Hydrodynamic Analysis for the Morphing Median Fins of Tuna during Yaw Motions

Abstract

Tuna can change the area and shape of the median fins, including the first dorsal, second dorsal, and anal fins. The morphing median fins have the ability of adjusting the hydrodynamic forces, thereby affecting the yaw mobility of tuna to a certain extent. In this paper, the hydrodynamic analysis of the median fins under different morphing states is carried out by the numerical method, so as to clarify the influence of the erected median fins on the yaw maneuvers. By comparing the two morphing states of erected and depressed, it can be concluded that the erected median fins can increase their own hydrodynamic forces during the yaw movement. However, the second dorsal and anal fins have limited influence on the yaw maneuverability, and they tend to maintain the stability of tuna. The first dorsal fin has more lift increment in the erection state, which can obviously affect the hydrodynamic performance of tuna. Moreover, as the median fins are erected, the hydrodynamic forces of the tuna’s body increase synchronously due to the interaction between the body and the median fins, which is also very beneficial to the yaw motion. This study indicates that tuna can use the morphing median fins to adjust its mobility and stability, which provides a new idea for the design of robotic fish.

Bioinspired morphing fins to provide optimal maneuverability, stability, and response to turbulence in rigid hull AUVs

“…The size of a 1.5× forward morphing fin is around 3% of the bare hull projected surface area. In comparison, the projected surface area of the first dorsal fin of tuna is approximately around 3%-5% of the projected surface area of the body [21]. The first dorsal fin of tuna has a lower aspect ratio as compared to the that of the Morpheus forward morphing fins, resulting in a smaller lift force.…”

“…However, their prey consists of smaller fish that have high body flexibility, and, hence, by employing significant body curvature, they can turn very rapidly [20]. As a result, tunas can utilize retractable dorsal fins to dynamically change the directional stability to conduct rapid maneuvers at high speed [21]; an intriguing method that can be applied to engineered underwater vehicles [10]. Aleyev [22] presented the first quantitative proof that maneuvering fish reduce the distance between their center of mass plus added mass and the center of pressure ('o'), defined as the center of the longitudinal projection of the fish shape, to reduce stability.…”

“…While dorsal and ventral fins have several functions [25,26], including counteracting roll and pitch moments in fast-starts [27], for stiffer-bodied cruising fish such as tuna, as shown by Li [21], extending the forward dorsal fins has a significant effect on the turning rate by adjusting their mobility and stability.…”