Experimental and numerical investigations of extreme wave impacting on a suspended structure

Abstract

Nonlinear interactions between an extreme wave and a cylindrical structure with its bottom being elevated above the still mean water level are investigated by a set of physical experiments, complemented by advanced CFD-type numerical simulations. The extreme wave is modelled as a solitary wave, which is widely applied as a simple model for tsunamis. Three horizontal (namely forward impacting, backward impacting and cyclic forces) and three vertical (namely uplifting, suction, and slamming forces) force modes are identified. The forward impacting force results from the wave crest impacting on the cylinder front face directly, and its force peak is found to have a quadratic relationship with the velocity of incoming water particle. The slamming force is however caused by the wave hitting on the cylinder bottom from beneath, and other modes are associated with the complex fluid behaviors around the cylinder, e.g. reverse flow and violent surface transformation. In addition, the effects of cylinder clearance (the vertical distance between the cylinder bottom and the mean water level), and the inclined angle are investigated in-depth. It is found that these two play significant roles in the slamming force mode. The former determines the total water momentum that is transferable, and the latter tells how much of this total transferable water momentum could effectively be transferred to the slamming force eventually. A planar collision occurs when the inclined angle is equal or close to the localized slope angle of the undisturbed wave surface, resulting in the largest momentum transfer, and in turn, the largest slamming force for a given cylinder clearance.