Vortices behind a bat wing
In the earlier notes Avenue de Henri Benard and Turbulence in Flow around Bodies we have discussed von Karman vortices in flow around solid bodies. Here is a nice video of computer simulation of vortices behind a bat in flight.
(Direct Link | video credit: Carl Zimmer | courtesy: Dan Riskin)
The video is part of extensive bat research of Dan Riskin of Brown University.
Looking carefully at the vortex that forms approximately at the center of the wing span behind the bat body, the spin direction seem to abruptly change from anti-clockwise to clockwise. I suspect this could be due to removal of computational frames for certain in between period, to squeeze short the video running time. Still the reversal of spin is intriguing.
On the other hand, the position of the vortex is in line with what I would expect. The flapping of the wings on either side causing the relatively stationary air to ‘churn’ up and down with a relatively undisturbed narrow passage of air at the center downstream, along the bat’s head and torso. The churning of air on either side would eventually shear this narrow air column into a spin – vortex.
Of course, the unsteady nature of the forces generated by the bat wing can result in the vortex to change direction. Also, due to the continuous bat flight, the subsequent second wave of twin vortex air column that raises upstream of the central vortex could also communicate a relative shear at the edges of the central vortex.
The computer simulation of vortices behind the bat won the 2007 Science (magazine) Best Visualization award (credit: Willis, D. J., Kostandov, M., Riskin, D. K., Peraire, J., Laidlaw, D. H., Swartz, S. M., and Breuer, K. S. 2007. Modeling the flight of a bat (science visualization feature). Science 317, 1860,(2007)). Here is a modified part picture from the winning poster with some explanation.
To put it mildly, the flow is complex for such heuristic predictions. Quoting from Carl Zimmer’s excellent post on this topic:
The shoulder of a bat starts rotating upwards before the wrist, which move up before the fingers. The fingers on each hand don’t move in sync with each other. A joint on the left wing is often out of sync with the corresponding joint on the right wing. Physicists like to treat wings as rigid surfaces because the math involved causes fewer headaches. But that’s a gross simplification when it comes to bats. The bones in a bat’s hands are surprisingly flexible, and the skin of the bat wing is never fully stretched out during its stroke. In fact, the region of the wing close to its body actually balloons out to double its surface area during each flight stroke. Bats probably use this ever-changing wing surface to control their lift and drag, so that they can make tight maneuvers without stalling.
Compared to this bat flight vortices, the wing vortex behind an airplane (given below) suddenly looks manageable.
[image credit]
There is more to the bat flight research. Recently the team that Dan Riskin was part of, have performed extensive controlled experiments to postulate how bats land on the ceiling. The outcome of this research is reported in a recent paper in the Journal of Experimental Biology. In short, bats go not head over heels but head under heels, for landing on the ceiling.
Further, Cynopterus brachyotis, a tree-roosting bat common in tropical parts of southeast Asia, backflips while swooping upward and landing with the hind legs and thumbs touching the landing spot simultaneously. This is termed by the researchers as a four-point landing, with an impact force more than four times the body weight of the species. On the other hand, cave-roosting species, Carollia perspicillata and Glossophaga soricina common in Central and South America, approach their landing target vertical (head at top and legs at bottom) and at the last moment rotate left or right to turn upside down to grasp the landing pad with only their hind legs. This is a two-point landing with an impact force of just one-third of their body weight. Certainly much gentler than the impact force exerted by the four point landing of the tree-roosting bats.
Read also the post by Carl Zimmer explaining the research for us with some great videos of bats performing a two point and four point landing on the ceiling. Dan Riskin’s website also carries all the necessary documents and videos of this research.
Citation
Lester, B. (2007). 2007 Visualization Challenge Winners Science, 317 (5846), 1858-1863 DOI: 10.1126/science.317.5846.1858
Related posts:
Why bats? Isn’t there another bird with a simpler wing planform to consider? And, then, the problem of bats landing on ceilings… This seems many steps removed from what I can grasp at a time
I think the CFD study is part of the much more broader one of evolutionary aspects of flight of mammals (bats) and their (in)ability to adapt to say, walking…
BTW, a simpler version of CFD for flapping wings is available here
K.D. von Ellenrieder, K. Parker, J. Soria, Fluid mechanics of flapping wings, Experimental Thermal and Fluid Science In Press, Accepted Manuscript, , Available online 20 May 2008. [ link ] [ doi:10.1016/j.expthermflusci.2008.05.003 ] – see this list of mine.
Also, at present, I am working on flapping of wing-like structure for another problem. The above note is more for self. More in person.