Fluid-structure interactions of a torsion spring pendulum at large initial amplitudes

The motion of a lamina of high aspect ratio suspended in a Newtonian fluid was studied experimentally and numerically. The damped oscillations for one rotational degree of freedom showed strong nonlinear fluid-structure interactions, mainly caused by the vortex structures forming at the lamina tips. The numerical results were obtained by a fully implicit Navier-Stokes solver, using partitioned coupling of the equations of motion of the fluid and suspended structure. Computations were carried out for different grid levels and time steps, providing information on the accuracy of the numerical results. For the fluid domain, a Langrangian-Euilerian finite-volume method was applied in order to solve the two-dimensional Navier-Stokes equation on grids moving with the oscillating lamina. The elastic motion of the lamina was computed as that of a torsion spring pendulum. The computed time traces of the angular position are in close agreement with corresponding experimental results. An equivalent empirical model which accounted for the fluid moments by empirical coefficients was much less successful in predicting the experimentally observed behaviour.