DPIV - Digital Particle Image Velicometry - |
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| Fog particles suspended in the airflow inside the test section are illuminated by laser light. | Geoff in action, setting up a test with an airfoil of the dimensions of a bird wing. | |
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Flapping wings of a flying animal transfer momentum to the wake, which organises itself as vortex structures. The geometry and energy of these vortices are fundamental properties to the aerodynamic characteristics. The aeroynamic forces can, in pronciple, be deduced from knowledge of the vortex structures and how they change with forward flight speed. Our lab has developed a DPIV technique for the visualisation of wake vortices for birds flying in he wind tunnel. This requires seeding of the air by small (1 microm) boyant particles (in relaity a thin fog), illumination of theses particles in a thin and very bright light sheet generated by a double-pulsed YAG laser see Fig), and finally the capturing of two exposures on digital CCD-arrays. Pairs of images are captured at a rate of 10 Hz, allowing sampling of all phases of a wingbeat because the flapping frequency is always out of phase with the laser repetition frequency. From processing of these image pairs using the procedure correlational image vlocimetry, a flow map is generated. The data also give vorticity from which circulation can be derived for local vortices. A grid of IR-sensitive diodes separates the bird from the laser sheet, and disrupt the if the bird accidentally drifts to far back in the wind tunnel test section. This set-up has recently been used to generate data accross natural flight speeds for live birds, such as thrush nightingale, house martin and robin.
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| Sketch of the flow visualisation set-up in the wind tunnel test section. The bird is flying one meter upstream of the camera field of view. As a safety precaution to the bird a grid of infrared LED-photodiode pairs interrupts the laser flashes when intersected by the bird. Seed particles are imaged in the light sheet onto two digital cameras. In the analysing procedure the background flow is removed and the reference frame of the data (left inset) is thus in still air, as if the bird were passing through it from right to left. | ||
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| Geoff and Anders are aligning the two laser beams. | Careful calibration of cameras with "Anders' grid", it took him a long time to prepare it. | |
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