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Arigid body B moves in an otherwise quiescent viscous liquid filling the whole space outside B, under the action of a time-periodic force f of period T applied to a given point of B and of fixed direction. We assume that the average of f over an interval of length T does not vanish, and that the amplitude, δ,off is sufficiently small. Our goal is to investigate when B executes a nonzero net motion; that is, B is able to cover any prescribed distance in a finite time. We show that, at the order δ, this happens if and only if f and B satisfy a certain condition. We also show that this is always the case if B is prevented from spinning. Finally, we provide explicit examples where the condition above is satisfied or not. All our analysis is performed in a general class of weak solutions to the coupled system body-liquid problem. 

We study the equilibrium configurations for several fluid-structure interaction problems. The fluid is confined in a 2D unbounded channel that contains a body, free to move inside the channel with rigid motions (transversal translations and rotations). The motion of the fluid is generated by a Poiseuille inflow/outflow at infinity and governed by the stationary Navier--Stokes equations. For a model where the fluid is the air and the body represents the cross-section of a sus-pension bridge, therefore also subject to restoring elastic forces, we prove that for small inflows there exists a unique equilibrium position, while for large inflows we numerically show the appearance of additional equilibria. A similar uniqueness result is also obtained for a discretized 3D bridge, consisting in a finite number of cross-sections interacting with the adjacent ones. The very same model, but without restoring forces, is used to describe the mechanism of the Leonardo da Vinci ferry, which is able to cross a river without engines. We numerically determine the optimal orientation of the ferry that allows it to cross the river in minimal time