One of my favorite things about spring and summer is lying in an open field watching the clouds go by. Every once and a while a plane will go by leaving an enormous wake of what looks like white clouds behind it. Only recently did I learn that these cloud trails are caused by vortexes of wind as it goes over the wings of the plane and creates lift. All things that fly have these vortexes in their wake, and depending on the flight pattern of the subject flying, the wake will have different vortexes. Because the hummingbird flies so different, they have a very unique type of wake vortexes.
Warrick, Tobalske, and Powers sampled the wake produced by rufous hummingbirds using something called the DPIV methodology. This technique involves putting a hummingbird into an idle wind tunnel with a a feeder ( a 1-ml syringe containing a 20% sucrose solution). The bird has been trained to fly to the feeder and hover while it drinks the ‘nectar’. This feeder has been positioned so that the bird will cross a light sheet of oil particles. The bird flaps its wings to hover in place, and the oil-particles are moved by the bird’s wings and airflow. By using a laser, they illuminate the oil particals so they can be seen. They then can record the movement of the oil particles by using high-speed cameras, taking two photos about two hundred nanoseconds apart. By examining where the oil particles move, we can learn the structure of the wake that the hummingbird creates.
The test was done by placing the laser light sheet at different angles so that they could observe different portions of the wake. Frontal-plane samples of the wake (1a, 1b) showed that vortexes were created at the tip of the wing. Parasagittal planes (1c, 1d) showed that the structure of the starting and stopping vortices of the downstroke, and the starting vorticies of the upstroke, and any leading-edge vorticity (LEV) shed at the end of the halfstroke. As the wing transitions from the downstroke to the upstroke, vortices are released spinning back towards the body.
The DVIP method has shown to be a very effictive strategy of finding how vortexes form in the wake of a hummingbird’s wing flapping. We can study different angles of the bird flying at to see how the vortexes are formed while it flies.