The Impact of a Flexible Plate on a Wavy Surface

The oblique impact of rectangular aluminum plates (length 108 cm and width 41 cm) on a wavy water surface is studied experimentally as a prototypical geometry for examining the strong nonlinear coupling present when an elastic object impacts a fluid domain. The plates are mounted at a pitch angle of 10 degrees (leading edge up) via a 6-component dynamometer frame to a carriage capable of horizontal and vertical translational motion. Our previous experiments were performed with plates impacting a quiescent water surface while varying the effective plate stiffness and impact time scale by changing the plate thickness and carriage horizontal and vertical velocities, U and W, respectively. It was found that the hydrodynamic loading and plate deformation were controlled by the plate's local pitch angle at the instantaneous position of the spray root, which is where the water surface overturns to form a spray sheet on the plate's lower surface. In the present experiments, oblique impacts on a wavy surface are explored. The wavy surface consists of a monochromatic wavetrain that has wavelength λ and wave height H and travels at wave phase speed c_p in the direction opposite to the plate horizontal motion. There are 11 wave conditions consisting of three wavelengths (1, 1.5 and 2 m) and a range of wave steepnesses (H/λ) from 0.021 to 0.064 at each wavelength. The plate horizontal velocity relative to the wave reference frame (U_rel=U+c_p) is held constant across all impacts by using the appropriate plate horizontal velocity for the wave phase speed of each wave condition. The plate vertical velocity and the resulting trajectory (U_rel/W) are held constant across all impacts. For each wave condition, the impact phase (φ) is set so the plate trailing edge first touches the water surface at the crest in one case and the trough in another. Initial experiments show oscillations in the time records of force normal to the plate surface (F_n) and spray root propagation along the plate's lower surface (ζ_r). Air entrainment and rapid changes in force are observed in cases when the plate surface is nearly parallel to the local water surface. This presentation will focus on the effects of λ, H/λ and φ on the time records of F_n, ζ_r and plate deflection.