Structural response of oscillating foil in water

Harmonic oscillations of NACA 0012 airfoils in water are numerically simulated to assess the corresponding structural loads due to the generated forces. An appropriately devised procedure estimates the unsteady effect caused by the foil acceleration, i.e. the added mass effect. This is found to play a very important role as the resulting inertia forces are largely enhanced in the range of analysed parameters. The influence of the wing mass is investigated and it is found that light wings generate forces larger than those generated by heavy wings, as light wings accelerate more than heavy wings. The resulting bending stresses and unsteady deflections are calculated by modelling the wings as elastic cantilevers with uniform distributed loads. The maximum unsteady deflection is found to be about 1% of the wing span, that is, the fluid–structure interaction problem can be considered decoupled in the present analysis. It is also shown that heavy, rigid wings appear to be more suitable for the swimming mode corresponding to steady cruise, as the applied stresses result smaller than those obtained for light, flexible wings. The added mass effect could instead be exploited when required, by using lighter propulsors, which generate larger forces.