Department of Zoology,
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Aerodynamics of micro-air vehicles.
This research provides aerodynamic insight for Micro Air Vehicle (MAV) designers. In addition, our experiments address imminent issues in insect flight research:
- Is the lift-enhancing spiral leading edge vortex (LEV), visualized on some hovering insects, a robust mechanism in laminar and turbulent flow?
- How does the strength of the spiral LEV depend on Reynolds number and wing parameters?
- Can the spiral LEV remain stable in forward flight?
A leading edge vortex, or LEV, is a region of high vorticity that can develop along the upper, front surface of a wing. The vortex creates a low pressure region, making the wing a lifting surface. The amount of lift produced depends on the dimensions, intensity (vorticity), and position of the vortex. But what determines these vortex characteristics?
Factors that potentially influence the LEV are:
- Reynolds number
- Wing shape, including aspect ratio, leading and trailing edge geometry and surface geometry
- Kinematics, including changes in angle of attack and wing velocity
Furthermore, interactions between these factors may affect the LEV.
We use a technique called Stereo Particle Image Velocimetry (PIV) to analyze the fluid motion around model wings. Stereo PIV uses a laser light sheet to illuminate particles in a fluid. Two cameras, focused on the light sheet, record still images of the illuminated particles. The images are captured in quick succession, such that particles displacement is barely visible between the two images. The images are analyzed by commercial software (ILA Gmbh.) that graphically reconstructs the flow fields, displaying velocity and vorticity. We can thus quantitatively and qualitatively study the flow around model wings.
Please see diagram of PIV setup below.
(Click for a larger picture)
This photo, taken from the left camera, shows particles illuminated around the wing.
(Click for a larger picture)
From particle displacement over a known time interval, the fluid velocities can be estimated. Here is a 2D velocity vector field at Re=5000. The wing is shown in cross section by the black line, with the leading edge indicated by a circle. The wing is moving from left to right in rotary motion. You can see the LEV above the leading front edge of the wing, indicated by the rotating velocity vectors.
(Click for a larger picture)
To visualise fluid rotation (vorticity), I have colour coded the vorticity magnitude (red=clockwise vorticity, green= no vorticity, blue=clockwise vorticity). You can watch an AVI of the vorticity along the wing surface (at Re=5000) by clicking on the image below. (This is only the component of the vorticity vector along the wing length, ie. parallel to the wingspan).
(AVI file, 1.0 Mb. © Gerda Nolan 2003. May take time to download.)
Funding: EPSRC (Engineering and Physical Sciences
Research Council);
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Last updated July 2004
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