Prof. A.H. Techet
Department of Ocean Engineering
Massachusetts Institute of Technology
Cambridge, MA 02139
To better understand the fluid forcing produced by fish when they are swimming it is necessary to first discuss the basic concept of a vortex and vorticity. A vortex arises when flow encircles a central point. Examples of where vortices occur in nature include: whirlpools, tornados, ocean eddies, blood flow through heart valves, and flow swirling behind rocks in a river current. Vortices appear in engineering fluid applications as well: flow around offshore platforms, tip-vortices from aircraft wings, helical vortices in propeller wakes and helicopter rotors. In general, vortices can form when flow passes any object with a non-streamlined shape or a sharp corner, and separates from the body.