The benefits of swimming together

Abstract

When a fish beats its tail, it produces vortices in the water that other fish could take advantage of to save energy while swimming.

Keywords: fish schooling; fluid dynamics; in-line swimming; physics of living systems; robotics; thrust wakes.

Figure 1.. Swimming in drag wakes and thrust wakes.

When water flows (grey arrows) past a physical object (light blue; top), it forms a pattern of swirling vortices in which the water rotates in either a clockwise direction (green arrows) or an anticlockwise direction (purple arrows). The arrangement of the vortices (which is called a ‘drag wake’ or a von Kárman vortex street) means that a fish swimming towards the object encounters water that is flowing slower on average than it would be if the object was not there. When a fish beats its tail (blue; bottom), it also produces a pattern of swirling vortices. In this case, however, the arrangement of the vortices (which is called a ‘thrust wake’ or a reverse von Kárman vortex street) means that a second fish swimming behind the first fish encounters water that is flowing faster on average than it would be if the first fish was not there. Thandiackal and Lauder used a hybrid robotic-animal experimental set-up to study fish swimming in thrust wakes.