV are
thinned by the increased velocities in that region making it darker, and the
stagnation point where the separatrix touches down moves aft from the forewing
onto the hindwing. In (F–H), as the downstroke ends and the wing
rotates, the LEV is shed into the wake. There is a saddle-point (red arrows)
in the wake where smoke-streams bifurcate in the shear layer between the
current LEV and the wake-vortex representing the LEV shed from the previous
wake. There is no evidence of any spanwise flow.
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Fig. 11. Smoke visualization of tethered dragonflies flapping, but not generating
any lift. (A) The dragonfly Aeshna grandis is flapping, but the
aerodynamic angle of attack is sufficiently close to zero to generate no lift
– as evidenced by the lack of any vertical displacement of the near-wake
(red arrows). The wake shows that the wings have swept a straight path during
the downstroke. In (B) (Aeshna mixta) the wake again shows that the
wings can maintain an angle of attack at, or close to, zero, even when the
forewing sweeps a curved path on the upstroke.
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Fig. 10. (A–E) Smoke visualization of static tethered dragonflies. The
dragonflies are still, and the images represent baseline data showing what the
flow around the dragonflies looks like when they are not flapping. The
successive images step from the right (far) wing hinge across the thorax and
out along the near wing. In (A) the smoke plane is aligned with the far wing
hinge, and smoke flows smoothly past the 5 mm diameter mount below the insect,
becoming incorporated in the Karman street (red arrow) behind the mount far
downstream. The flow over the thorax is attached back to the hinge of the
hindwings, and then separates to form an unstructured wake behind the body. In
(B) the smokeplane is on the midline, and the smoke hits the dragonfly between
the eyes. Below the dragonfly the smoke is entrained into the Karman street
(red arrow) behind the mount support. Smoke streams flowing over the top of
the thorax are attached back to a point behind the forewing hinge, but then
separate as the top surface of the thorax descends towards the abdomen. Flow
above the thorax is essentially linear and undisturbed. Flow behind the thorax
is separated forming an unorganised bluff-body wake. In (C) the smoke
intersects the wing at 1/4 wing length. The wings are stationary, but a Karman
street behind the wings (red arrow), and slight downwards deflection of the
smoke-streams indicates that they are held at some small positive static angle
of attack. The flow below the insect is disturbed by the Karman street behind
the mount support at the far downstream end of the image. In (D) the smoke
intersects the wings at 3/4 wing length. As in (C) the flow over the wings
themselves is attached, but a Karman street (red arrow) behind the trailing
edge shows that the wings are held at some small positive static angle of
attack. The flow is otherwise apparently laminar. (E) Here smoke hits the wing
near the wingtip. The flow pattern remains similar to that seen further
inboard in C and D, with a trailing Karman vortex street (red arrow).
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Fig. 13. Smoke visualizations stepping across the thorax of Aeshna grandis
in tethered flight. The flow pattern, shape, size and structure of the LEV is
consistent at all positions across the thorax, and from wingbeat to wingbeat.
(A–L) Oblique front views in which the dragonfly is traversed through
the smoke plane in 1 mm steps from the far wing hinge across the thorax and
out onto the near wing. There is a leading edge vortex in all images, and the
shape and size of the LEV is consistent across the thorax and out onto the
wing. (I-VI) Higher resolution side images. The dragonfly is traversed through
the smoke plane in 2 mm steps so that the smoke impinges on the far side of
the thorax in I, is on the midline and hits the dragonfly between the eyes in
IV, and is out on the near wing base in VI. The blue arrows show the
stagnation point where the separatrix touches down on the top of the thorax or
hindwing. The shape and size of the leading edge vortex are strikingly
consistent, even though the wing chord and velocity change dramatically as we
step along the wing, across the narrow wing base onto the thorax. This is a
remarkable result, suggesting that while the wings form the LEV the details of
their shape, size and motion are not amongst the principle parameters
controlling LEV morphology.
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Fig. 16. LEV formation and growth in dragonflies. (A–D) Composite sequence of
high-resolution centreline flow visualizations of tethered flight in
Aeshna grandis. At the top of the forewing upstroke (A) the LEV shed
after the previous downstroke is visible behind the wings in the wake (yellow
arrow). There is a smoke bifurcation in the smoke streams behind the LEV (red
arrow). In (B) at the start of the downstroke a LEV has formed between the
forewings (left yellow arrow), and there is a second vortex in the wake (right
yellow arrow), but this has the same sense of rotation as the LEV – as
is clearly demonstrated by the pattern of smoke at the red arrow. Thus this
second vortex is the shed LEV from the previous downstroke –
representing a stopping vortex – and there is no evidence of the
existence of any form of starting vortex. The wings clearly operate in a
region influenced by the upwards flow to the left of the clockwise rotating
shed vortex in the wake. By mid-downstroke (C), the LEV extends over the
entire wing chord, and again there are only two coherent vortex structures
visible (yellow arrows), and they have the same clockwise sense of rotation
(as evidenced by the smoke at the red arrows). The LEV is transferred from
forewing to hindwing at the end of the downstroke (D).
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Fig. 15. LEV formation at the start of the downstroke in Aeshna grandis in
counterstroking tethered flight. Yellow arrows point to the LEV throughout. In
(A), a separation bubble can be seen on the top surface of the wing early in
the phase of rotation (pronation) at the top of the upstroke prior to the
beginning of the downstroke. The separation bubble begins at the leading edge,
and flow reattaches at a point on the top surface between 1/4 and 1/2 of the
way to the trailing edge. In (B) the wing has rotated further and begun to
descend. The separation bubble is larger, with the separatrix reattaching on
the top surface about 3/4 of the way from the leading edge to the trailing
edge. In (C) the LEV has grown to cover the entire top surface of the wing,
and shear is apparent behind the trailing edge between the forwards moving
flow of the LEV and the backwards moving flow that has passed underneath the
wing. The LEV is fully formed in (D).
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© The Company of Biologists Ltd 2004
, as in
real dragonflies. No starting vortex is produced: instead a vortex sheet forms
in the shear layer behind the trailing edge (B–F), and transverse
vortices of circulation opposite to the circulation of the LEV roll up under
Kelvin–Helmholtz instability (C–G). The LEV grows through the
downstroke (B–E) and translates back across the wing chord at the end of
the downstroke (F). The LEV is eventually shed into the wake on the upstroke
(G–J). Video S3 in