| Themes > Science > Physics > Fluid Dynamics > Flying the flag for fluid dynamics > Aerodynamics > Hydro- and Aerodynamics of Animal Swimming and Flight > Why fish and birds are not like submarines and planes? |
Animals do not have wheels. They do not, in general, have rotating parts that describe a full circular motion. Exceptions to this rule exist; rotating flagellae come to mind, as does the fact that a circular motion can be described quite well by a simple jointed limb (the drive system of old steam engines we see in movies illustrate this as linear motion is converted to circular motion at the wheel rim). Tumble weeds tumble round and round, woodlice curl into a ball, and hedgehogs can probably roll downhill. But these exceptions pretty much prove the general rule, and rotating machinery does not exist to provide a separate power plant for animal locomotion. Powered thrust and lift must be generated by some kind of oscillatory motion. The resulting fluid flows are quite complex, characterised by large scale separation bubbles, highly unsteady flow fields and surface pressures, and very complex geometries. In this way, students of biofluid dynamics frequently find themselves at the forefront of fluid dynamics of unsteady, complex viscous flows. The Reynolds numbers of interest can range from 10 to 10^5, and in such situations, we are still in the realm of modelling efforts, as full direct Navier-Stokes simulations are quite impractical for even toy problems. This is perhaps part of the allure, because the smart models can teach us things that might not be appreciated if we could just turn on our computer code and return for the results in a couple of weeks. The issues discussed in the next sections touch on state-of-the-art modelling efforts and sophisticated and painstaking experimental work. Together, they shed light on some fundamental questions concerning efficiency of unsteady lift and propulsion mechanisms, and on integration of form and function in design. The persistent student will also be rewarded by an increased depth of appreciation of the every day sights of swimming and flying in nature. The humble housefly can teach us much. |
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