Study on Water Impact of Elastic Plate with Small Deadrise Angles

Abstract

It is well known that in the case of the collision between a flat body with very small deadrise angle and a water surface, Wagner's theory is not applicable because of the trapped air effect. This paper describes the study on this trapped air affected region of the water impact problem. In order to investigate the transient situation from the air trapped impact to the Wagner type impact, we have carried out the pressure and the stain measurement at the water impact by dropping an aluminum flat plate of 200 mm breadth and 5 mm thick in the condition of attack angles from zero to 4.0 degrees at intervals of 0.5 degrees.Based on the experimental results, the following conclusions have been derived.(1) Any pressure pattern in time space is clearly identified with the Wagner type or the air trapped type.(2) The impact situation is classified to the following three regions according to the attack angle β. (a) Air trapped type impact region : 0≤5β< 1°The pressure pattern is the air trapped type all over the impact surface. Flat pressure distribution rises and falls smoothly.(b) Transient region : 1°≤β<3°The Wagner pattern around the keel of the plate and the air trapped pattern at the edge side are observed. The Wagner pressure pattern expands toward the edge with increasing β. Sharp peak pressure is observed.(c) Wagner type impact region : β≥3°The pressure pattern is the well-known Wagner type all over the plate. (3) The maximum strain that has the maximum value around β=0 is moderately depend on β from zero to 4 degrees. The sharp peak pressure hardly affects the strain.(4) The strain calculated statically by loading the measured average pressure is in good agreement with the measured strain.The structural response at impact depends on not only the load but its own structural characteristics. Therefore, further research is needed to evaluate the strength of more complicated structures in practice.