The controlled impact of elastic plates on a quiescent water surface

The impact between a structure and a water surface is a complex phenomenon involving many physical processes. Such impact is often associated with structural deformation, highly transient impact loads and intense water surface motion. The interaction between these physical processes and the related rate of change in physical quantities are often substantial. Therefore, the interaction during the impact can significantly affect the integrity of the structure. Thus, understanding the physics of the impact phenomena is of paramount importance.

The impact of Rigid objects on a water surface has been extensively Studied theoretically in the literature The experimental studies on the impact of rigid objects on water surfaces have supported most of the theoretical findings regarding the dynamics of water entry flows. Despite the numerous research efforts, the impact of elastic plates on a water surface is yet to be fully explored in detail.

On this account, Dr. An Wang, Dr. Kit Pan Wong, Dr. Miao Yu, Dr. Kenneth Kiger and Dr. James Duncan from the University of Maryland conducted elegant experimental studies to investigate the oblique and vertical impacts of flexible aluminum plates on quiescent water surface. The flexibility of the plate ranges from very stiff to highly flexible in this experiment. The selection of the impact conditions was in accordance with the carriage velocity component in the direction normal to the undeformed plate surface and with the ratio of the horizontal carriage to vertical velocity components. The work is currently published in the Journal of Fluid Mechanics.

In their approach, the plates were mounted to the two-axis instrument carriage at the leading and trailing edges using pinned supports. It was then driven into the water surface at a constant velocity and at various pitch angles for the time interval representing the passage of the leading and trailing edges of the plate through the still water. The moment and force needed to maintain the steady plate motion were measured during the impact. The experimental conditions comprised Froude number varied from 0.17 – 0.43, the plate stiffness ratio from 0.027 – 1.33 and the submergence time ratio from 1.58 – 10.7.

The authors showed that the stiffness ratio is the main dimensionless parameter responsible for controlling the role of flexibility during impacts. For low plate stiffness ratio, maximum plate deflections on the order of 1 mm were observed and the dimensional form of the transverse moment, plate deflection, spray root position and hydrodynamic normal force was nearly identical. For all these scenarios, the impacts occurring over time scales were considerably greater than the natural period of the plate and the quasi-static response occurred with a maximum deflection midway through the impact.

For conditions with high plate stiffness ratio, the effects of the dimensionless quantities strongly depended on the stiffness ratio. As a result, two-way fluid-structure interaction and dynamic plate response were observed. In this dynamic response, the plate deformation caused significant changes in the moment and hydrodynamic force, with the largest deflection occurring after the spray root reached the leading edge of the plate. Furthermore, Froude numbers exhibited little influence on the results except for the lowest values < 0.18, where the spray root system under the plate collapsed towards the end of the impact.

In summary, Dr. An Wang and colleagues investigated the controlled impact of elastic plates on a quiescent water surface. The results of the impact conditions allowed for the determination of the effects of different dimensionless parameters The study was significant advancement from the previous studies. In a joint statement to Advances in Engineering, the authors explained their findings would contribute to resolving the complexity of the impact of structures on a quiescent water surface.