Experimental And Numerical Study On Nonlinear Coupling Of Pulsating Bubbles And Floating Bodies

The nonlinear coupling effect between bubbles and structures has important applications in ship and ocean engineering(e.g.,underwater explosion,seabed resource exploration,cavitation phenomenon in silt-laden flow,cavitation of the propeller and other propulsion devices,ultrasonic cleaning,medical treatment).The coupling between a pulsating bubble and a floating body with the free surface is a worthy investigation.However,there is little knowledge in this specified field of bubble dynamics.This paper focuses on the non-linear coupling effect between a bubble and a floating body.The research work is based on the combination of experiment and numerical method,the numerical model is established via boundary integral method(BIM),and the correctness of the numerical model is verified by spark-generated bubble experiments.The coupling effect of the bubble and the floating body under different conditions was studied,and the response of the floating body and the motion characteristics of the bubble were compared and analyzed.First,this paper starts with the current research status of bubble dynamics at home and abroad,summarizes the theoretical and experimental research methods of bubble dynamics,and then discusses the existing problems and deficiencies in the existing research work:the former numerical models of bubble dynamics mainly focus on the bubble movement characteristics with the free field,free surface,and rigid wall,but the strong coupling between the bubble and structure,especially the research on the coupling between bubbles and floating bodies at the free surface,is relatively scarce;Most of the previous studies on the coupling between bubbles and floating bodies are based on the "loose coupling" method,which inevitably introduces errors into the calculation,and this method cannot solve the strong nonlinear problems involved in the strong coupling;Most of the previous numerical studies on bubbles are focused on the period that before the bubble jet,there are few studies on the annular bubble stage,and the coupling effect between the bubble and the floating body in the annular bubble stage is even rarer;most of the previous experimental studies on bubble dynamics are based on simple boundaries such as free surface,and rigid wall,and the systematic research on the coupling movement of the bubble and the floating body is relatively scarce.In view of these existing problems and deficiencies,this paper will carry out a series of research work.Then,in this paper,a fully nonlinear fluid-solid coupling model of the bubble and the floating body is established.The calculation method of the pressure field and the velocity field of the flow field is proposed.The auxiliary function method is used to decouple the force and motion of the floating body,which improves the program's performance,stability,and calculation accuracy.At the same time,this paper also proposes a number of numerical optimization methods based on the fully coupled model,such as simplified treatment of fluidsolid boundary junction points under cylindrical floating body conditions,distributed processing of free surface variable nodes,water layer removal treatment in the case of bubble contact with the structure and node removal in the case of small bubble splitting,etc.These technologies enable the model to handle the coupled simulation of bubbles and floating bodies at arbitrary distance parameters.Experiment is one of the powerful tools to study the bubble dynamics.In this paper,the coupling effect of the bubble and floating body is studied systematically through the sparkgenerated bubble experiment.The bubble motion characteristics and floating body response under different distance parameters are studied.The correctness of the numerical model is verified by the experimental results,and the coupling effect under different distance parameters is divided into three categories:coupling at long distances,coupling at middle distances,and coupling at close distances under contact conditions;meanwhile,the mechanism behind some special physical phenomena in the experiment is revealed by numerical simulation,and some rules are summarized.Then,in this paper,we use numerical simulation to further study the coupling effect of bubble and floating body under different proportion parameters and buoyancy parameters,and investigate the movement characteristics of a bubble and floating body under different dimensionless parameters.We also study the evolution rule of the jet in the bubble collapse stage and the bubble movement in the annular bubble stage after bubble jet.The time history changes of some physical parameters such as displacement,velocity,acceleration,structure bottom pressure,bubble centroid,and volume are compared and analyzed.Finally,this paper studies the special physical phenomenon that the bubble drives the floating body out of water.When the coupling effect of bubble and floating body is experimentally studied,the floating body appears "out of water" after bubble collapse under some conditions:when the coupling effect of bubble and floating body is very strong(such as the contact condition between bubble and floating body),the floating body may continue to rise after bubble collapse and structure out-of-water phenomenon happens.In this paper,a detailed experimental study is carried out to study this phenomenon,and the rule of the out-of-water phenomenon of the floating body under different distance parameters and proportion parameters is summarized.In addition,a numerical model of floating body out-of-water is established by BIM,and the calculated results are in good agreement with the experimental results.