Study On Evolutionary Process And Load Characteristics Of The Bubble Near A Free Surface In Confined Fluid Domain

Bubble dynamics has widely applications in the fields containing the naval architecture and ocean engineering and biomedical.in which the typical applications are the underwater explosion bubble,cavitation bubbles in ship propeller,bubbles for drag reduction of ships and technology of ultrasonic bubbles for cleaning structure surface,respectively.Since the middle of the last century,a lot of work on bubble dynamics had been carried out by researchers all over the world and a wealth of research results were achieved,but for the problems of bubble oscillating in a confined fluid domain such as 'shallow underwater explosion'or‘pool explosion test'.the behaviours of bubble under the combined actions of a free surface and several rigid walls surrounding the fluid become more complex,and up to now,an effective solution to investigate the dynamic behaviours and load characteristics of a bubble oscillating in a confined fluid domain is still deficient.On this basis,incompressible potential-flow theory is adopted in this paper to establish the numerical models of bubble dynamics in a confined fluid domain by using the boundary integral method.Combining the experimental methods of both the spark-generated bubble and ultrasonic cavitation bubbles,the dynamic behaviours of bubble(s)in a confined fluid domain are studied,from which some new physical phenomena are discovered and the load characteristics in the fluid field which is induced by bubble motions are obtained.The conclusions,on one hand,reveal the relevant mechanical mechanisms of bubble motion in confined fluid domain and.on the other hand,aim to provide basic technique support and feasible reference for the optimization of such engineering applications.On the basis of equations of mass conservation and momentum conservation,and by using Green's formula,the fundamental equations of boundary integral method(BIM)are derived.The indirect boundary element method is adopted to calculate the pressure load in fluid field which is induced by the bubble oscillation.In previous researches,the fluid field surrounding the oscillating bubble is usually assumed to be infinite,but for the problems of bubble oscillating in a confined fluid domain,the effects of rigid walls encircling the fluid domain on the bubble motion require for a consideration.Thereafter,the numerial method of the interaction between bubble(s)and free surface in a confined domain is proposed in the paper.With regard to the arising difficulties of calculating the singular integral and velocity of intersection point connecting three phases of gas,liquid and solid,the corrected 4 ? rule and node-separation technique are proposed,respectively,to address these problems.The present numerical model is proved to be sufficiently correct and valid by the comparison results from conducted spark-generated bubble experiments.When a bubble is close to a free surface,there will be strong nonlinear interactions between them during the bubble oscillating,then a high-speed jet directing away from the free surface generates on the bubble surface during the bubble collapse,meanwhile,a high water spike arises from the free surface.In the case of confined fluid domain with significant influence of the rigid wall on the bubble and free surface,the water skirt' phenomenon will occur at the stage of bubble expanding,but this feature will not happen until the rebounding phase of toroidal bubble oscillating in the infinite field.With different parameters of buoyancy and characteristic distance,the interactions between bubble and free surface are discussed,and the characteristic of pressure loads both on the horizontal rigid wall and in fluid field surrounding this oscillating bubble are obtained.On this basis,the numerical models of bubble bursting at a free surface and double-bubble interacting with free surface in a confined fluid field are established.The propagating characteristics of the bursting wave after the bubble bursting at the free surface is studied and the interesting phenomenon called‘bottom exposure'is discovered.During the interaction between two bubbles with the free surface,the oscillation cycle of upper bubble near the free surface as well as its maximum expanding volume are observed to be much shorter than the lower bubble,meanwhile,there is a crown-shape spike occurring at the free surface.All these particular physical phenomena observed in numerical simulations are well validated from results of our spark bubble experiments.By discussing the effect of bubble-rigid wall distance,a critical size of the fluid field for negligible effects on bubble oscillations is obtained.When the ship structure suffers from shock wave load of underwater explosion and subsequently leading to the local damage with a breach,this damaged structure with the breach is likely to be attacked again by the bubble load.On the background of this engineering application,a numerical model of bubble interacting with both a non-integral structure and free surface is established.With regard to the problem of numerical divergence resulted from the bubble wall infinitely approaching the breach boundary,a numerical technique called 'fluid domain cut' is proposed,and then the fluid field above and below the rigid wall are numerically calculated,respectively.When the effect of buoyancy on bubble motion is small,the free surface will render the bubble to form a downward jet,while the breach on the rigid wall renders the bubble to form an upward jet.Via discussions of several cases with different characteristic parameters,three typical cases of upward jet,downward jet and counter jet are analysed,respectively.The dynamic behaviours and load characteristics of bubble under combined action of the free surface above and rigid wall with a breach below are summarized,which preliminarily solves the problem of calculation of the bubble second-attacking damaged structure in shallow underwater explosions.Cavitation is the primary cause which leads to the erosion of ship propeller.Therefore,the experimental method of ultrasonic cavitation is used for systematically investigating the dynamic behaviours of cavitation bubbles and the characteristics of acoustic pressure load in a high intensity focused ultrasound(HIFU)field.The experimental results show that the structure of cavitation bubbles exhibits a feature of layer-distribution,and the gap between adjacent layers is determined by the driving frequency of ultrasound.Via a comparison of the gap between adjacent bubble layers with different driving frequency,the deviation between the experimental and theoretical results are all less than 3%.The trajectory of bubble moving indicates that there exist three typical moving states between the bubbles which are attraction,repulsion and relative stability,respectively.If two bubble are attracted to each other and finally merges,the volume of merged bubble increases,and the subsequently increasing buoyancy of this merged bubble is prone to render it jumping to the higher layer close to the water surface.Finally,the damage characteristics and mechanisms of elastic material subject to the pressure load generated by HIFU are investigated with the help of infrared thermometer and infrared camera,indicating that the damage of elastic material in a HIFU field is caused by thermal mechanism.The conclusions summarized in this chapter further reveal the moving characteristics of acoustic cavitation bubbles as well as the mechanical mechanisms of bubble oscillations,which aims to provide relevant references for the engineering applications of ultrasound cavitations.