Fishes are unmatched when it comes to motion control and flexibility, swimming in complex underwater environments. Scientists have been inspired to copy nature’s most gifted swimmers to optimize underwater vehicle propulsion and maneuverability.
The relation between the movement of the tail, transient shapes and motion of the water environment is well known in fish mechanics. There has been little focus on how pressure fields affect thrust generation and control to propel fish.
Harbin Engineering University in China scientists showed that fish generate movable vortex pairs of high and low-pressure regions that enable them to swim, through precise control of body fluctuations. The study provides the groundwork in the design of flexible structures for a high-performance underwater bionic propeller.
Scientists used particle image velocimetry and high-speed cameras to analyze the spontaneous swimming of zebrafish in a tank. A fish accelerates when it bends its caudal fin to one side and then returns to the neutral position as the fish straightens its body. Scientists discovered the formation of two vortex cores at the wake spinning in opposite directions, in this completion of a single tail swing.
These cores form a low-pressure and high-pressure region on opposite sides of the fish. Scientists found the pull generated by the low-pressure area and the thrust produced by the high-pressure area together provide the propulsion power of the zebrafish.
The movement of these high and low pressure regions together promoted the acceleration of the fluid mass backward while pushing fluid outward at the tip of the caudal fin. When the fish body was J-shaped, the high-pressure area slid to the rear of the caudal fin. The low-pressure area slid to the front of the caudal fin.
The caudal fin used the low-pressure area to drive the fluid toward the body. It will generate a vertical upward pull on the fin. The high-pressure area pushed the fluid away at the crest. It generated an upward thrust on the caudal fin. Zebrafish can move continuously because of the repetition of this process.