The
History of Movement in Water
Animals can't rotate like wheels on a car. They must move by oscillatory motion. The resulting flow of fluids are complex and create agonizing studies for biofluid students. These students often find themselves at the forefront on research in fluid dynamics of unsteady, complex viscous flows.
Often men have marveled at the
dolphins and whale and at how gracefully they moved through the water.
Jim Rohr, a fluid dynamicist working for the US Navy was on an evening
cruise in the waters near San Diego when he saw nature doing what scientists had
failed to do in the lab: reveal
water motion to the naked eye. Watching
the plankton bioluminesce as the boat moved by he realized that if he could
measure that luminescing he could measure fuid dynamics, turbulence and laminar
flow.
But how do the plakton know
that the water is moving? And what
forces act on animals as they swim?
The answer is mechanical strain of relative motion. Fluids flow around walls slower than they flow in the middle. As the fast molecules tug on the slow ones this creates shear stress. Since velocity is always zero, the faster the liquid, or the animal moving through it, the greater the sheer stress. This is laminar flow. But at some point laminar flow turns to turbulence. Turbulence decreases speed because it increases drag. The plakton were feeling the pull of the drag gainst their cells and luminescing.