LAUSR

Abstract The dynamics of a fully-passive flapping-foil turbine, operating at a Reynolds number of 3.9 × 10 6 , is studied via two-dimensional fluid-structure numerical simulations. The foil is allowed to move freely, but only, in heave and in pitch by being simply attached with springs and dampers. These elastic supports eliminate the need for the more complex mechanisms that are traditionally used to prescribe specific foil motions. This study demonstrates that the optimal performance of fully-constrained flapping-foil turbines can be matched with this simpler concept when the structural parameters are adequately adjusted. An efficiency reaching 53.8% has been achieved. Also, the effects of varying the heaving mass and the heave stiffness can be effectively characterized by a single parameter, which is not the heave natural frequency. On the other hand, the pitch dynamics is appropriately characterized by the pitch natural frequency, which combines the moment of inertia and the pitch stiffness. An optimal efficiency can be maintained over large variations of the inertial and stiffness properties when the effective parameters are kept constant. It is also found that the presence of viscous friction in pitch is detrimental to the turbine performance, but its effect remains small with a realistic friction level.