Computational Analysis of 3d Fin-fin Interaction in Fish’s Steady Swimming

Three-dimensional numerical simulations are used to investigate the hydrodynamic performance and the wake patterns of a sunfish in steady swimming. Immersed boundary method for deformable attaching bodies (IBM-DAB) are used to handle complex moving boundaries of one solid body (fish body) attached with several membranes (fins). The effects of the vortices shed from both the dorsal and anal fins on the hydrodynamic performance of the caudal fin are analyzed by prescribing an undulatory swimming kinematics to a full body sunfish model. Results show that both the dorsal fin vortices and the anal fin vortices can increase the thrust and efficiency of the caudal fin comparing to caudal fin only case. This is because the dorsal/anal fin not only can feed vorticity into the caudal fin wake via vortex shedding, but also can modulate the flow in the downstream in a way of forming a jet with stronger backward component. NOMENCLATURE C chord length CT thrust coefficient C?̅? overall thrust coefficient CP power coefficient C?̅? overall power coefficient Re Reynolds number St Strouhal number f flapping frequency k wave number η propulsive efficiency BC model that contains body and caudal fin BDC model that contains body, dorsal fin and caudal fin BAC model that contains body, anal fin and caudal fin BDAC model that contains body, dorsal fin, anal fin and caudal fin INTRODUCTION Engineers and researchers look for inspirations from nature for superior designs of propulsion system of Autonomous Underwater Vehicles (AUVs)[1-4]. Many studies were performed previously, holding the purpose of understanding the fundamental of flapping propulsion. The hydrodynamic performance of an isolated flapping foil was well discussed previously[5-9]. Recent studies also focus on the hydrodynamics of multi-propulsor systems, especially tandem flapping foils. Among those works, Lan & Sun studied two tandem airfoils performing flapping motions and found the enhancement of both vertical force and horizontal force in different cases[10]. Warkentin & DeLaurier found that the tandem arrangement can result in thrust and efficiency increases by choosing proper relative phase angles and longitudinal spacing between the wing sets[11]. Simulations conducted by Akhtar et al. [12] indicated that the thrust of the downstream foil can be increased significantly by capturing the vortices shed from the upstream foil, and the increment is quite sensitive to the phase difference between the two tandem foils. Liang & Dong used realistic morphologies of dragonfly to explore the wing-wake interactions between ipsilateral wings and found that both the lift and thrust of hindwings could be increased significantly due to the wingwing interaction[13]. Broering et al. investigated the effect of phase angle and wing spacing on tandem flapping wings, as well