Air-cushioning below an impacting wave-structured disk: Free-surface deformation and slamming load

Prior to the impact of a horizontal disk onto a liquid surface, the air underneath flows radially outward across the liquid surface to escape from below the edge of the disk. Such airflow causes the surface to be elevated near the disk edge, creating a free-surface condition that influences the details of the subsequent impact dynamics. In this work, the nature of the surface elevation under an impacting disk is investigated by modulating the forcing of the free surface: The airflow below the disk is altered by imposing a radially symmetric wave structure of varying wavelength on the impacting disk surface. Subsequently, the liquid surface deformation before impact is measured experimentally using a total internal reflection technique. The experiments provide convincing evidence that supports the argument that the surface elevation is an instability of the Kelvin-Helmholtz type. In addition, the impact force exerted on the wave-structured disks is measured using a load cell. Due to the macroscopic wave structure on the disk, the maximum impact force is significantly reduced, and the results indicate that both the free-surface deformation before impact and the way in which the impacting surface is subsequently wetted influence the maximum impact force.