A Combined IB-LB Method for Predicting the Hydrodynamics of Bionic Undulating Fin Thrusters

A novel method combining Immersed Boundary and Lattice Boltzmann (IB-LB) is proposed and applied to the hydrodynamics prediction of bionic undulating fin thrusters (BUFTs). The IB concept is introduced within the LB framework, and modifications are made to the computation format of body forces to enhance efficiency while ensuring accuracy. Sub-lattice models are further introduced to make the method adaptable to high Reynolds number (Re) cases. After validation in two classical cases, the IB-LB method is employed to predict the hydrodynamics of BUFTs. The results show that the propulsive force and efficiency increase with the increase of Re, which is confirmed by the spatial distribution of vortex structures. In addition, as the undulating frequency and amplitude coefficient increase, the longitudinal spacing between the wake vortices decreases and the vertical distance increases, and this effect can be regarded as increasing the effective striking area of the fins in one cycle, and thus propulsion are bound to increase. When the frequency is five times of the basic frequency, amplitude coefficient is 1, and wavelength is 0.53 times of the body length, the BUFT achieves optimal propulsion performance. The findings show that the method can well simulate the interaction between moving objects and fluids, accurately capture the vortex structures around complex moving boundaries.