Fish generate movable pairs of vortices to propel them forward like body
waves
Study reveals the importance of underwater pressure fields in enabling
fish to swim.

Date:
February 22, 2022
Source:
American Institute of Physics
Summary:
Researchers show that fish, through precise control of
body fluctuations, generate movable vortex pairs of high-
and low-pressure regions that enable them to swim. They used
particle image velocimetry and high-speed cameras to analyze the
spontaneous swimming of zebrafish in a tank, and the findings
provide the groundwork in the design of flexible structures for
a high-performance underwater bionic propeller.



FULL STORY ========================================================================== Swimming in complex underwater environments, fish are unmatched when
it comes to motion control and flexibility. For decades, researchers
have been inspired to copy nature's most gifted swimmers to optimize
underwater vehicle propulsion and maneuverability.


========================================================================== Although the relation between the movement of the tail, or caudal fin,
and the 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.

In Physics of Fluids, published by AIP Publishing, researchers at the
Harbin Engineering University in China showed that fish, through precise control of body fluctuations, generate movable vortex pairs of high- and low-pressure regions that enable them to swim. The findings provide the groundwork in the design of flexible structures for a high-performance underwater bionic propeller.

The researchers 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 -- attached to the vertebral
column -- to one side and then returns to the neutral position as the
fish straightens its body. In this completion of a single tail swing,
the researchers discovered the formation of two vortex cores at the wake spinning in opposite directions.

These cores constitute a low-pressure and high-pressure region on
opposite sides of the fish. The researchers 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 jointly 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, and 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 and generate a vertical upward pull on the fin. The high-pressure
area pushed the fluid away at the crest and generated an upward thrust
on the caudal fin.

Repetition of this process enabled the zebrafish to move continuously.

"The entire zebrafish in the swimming process is regarded as a
body wave," co- author Yang Han said. "Whether they accelerated
forward or changed directions, the fish maintained a wavelike
motion at all points of the body from the start of movement." ========================================================================== Story Source: Materials provided by American_Institute_of_Physics. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Chun-yu Guo, Yun-fei Kuai, Yang Han, Peng Xu, Yi-wei Fan,
Chang-dong Yu.

Hydrodynamic analysis of propulsion process of zebrafish. Physics
of Fluids, 2022; 34 (2): 021910 DOI: 10.1063/5.0076561 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/02/220222121224.htm

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