Dynamics of flexible plates and flow under impulsive oscillation

The induced flow and the dynamics of flexible plates under heaving characterized by impulsive acceleration patterns Π(t)were experimentally inspected for various Cauchy numbers Ca and fractions τ_a of the time under acceleration with respect to the period of oscillations T. Simultaneous measurements of the plate motion along their span and the surrounding flow were obtained with particle tracking velocimetry (PTV)and particle image velocimetry (PIV). The Π(t)function was imposed at the root of a low-aspect-ratio plate vertically submerged in a quiescent water tank. Results show that Ca and τ_a significantly modulated the plate dynamics as well as the induced flow and coherent motions. We developed a simple model to estimate the velocity difference Δu between the root and tip of the plates; it is based on the damped harmonic oscillator and exhibits good performance across the fifteen scenarios inspected (three Ca by five τ_a). In general, the plate dynamics and flow exhibited three distinctive patterns characterized by flapping-like motions, translation-like motions and another intermediate state. The type of motion was induced by the relative instant at the reverse direction within the forced Π(t)-heaving within one of three characteristic stages; two of them depending on the sign of Δu and the other when Δu→0, i.e., in the limit of pure translation.