Propulsion secrets of forked caudal fins numerically investigated

The diverse and unique abilities of marine life have inspired engineering designs throughout human history. Scientists are studying the propulsion power of fish fins to construct better underwater vehicles and biomimetic propellers.

Xiong et al. numerically studied forked caudal fins, such as one might see on a shark, tuna or mackerel. This fin, located on the rear of the sea creature, produces more than 90% of its swimming thrust.

The authors investigated the propulsive performance and thrust of three different forked caudal fins with different chord lengths. They examined the evolutionary characteristics of leading-edge vortexes created by the fins and how they can lead to vortex-augmented thrust.

“These findings have implications for understanding how the locomotive force is derived from the motion of the caudal fin and its shape,” said author Zhongying Xiong. “A more complete understanding of the potential physics of the evolutionary characteristics of leading edge vortexes will provide a reference for biomimetic roboticists to choose appropriate propellers for underwater vehicles.”

The numerical results showed the emergence and development of the leading-edge vortexes are directly related to the distribution law of the angle of attack caused by the leading-edge configuration.

“In the future, when designing biomimetic propellers like caudal fins, we should consider the leading-edge configuration, motion mode, pitching axis and Strouhal number, because this impacts the emergence and development of leading-edge vortex and thrust generation,” said author Xiaomin Liu.

Source: “Numerical investigation on evolutionary characteristics of the leading-edge vortex induced by flapping caudal fin,” by Zhongying Xiong and Xiaomin Liu, Physics of Fluids (2019). The article can be accessed at https://doi.org/10.1063/1.5128701 .