Faculty of Biological Sciences

Modelling Flapping Flight

Faculty of Bio, University of Leeds

Enquiries to Professor Jeremy Rayner

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This project is funded by the BBSRC, and aims to investigate vortex structures in the wake of flying birds, both theoretically and experimentally. This provides valuable information on the forces and moments acting on the wings and to the mechanical energy required in flight, thereby allowing flight performance to be quantified. To visualize and measure the wake structure, experimental and numerical techniques are being developed by Professor Jeremy Rayner and Dr Robert Gordon.

Experimental Work

Currently the experimental work involves flying a Starling (Sturnus vulgaris) through a cloud of neutrally buoyant helium filled soap bubbles. Multiple exposure stereo photographs are obtained using flashguns to illuminate the bubble field. By tracking the bubble images between exposures and applying stereo photogrammetric techniques, it is possible to determine 3-D air flow velocities throughout the wake. Similarly by using a 2-D light sheet, cross-sectional slices of the wake are photographed using a single camera. These images are then analysed to obtain 2-D velocity information throughout a number of wake slices.

Side View Side View
Front View

Front View

Front View

Numerical Work

To complement the flow visualisation work, the large amplitude, unsteady, vortex panel method will be implemented on an INDIGO workstation to investigate the effect of speed, kinematics and morphology on the flight power and forces and moments on birds wings. Work on the pannel method is just beginning and will be discussed more fully at a later date. However, in order to determine the velocity induced on the birds wing by the wake, a wake model has been devloped and animated using 3-D graphics and visualisation techniques. These techniques are also being employed to create a realistic animation of a pigeon (Columba livia) in flight.

Pigeon Model

Morphological measurements were obtained by measuring cross-sections of a pigeons wing and body at 5mm intervals. The sections were digitised and smoothed using an averaging technique. Initially the wings were constructed from ploygons using the OpenGL programming library in conjunction with the C-programming language on an Indigo-2 IMPACT workstation. As the polygon rendering technique used large arrays of co-ordinate data, it was quite inefficient in memory usage. However, this was outweighed by the minimal computation required durring rendering, making this technique suitable for high speed animation within the OpenGL environment. Later models imporved on this by employing NURBS (non-uniform B-splines). This technique was used to construct a more realistic model of the pigeon wings and body.

Image of polygon wings Image of NURB Pigeon Image of shaded Pigeon

Wire Polygon Wings

Wire NURB Pigeon

Solid NURB Pigeon

Pigeon Animation

Previously recorded kinematic data of the pigeon wingtip and wrist during level flight (from Tobalske & Dial, 1996) were used as nodes for B-spline curve fitting. The 3-D constructed curves obtained allow the kinematics at any point in the wing movement to be predicted. The resulting wingbeat sequence for level flight at 8 m/s, can be downloaded in quicktime movie format by clicking on the animated gif below.In this example only selected frames are shown. The original animation contains 300 frames pre wingbeat.

Image of polygon wings Image of NURB Pigeon Wing trace side elevation Quicktime Movie (300K)

Plan View

Front View

Side View

Quicktime Movie

Wake Model and Animations

The wake model is currently under development, and is being implemented with mathematicl modelling techniques. We are using to separate approaches: the first - implemented on an SGI workstation - is intended to generate a realistic looking bird; this work is still in progress. The second - implemented in DOS (with Microsoft Profes sional Basic) - is designed for the development of analytical expressions for the location of the wings and the vortex wake; this phase of the project has now been successfully completed. The theoretical and experimental background to this work has been s et out in papers by Professor Jeremy Rayner: in brief, there are two wake patterns or gaits in flying birds in steady level flight - the vortex ring and the continuous vortex - and these differ primarily in the aerodynamic function of the upstroke. A recent paper reviewing wake patterns is available here in Word97 format. This describes some of our experimental and theoretical results, and includes a selected bibliography.

On the SGI system, because of the extent of the computations required to determine the wake geometry at each stage of the wingbeat cycle, we have chosen to use polygon rendering for both the bird and wake in order to obtain reasonable animation speeds. Two example stills from these animations showing the vortex wake in different forms are given below.

Line Wake Image

Lines

Ribbons

Images from the Basic animation are simpler, and show the bird only as a wire-frame outline (of perhaps questionable accuracy). With relatively simple mathematical algorithms the wing movements mimic those measured or predicted remarkably well, and the vo rtices indicate quite accurately the location of the vortex loops or lines generated by the wing aerofoils. Some example animations in the form of animated .GIF files and .AVI movie files are given below. Click on the image to download the file. These are very much work in progress, and better examples will be posted here as they become available. The examples for Archaeopteryx are based on the body and wing reconstruction of Yalden, with kinematics predicted by aerodynamic modelling of flapping fl ight (after Rayner 1985): the wakes of this animal have not been observed, but it probably only used the continuous vortex gait, and could not generate the vortex ring wake.

Archaeopteryx
vortex ring wake
GIF
AVI
GIF
AVI
Archaeopteryx
continuous vortex wake
GIF
AVI
GIF
AVI
Pigeon
vortex ring wake
GIF
AVI
GIF
AVI
Pigeon
continuous vortex wake
GIF
AVI
GIF
AVI
Kestrel
continuous vortex wake
GIF
AVI
GIF
AVI
Magpie
vortex ring wake
GIF
AVI
GIF
AVI
Gull
continuous vortex wake
GIF
AVI
GIF
AVI
Goshawk
continuous vortex wake
GIF
AVI
GIF
AVI
Eagle
continuous vortex wake
GIF
AVI
GIF
AVI

Click on 'GIF' or 'AVI' to download the appropriate animation. If your Browser is capable of disaplying it, the GIF format is probably preferable. If there is no thumbnail in the list above, then the animation has not yet been created or mounted here: look back soon! All of the animations have been created to a uniform 640*480 size with black background.

These animations represent work in progress, and all the work is copyright to Professor J. M. V. Rayner and the University of Leeds: please feel free to view the files, but do not copy, link or use them without specific permission. Executables of the program and data files which generates these animations will be available shortly.

Faculty of Biological Sciences, University of Leeds, L. C. Miall Building, Clarendon Way, Leeds LS2 9JT, U.K.
Telephone +44 113 233 2873 / 2823, Fax +44 113 133 2835