Deformation of structures upon impact with a liquid free surface

Abstract

This thesis study is intended to research the effects on peak pressure during impact of a deformable structure with a liquid. To find an answer to this question, a novel wedge was designed to perform free fall wedge experiments at the TU Delft. This report describes the progress that lead to answering the research question:

What is the effect of dynamic deformation of a structure on the maximum pressure at the wedge surface during impact?

To create an overview of the problem, the various systems and knowledge required to answer the research question are discussed. Among others, a decision on wedge deformation type is made, the importance of multiple impact velocities and the sensors required to measure data of the impact are discussed. The proposed concept to perform experiments is with a deformable wedge that is allowed to fold around the keel upon impact with the free surface. Hypothesis is that the deformation on impact could lead to initial lower impact pressure but dynamic response could lead to higher peak pressures at later stage of impact.
In the next phase, estimations of impact pressures, forces and accelerations are used to finalize the design of the wedge. Strength calculations were performed on the critical parts of the wedge. It is concluded that the analysed parts are strong enough to withstand repeated loading of the structure. Also during this phase, an extra experiment into dynamic calibration of the accelerometer was performed. By curve fitting the expected acceleration to the experiment data, the error of the accelerometer is determined. The accelerometer used in this experiment has an error of 0.48% which is within the limits of the manufacturers requirement of ≤1%. Before the start of the experiments the limitations of the setup are discussed and an uncertainty analysis is performed on the sensors.
The experiment consists of a series of free falls with various wedge setups, either rigid or deformable. A rigid setup of the wedge is tested at impact velocities of 2, 4 and 6m/s and deadrise angles of 10 and 20 degrees to create a base line for maximum impact pressures. The results of the rigid experiment are compared against previously tested rigid wedges. It is concluded that the novel wedge showed similar peak pressures compared to previously tested rigid wedges. The deformable wedge is dropped at all impact velocities and deadrise angles with three different springs, varying in stiffness.
Deformation of the wedge during impact resulted in decrease in peak pressure. Decreasing spring stiffness and increasing impact velocity resulted in increasing deformation of the wedge. With these results the research question set in the first chapter of the report is answered.
The answer to the research question is that impact induced deformation of the structure leads to lower peak pressures. Dynamic effects were observed but did not lead to higher pressure than the rigid experiments. While further research into the matter is required, this result may already lead to improved ship design which could lead to lower manufacturing costs and lower emissions.