CLAS: Colby Liberal Arts Symposium

Using Precise Magnetic Fields to Measure the Fluid Dynamics of Swimming Fish

Davis 301

When the blade of a propeller in a jet engine spins, when the wing of a bird flaps, and when the tail of a fish wags through the water, horizontal torques and forces through a fluid somehow create a forward thrust. Forces through fluids are mysterious and complex; they are ubiquitous, yet invisible. Animals since the dawn of life have contorted their bodies in complicated twists and bends to propel themselves using nothing but their surrounding fluid medium. How is the efficiency of this motion measured, and how can it be maximized? With the power of modern lensing and image processing software, we are now able to precisely track the motion of a swimmer across the surface of water. More impressive still, by taking advantage of the Biot-Savart Law and computer controlled currents, we are able to apply a precise torque on a bar magnet to manipulate its motion in water. By knowing the exact motion of a magnetic swimmer in water through a high-speed camera and knowing the exact magnetic forces on this swimmer, we are able to isolate the hydrodynamic forces acting on the magnet in every frame of film. We assert that neither the frequency nor the amplitude (effort) of tail wagging kicks have a linear relationship with net swimming speed. In fact, there is an optimal frequency and amplitude of kicks, dependent on the fluid, that propels the swimmer at its highest speed. Faster and stronger kicking leads to slower swimming above or below this optimum. The efficiency swimming can be measured as the power associated with velocity divided by the magnetic power input to the magnet. We believe that our method could be used by future physicists to study the results of manipulating the abundance of other variables that affect this intriguing and primitive swimming motion.

https://digitalcommons.colby.edu/clas/2015/program/79