Robotic fish with Compliant Caudal fin

The compliance of a fin affects the thrust of underwater vehicles mimicking the undulatory motion of fish. Determining the optimal compliance of a fin to maximize thrust is an important issue in designing robotic fish using a compliant fin. We found a simple method to identify the condition for maximizing the thrust generated by a compliant fin propulsion system. By observing the relationship between the thrust and phase difference, we conclude that while satisfying the zero velocity condition, the maximum thrust is obtained when a compliance creates a phase difference of approximately π/2 at a certain undulation frequency. This half-pi phase delay condition is proven by the fluid dynamic analysis using a digital particle image velocimetry. This condition can be used as a guideline to select the proper compliance of a fin when designing a robotic fish. Using this condition and an in-house variable stiffness mechanism, we developed and tested a novel variable stiffness flapping mechanism for a robotic dolphin system. 


Related papers and patents

Yong-Jai Park, Useok Jeong, Jeongsu Lee, Seok-Ryung. Kwon, Ho-Young. Kim, and Kyu-Jin Cho, “Kinematic Condition for Maximizing the Thrust of a Robotic Fish Using a Compliant Caudal Fin,” IEEE Transactions on Robotics, Vol. 28, No. 6, pp. 1216 –1227, Dec. 2012.


Jeongsu Lee, Yong-Jai Park, Useok Jeong, Kyu-Jin Cho, and Ho-Young Kim, "Wake and thrust of an angularly reciprocating plate," Journal of Fluid Mechanics, Vol. 720, pp. 545-557, Apr. 2013. [PDF]


Yong-Jai Park, Tae Myung Huh, Daegeun Park, and Kyu-Jin Cho, “Design of a variable-stiffness flapping mechanism for maximizing the thrust of a bio-inspired underwater robot,” Bioinspiration & Biomimetics, Vol. 9, No. 3, p. 036002, Mar. 2014. [PDF]


Hyungmin Park, Yong-Jai Park, Boogeon Lee, Kyu-Jin Cho, and Haecheon Choi, “Vortical structures around a flexible oscillating panel for maximum thrust in a quiescent fluid,” Journal of Fluids and Structures, Vol. 67, pp. 241–260, Nov. 2016. [PDF]


Jeongsu Lee, Yong-Jai Park, Kyu-Jin Cho, Daegyoum Kim, and Ho-Young Kim, “Hydrodynamic advantages of a low aspect-ratio flapping foil,” Journal of Fluids and Structures, Vol. 71, pp. 70–77, May 2017. [PDF]

Related papers and patents

Yong-Jai Park.jpg

· E-mail : yjlone at
· Affiliation : Professor of Kangwon National University(강원대학교)


Yong-Jai Park