Numerical Research On The Piston And Sloshing Modes In Moonpools

Moonpools are used on many types of vessels, cable-laying vessel, exploration and drilling vessel, production barges, research and offshore support vessels. There are used to launch and retrieve equipment, divers or diving bells, or lay cables or risers, in an environment protected from the waves. moonpool is a large wall-sided hole in the bottom of a ship through which for instance equipment can be lowered into the sea or through which pipes (riser , cables or drills) are going. The structure of the moonpool is in the middle of the vessel. There is free surface in the cavity. The water in the moonpool flows nonlinear and random. The flow will bring flow noise near the cavity of the moonpool. When the fluid flows along the bottom of the ship and flows by the upstream of the moonpool, the shear layer oscillates increasingly .that brings shear layer self-oscillate in the moonpool. Maybe there is sloshing phenomena in the moonpool and the fluid fluctuations pressure also can bring vibration response at the part structure of the moonpool. These phenomenon will effect the normal work and sail. Consequently ,we must study the hydrodynamic characteristic of the cross-flow in the moonpool. And than we can make sure that the sail and the oil storage system works safe.For this phenomena, this paper studied the hydrodynamic characteristic on the sloshing phenomena of the moonpools by numerical simulation. This paper uses the CFX which is a CFD software to simulate the sloshing phenomena in the moonpool fluid. Under the condition of different flow speed this paper analysed the changing of the free surface, sloshing phenomena and the changing of the vibration frequency. After that this paper compares the results with the physic experiments. The results indicates that there are two kinds of oscillate phenomenon in the moonpool fluid. One is inherence oscillate along the deep of the moonpool, it was named piston .And the other kind of phenomena is that the free surface in the moonpool oscillates right and left, it was named sloshing. These two kinds of phenomenon relate to the structure style , size and the level of the free surface. When the incoming flow frequency is the same as the piston or sloshing, it will bring the fluid in the moonpool resonance. In these two kinds of oscillate, the piston resonance comes first. When the inspirit frequency of coming flow increase, it will inspirit sloshing phenomena, and these two kinds of oscillate always become coupling. When the there is piston phenomena there is also sloshing phenomena appears. When there is sloshing we can see pistonphenomena.In this dissertation, the computational fluid dynamics software CFX isapplied to calculate the experimental model at first in order to determine the most reasonable model scale and the fixed position of the equipment. Afterwards, the model experiment is developed in the towing tank using circle and square shaped moonpool under different uniform flow conditions. The motion rule of the liquid inside the moonpool of two different cross-sections and the liquid's action on the structure of moonpool are gained. It is discovered that under low speed flow conditions, the surge phenomena inside the moonpool may result in the distinctive change of the pressure distribution of the side wall of the moonpool.Finally, the computational fluid dynamics software CFX is applied to calculate the pressure distribution at the bottom of the moonpool under some typical flow conditions. The numerical result is compared with the experimental result and it is shown that the constant pressure result of the numerical calculation is basically close to the experimental result.