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Flow visualisation

For flow visualisations we use Particle Image Velocimetry (PIV) which is a method to both qualitatively and quantitatively measure fluid motions. We use various configurations of this method to study the aerodynamics of animal flight.

Flying animals generate flight forces by transferring momentum to the wake. The resulting induced air velocities can be measured using a PIV-system and based on these results the aerodynamic forces can be deduced and flight performance and efficiency can be estimated.

We installed our first generation PIV-system in 2000, which was a 10Hz single camera system that was used to study the wake dynamics of several different bird species and of one bat species.

Currently, we run two systems in parallel:

Our latest and main system, installed in 2013, is a tomographic PIV-system, which allows us to measure three-dimensional flow inside a volume at 1000Hz. It uses a Litron LDY304 PIV laser (Nd:YLF, 527 nm) and four Imager pro HS4M cameras (2016x2016 pixels). Due to the high demand for light in a volumetric setup the field of view is somewhat limited. Therefore, currently the tomographic system is mainly used to study insect flight, but also that of smaller bats and birds.

In addition to the tomographic system we use a 200Hz stereoscopic system when a larger field of view is needed. This was installed 2008 and uses a Litron LPY732 laser (Nd:YAG, 532 nm) and two HighSpeedStar3 cameras (1024x1024 pixels). 

Particle Image Velocimetry principle explained
Schematic of the PIV method. The high-speed camera(s) take two successive pictures of the fog particles that are illuminated by the laser. From these images, a correlation routine is used to calculate velocity vectors.
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The laser light shining in the PIV setup
The PIV laser lighting up fog particles in the wind tunnel test section.