Physics of Whales Swimming :: physics whale swim

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. Whales create spectacular sights as they cruise through the ocean. We have observed them as they thrust with powerful tails and direct themselves with stiff pectoral flukes. To move forward they swing their tail in vertical motions. This powerful motion can push them for hundreds of miles during migration or jettison them out of the water in amazing aerial displays. Below is an animation of an orca as it moves its tail to go forward. Notice that the front of the body is fairly rigid as a planning surface. Other whales, like the Grey Whale, are more streamline for longer journeys. The gray whale migrates from Alaska to Baja twice a year in the spring and fall. Rohr believed that bioluminescence could solve a very large question: do whales and dolphins have frictionless skin and can we imitate it? Laminar flow creates much less shear stress than turbulent flow at the same velocity because there is no swirling or random motion.