The Influence of Aeration and Deadrise Angle on Impact

Abstract

When sailing in heavy seas, it happens that the water washes over the bow. This shipping of water is generally divided into two types. The term white water is used when the water is more of a spray and is generally harmless. Green water is more violent and can pose a danger to the crew and structures on board.

Research on green water has mainly focused on the arrival of green water on deck and the different types. Impacts are generally considered as result of green water wave and tested by a dam-break problem. The impact of green water has not been studied as solely the impact of water on a structure. Drop test experiments are conducted in different ways for different shapes in the past. However, there have not been tests before with green water using drop tests. Drop tests are deemed suitable to test shapes with different deadrise angles and create aerated water.

The aim of this work is to investigate the effects of green water on deck structures by studying the maximum impact pressure when considering the deadrise angle and the aeration.

In this thesis the influence of shape and aeration on the maximum pressure at impact is investigated. To investigate the effect of aeration and shape, the impact tests are conducted using a drop tower experiment. For this experiment different increasing deadrise angles are chosen from 0± or a flat plate up to 30±. In addition test with aerated water are done up to 4% which is in range of real green water waves. The aeration and the shapes are chosen in such a way that non-compressible, linear compressible and non-linear compressible effects take place. It can be seen that with a flat plate, the impact increases almost instantaneously and produces a high peak. Adding aeration reduces this peak considerably. Fromexamining the impact of the wedges, it can be concluded that the pressure rises more slowly at a higher deadrise angle. The almost instantaneous drop in pressure seen with a flat plate changes to a smoother rise at a high deadrise angle.

In addition to the experiment, numerical simulations are carried out to verify the solution. The numerical simulations also give a better insight into the pressure across the width of the object. Upon examining the numerical simulations, it is concluded that the results for deadrise angles of 15± and 30± are equivalent to the results of the results of the experiment. However, the simulations for a flat plate result in an high pressure which is probably due to a low number of data points which capture the peak.

It can be concluded that the shape of the structure and aeration have a significant influence on the maximum pressure. However, the decrease due to a higher deadrise angle is more significant than the effect of additional aeration. With higher aeration, the maximum pressures are closer together and the difference in deadrise angle has less effect. When examining the impact pressures the effect of both deadrise angle and aeration can be seen. The maximum impact for a non aerated flat plate impact measured is 9.93[bar ] and is decreased by 8.8[bar ] for a wedge impact with a deadrise angle of 30± which drops in water that is 4% aerated. The influence of both shape and aeration can be reviewed independently which leads to an reduction of impact pressure for a flat plate of 59 percent when 4% air is added to the system. When only changing the deadrise angle to 30± a reduction of 86% is found.