Flapping and Clapping Propulsion

There are two primary mechanisms in nature for underwater locomotion: jet propulsion (as used by jellyfish and squid) and flapping propulsion (as used by fish).

Why did Nature select two different mechanisms for underwater propulsion? In what circumstances is one mechanism superior to the other? In order to answer these questions, we built an experiment to provide a clear and fair comparison between the two mechanisms. The motions for each mechanism is simplified: jet propulsion is simplified to a clapping motion where two connected flat plates starts fully closed, open to a given angle, and then close again, creating a jet of water; flapping propulsion is simplified to a two dimensional flapping plate.

 Jet propulsion is simplified to a clapping motion (left) while flapping propulsion is simplified to a two-dimensional flapping plate (right)

Jet propulsion is simplified to a clapping motion (left) while flapping propulsion is simplified to a two-dimensional flapping plate (right)

In order to provide a fair comparison between the two mechanisms, a single-axis experimental apparatus,  which can function in jet propulsion or flapping propulsion, is used. In addition, the total swept angle and sweep time are kept the same for both types of propulsion. Force data is obtained to determine which produces greater thrust, and torque data is obtained to measure which requires more power to use. Comparing the force produced to the torque required gives a measure of the mechanisms' effectiveness in various flow conditions.

This experiment is currently underway for various sweep times, sweep angles, blade flexibilities, and free stream velocities. Check back for updates!