Flow past a propulsion system with multiple (up to five) in-line foils under pitching motion has been computationally investigated by an immersed-boundary-method based computational fluid dynamics (CFD) solver. The hydrodynamic performance and the vortex dynamics were studied by varying the phase differences between adjacent foils as well as number of foils at a fixed Reynolds number (Re), Strouhal number (St) and the foil spacing. It was found that by choosing appropriate phase difference between the foils, the optimal propulsive performance of the propulsion system was improved significantly with the increasing of the number of foils when it was less than three. However, when the number of foils exceeded three, the overall average performance converged regardless of the number of foils. In the best performed cases, the interaction between the leading-edge vorticies (LEVs) and the trailing-edge vorticies (TEVs) of adjacent foils was found much stronger when the number of foils exceeded two. The associated vortex dynamics was discussed in this paper.
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