Spray Formation and Structural Deformation During the Oblique Impact of a Flexible Plate on a Quiescent Water Surface

Abstract

The oblique impact of a flexible and a nearly rigid plate on a quiescent water surface is studied experimentally. Both plates are 122 cm long by 38 cm wide and are mounted with a 5° upward pitch angle and a 10° lateral roll angle. The plates are attached to a dual-axis instrument carriage. The horizontal and vertical components of the carriage (plate) motion are driven by servo motors and controlled by a single computer-based feedback system, which is set with a single trajectory that is traversed for all impact speeds. The transient strain at multiple locations on the upper surface of the plate is measured with optical fiber Bragg grating sensors and the out-of-plane deformation is measured with a photographic method. A cinematic laser-induced fluorescence technique is used to measure the water spray generated during the impact. Two types of spray are observed and several aspects of the spray behavior are found to be noticeably affected by the deformation of the plate. The maximum deflection along the plate's upper long edge is found to increase almost linearly with impact velocity. 1. Introduction In rough seas, planing boats moving at high speed frequently slam into the water surface. The slamming process involves large highly transient pressures and forces on the hull, rapid accelerations of the boat and the water, the generation of water spray, and significant structural responses. This phenomenon is difficult to study numerically and experimentally because of the large motions of the hull and the violent motions of the water free surface, including the formation of spray sheets and the structural responses coupled with the flow dynamics. The problem of slamming (water entry) has received significant attention in the past. Many of the previous studies on this subject examine fundamental problems, such as the impact of a wedge or a flat plate on water surface. Some early theoretical studies on the water entry of a rigid wedge include, e.g., Von Karman (1929) and Wagner (1932). Wagner's model considers the water rising adjacent to the surface of a vertically moving wedge with small deadrise angle. This model was later extended to higher order (e.g., Oliver 2007) and other geometries (e.g., Howison et al. 1991). Based on Wagner's theory, Dobrovol'Skaya (1969) derived a similarity solution to the water entry of a wedge and the solution at small deadrise angles (down to 4°) was computed numerically by Zhao and Faltinsen (1993) using a nonlinear boundary element method. De Divitiis and de Socio (2002) studied the water entry of symmetric and asymmetric wedges. In their method, the flow field is represented by potential flow singularities whose intensities are determined as part of the solution. Moore et al. (2012) did a numerical study of normal and oblique water entry of a threedimensional rigid body with its bottom surface nearly parallel to the water surface.