Experimental Investigation of Water Impact on a Disk in Pure and Aerated Water

Hong, Yao (Shanghai Jiao Tong University) | Wang, Benlong (Shanghai Jiao Tong University) | Liu, Hua (Shanghai Jiao Tong University)

OnePetro 

ABSTRACT

The impact loads of a disk slamming into both pure and aerated water are investigated experimentally. The impact velocity of the disk (diameter = 140 mm) is 2 m/s. The pressure distribution and evolution of impact loads are recorded with the help of three mini pressure gauges and a force sensor. To minimize the influence of bubbles on the calmness of free surface, the bubble generator is improved with quartz covered on its surface and a flow mass controller is utilized to control the flux of gas introduced into the water precisely. The experimental results show the central region of the disk endures higher impact loads and the decline of impact loads along the radius is not uniform in pure water. For impact tests in aerated water, not only a reduction of impact loads but also a longer duration of impact loading is observed.

INTRODUCTION

The water entry problem has been studied for nearly a century and is still of interest in the fields of hydrodynamics and ocean engineering. The impact load is characterized by its high local pressure and short duration, which will significantly threaten the safety and integrity of a structure. For this reason, the prediction of impact loads is needed in the design of naval and offshore structures, ships even aerospace structures.

Commonly the fluid is assumed to be incompressible and inviscid for convenient to study. The seminal theoretical work of impact loads is conducted by Von Karman (1929) when investigating the landing of a seaplane. Based on the conservation of momentum, Von Karman derived a formula to predict the impact force. Wagner (1932) proposed a refined theory to predict the impact loads later. After that, some researchers improved Wagner's theory to predict the impact loads more accurately. For example, Zhao and Faltinsen (1993), Korobkin (2005) and Oliver (2007).

However, in some cases the compressibility of fluid plays an important role, especially in the initial stage of water entry. It is well known that the sound speed of the water, which is important to the prediction of impact loads, can drop quickly when the water is involved with gas. Moreover, the work of Lamarre and Melville (1991) shows that in the upper layer of the ocean, the void fraction of air bubbles can reach an order of 0.5%. Thus the evaluation of the influence of aeration effect on impact loads is necessary for the design of naval and ship design. Recently, this point is attracting the attention of researchers. Ma et al. (2016) and Elhimer (2017) respectively studied the aeration effect on impact loads of a plate and a cone. The reduction of impact force is verified by experimental results. To have a better understanding of the aeration effect on impact loads, a series of water entry experiments of a disk are conducted in this paper. Except for the relation between maximum impact loads and void fraction in water, special attention is given to the pressure distribution on the bottom surface of a disk.