Fluid dynamics of a self-propelled biomimetic underwater vehicle with pectoral fins

Abstract Fluid dynamics of a self-propelled biomimetic underwater vehicle (BUV) with pectoral fins is investigated by an immersed boundary (IB) method. Typically, the BUV with a pair of pectoral fins starts from rest and attains a constant mean velocity as the mean longitudinal force is zero. The capability and accuracy of the IB method to deal with the interaction between the fluid and complex moving body are firstly validated. Then we carry out a parametric study to understand the effect of key governing parameters on the dynamic response of the BUV. It is found that with the increase of motion frequency or rolling amplitude, the pectoral fin propulsors can induce larger forward velocity so that the BUV takes less time to attain its stable periodic swimming state. Although the pectoral fin is a very complicated lifting surface, a linear relationship between forward Reynolds number (final swimming velocity is used as velocity scale) and frequency Reynolds number (product of motion frequency and fin chord length is used as velocity scale) can be established when the frequency Reynolds number is above a critical value. A linear relationship between forward Reynolds number and rolling amplitude is also found within the studied range of rolling amplitude. Furthermore, a small-density-ratio BUV is sensitive to the surrounding flow with more rapid evolution process of self-propulsion. Whereas, BUV with a large density ratio is more stable. The implications of the hydrodynamic analysis on the bio-inspired engineering design of BUV with pectoral fins are also discussed.