Numerical Study On The Dynamics Of Typical Bubbles In Shipbuilding And Ocean Engineering

In naval architecture and ocean engineering,there are various kinds of bubbles.Four typ-ical kinds of them are studied in this thesis,including the air gun bubble（s）,the underwater explosion（UNDEX）bubble,the acoustic cavities as well as the rising bubbles.All of them are very critical in this field,i.e.to explore the seabed resources,to destroy enemy warships/sub-marines,to clean the wet surface of the ship and to be used in the air lubrication system of ocean range vessel.Dynamics of these bubbles as well as the interaction between the bubble and dif-ferent kinds of boundaries in their neighbourhood are studied,in order to have a better knowledge of the mechanical mechanism behind the phenomena,and also,to offer some useful suggestions for the engineering applications.The dual fast multipole boundary element method（dual FMBEM）is used to deal with large scale problems,e.g.the dynamics of multiple bubbles and/or the coupling effects between bubble and complex boundaries.Through applying the fast multipole method（FMM）to the dual boundary integral equations（dual BIE）,a normal desktop computer can be used to solve problems with greater than 100 thousand elements.The dual BIE is combined linearly with the conventional BIE and the hypersingular BIE with the weights x=1.0 and h=0.4 re-spectively.Then,for a problem with N elements,the CPU time consumed by the conven-tional BEM isO(N^2.4)while that of the dual FMBEM isO(N^1.1).And also,the cost memory is cut down fromO（N ²）to O（N）.The accuracy of the fast method can be con-trolled by the number of expansion p of the kernel function of the BIEs,sometimes may even better than the conventional BEM.Dynamics of the multiple air-gun bubbles with 1D,2D and3D configurations are studied through the fast method.Behaviour of each bubble is influenced by the gravity and the Bjerknes force from other bubbles.The motion of one bubble（expansion or collapse）resists the change of the pressure of the flow field,which is similar to the Lenz’s law in the electromagnetism.The bubbles inside the group is shielded by the bubbles outside,and have longer oscillation periods and larger maximum volumes.For a uniform distributed 3D bubble array,it behaves like a single bubble.Then,with the BEM,dynamics of an UNDEX bubble near a full scale warship and a sub-marine is studied.Due to the block of the warship,the water spike at the free surface above the bubble is 22%higher than that produced by a same bubble without any structures nearby.And the study also finds out that the ground effect brings both advantage and disadvantage to a submarine above the seabed when suffering the UNDEX bubble attack.On one hand,the seabed absorbs the energy from the bubble,which makes the pressure on the submarine from the bubble smaller than that from a bubble in the free field.Nonetheless,on the other hand,the ground effect makes the liquid jet of the bubble“prefer”to shoot towards the submarine.For a bubble with the distance of _mR（_mR is the maximum radius that a bubble attains in an infinite field）from the submarine,and with the seabed2R_m beneath the submarine,the range of the dan-gerous attack angle is 30°larger than that of the same case in the free field.Hence,when a submarine is under attack,it’s better to sink to the seabed to protect itself.However,it should prevent the bubble being triggered in its range of attack angle.The situation of a bubble near a plate with an opening that is produced by the UNDEX shock wave is also investigated.The interaction between the bubble and all other air-water interfaces is complex.Usually,the water jet of the bubble will not strike onto the structure due to the existence of the opening.A high speed inrush water spike with 30 m/s appears due to the motion of the bubble,and it will reach the depth of 6 m into the cabin.This will bring a severe hit onto the facilities and structures inside the cabin.The suction effect at the opening and the shrinking at the base of the inrush water spike due to the evolution of the bubble are also observed.Subsequently,the study moves from the large,high pressure,transient and traditional bub-bles to ones with small sizes,normal pressure,enduring and promising applications.One of the latter is the bubble pair in the sound field.This thesis focus on the two-bubble system because it is the basic unit of bubble clusters.3 categories of bubble behaviour are studied in terms of the amplitude of the acoustic wave _ap.When p_a<0.5p₀（₀p is the initial pressure in the flow field）,the weak wave makes the bubble oscillate for many cycles before collapsing or coalescencing.The two bubbles are attracted,repelled or even stand at a relatively stable dis-tance with each other,due to the wave frequency and nature frequencies of the bubbles.Besides,there is periodical pressure released from the two bubbles.For the strong wave,e.g.p_a>p₀,both bubbles collapse in its first oscillating cycle,and the liquid jets with a speed of more than200 m/s appear.The pressure induced by the bubble pair at this time is one order greater than the amplitude of the wave.Both the bubbles in the weak wave and the strong wave are suitable for the surface cleaning of the subsea structures.Firstly,one may use the weak wave to let the periodical bubble pressure loosen the attachment;then enhance the wave and let the high speed liquid jet wash it away.This will save much time and labor,which is worth considering in the engineering.In the last chapter,the lattice Boltzmann method coupled with the free energy model is adopted to simulate the rising bubble with high liquid-gas density ratio.The bubble grows and departures from a small opening is studied.The motion of the first bubble may disturb the flow field,resulting in the faster growth and detachment of the following bubbles.Bubbles grow from a wall on the top in a cross flow domain is investigated,with the aim of applying to air lubrication system.In this case,the buoyancy and the cross flow together make the air cover the wall.In this chapter,the speed ratio of the cross flow and the outlet gas l is studied.Re-sults reveal that small l may result in a better coverage of the gas to the wall,i.e.the hull bottom.However,in reality,the speed of the ship is certain,which means an optimal gas flow speed should be chosen for realizing the best lubricant and cost as few as possible energy to blow out the gas.