In this study, a reciprocating-type water turbine model that applies the principle of the Weis-Fogh mechanism was proposed, and the model's unsteady flow field was calculated by an advanced vortex method. The primary conditions were as follows: wing chord $C=1$, wing shaft stroke length $hs=2.5C$, and the maximum opening angle of the wing $α=36 deg$. The dynamic characteristics and unsteady flow fields of a Weis-Fogh type water turbine were investigated with velocity ratios $U/V$ = 1.0 ∼ 3.0. Force coefficients $Cu$ and $Cv$ acting on the wing in the $U$ and $V$ directions, respectively, were found to have a strong correlation each other. The size of a separated region on the back face of the wing increased as the velocity ratio increased and as the wing approached the opposite wall. The rapid drop in $Cv$ during a stroke increased as the velocity ratio increased, and the average $Cu$ and $Cv$ increased as the velocity ratio increased. The maximum efficiency of this water turbine was 14.1% at $U/V$ = 2.0 for one wing.

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