| Ship bubble drag reduction and air layer drag reduction have been highly expected energy saving technology for a long time.In order to make full use of buoyancy to enhance the interaction between air flow and the water flow field near the hull surface,the application of this technology mainly focuses on flat-bottom-ship.In the physical processes of air injection drag reduction,microbubble mixed flow and air-water stratified flow are the two most typical flow states.There are both obvious differences and mutual coupling transformation between them,resulting in a complex multi-scale two-phase flow system.At present,the flow characteristics have not been fully understanded,which hinders the practical development of bubble/air-layer drag reduction technology.The bottleneck problems in fundamental research mainly include two aspects: First,effective numerical method for the simulation of bubble/air-layer multi-scale coupled two-phase flow is lacking.Most of the previous studies focused on the single flow phenomenon and took experiment as the main research method.Second,the multi-factor influences laws of bubble/air-layer and its coupling flow are not clearly understood.The effectiveness of air injection drag reduction has been proved by many previous studies,and the corresponding mechanism of drag reduction has also been discussed.However,the difficulties in practical application are flow control and design.It is necessary to understand the evolution characteristics of the bubble/air-layer two-phase flow and the influence law of multiple factors,which can provide theoretical foundation for practical system design.In this thesis,both numerical simulation and model experiment are adopted as research methods.In terms of the numerical simulation,a physical model describing multiscale twophase flow is proposed according to bubble/air-layer flow phenomena.On this basis,a numerical method for the coupling of discrete particles with interface capture is developed.The solver consists of three modules.Macro-scale flow simulation module uses VOF(Volume of Fluid)method to capture large-scale interface deformation,which can be used to simulate air-layer drag reduction flow.Microbubble simulation module uses Lagrange method to track each bubble,which can be used to simulate the evolution of large number of bubbles in microbubble drag reduction.Multi-scale coupling module can realize the mixed simulation of two flow phenomena.A new multi-scale transformation algorithm based on curvature proposed in this doctoral thesis is adopted in this module,which significantly improves the efficiency and simulation accuracy.In terms of the experimental study,bubble/air-layer patterns experiment is carried out based on cavitation tunnel facility.Both high-speed photography technology and PIV(Particle Image Velocimetry)flow measurement technology are adopted in the experiment,which provided abundant data for the study of flow characteristics.In order to verify the physical model and the numerical method developed in this thesis,a series of numerical simulations of standard multiscale two-phase flow problems are carried out,including microbubble rise up and merge with free-surface,large bubble rise up and breakup,vertical plunging jet flow.Three aspects are verified: First,the bidirectional transformation process of Lagrange bubbles and VOF interface can be simulated successfully and smoothly,physical laws such as the conservation of mass and momentum are consistent.Second,the new multi-scale transformation algorithm can accurately capture the multi-scale flow phenomenon naturally formed during air flow evolution,and has significantly higher computational efficiency than the traditional method.Thirdly,the macro-scale air-water interface evolution and micro-scale bubble movement and size distribution spectrum are verified by benchmark results and experimental results.In this thesis,the bottom plate of flat-bottom-ship is taken as the basic geometric object.Three parts of air injection drag reduction flow are further studied,which focuses on microbubble drag reduction two-phase flow characteristics,complete air-layer drag reduction two-phase flow characteristics and bubble/air-layer co-existence and transformation flow characteristics,respectively.In the first part of the study,the interaction of wall-bound turbulence flow and microbubble cloud is focused.Microbubble drag reduction in both turbulent channel flow and turbulent boundary flow are studied.Results indicate that the contribution of microbubbles to drag reduction is reflected in near-wall void fraction and Reynolds stress,and there are obvious attraction characteristics of strip vortex structure on the flat plate surface.The migration of microbubbles under the action of turbulence in the boundary layer is believed to be the key reason for the limitation of effective drag reduction area.Fluid acceleration force acting on bubble provides the dominate motivation for bubble migration.The spatiotemporal distribution analysis of bubble size distribution shows that the smaller microbubbles are easier to enter the inner of turbulent boundary layer.In the second part,flow characteristics of air-water interface of a complete air layer filled with air cavity are focused.The air layer surface wave pattern observed in the experiment is successfully reproduced,and the instability phenomenon of upstream air layer caused by the uplift of air-water interface is explained from the perspective of two-dimensional and three-dimensional vortex flow field.Influence laws of multiple factors are studied by numerical simulation,variables including inflow velocity,air flow rate,air injector position,air injector size,air cavity height,air cavity width and bow structure.The relationship between wave height,wave length of air-water interface and flow parameters is quantificationally analyzed,and the main influencing conditions of stable attached air layer are summarized.In the third part,the problems of bubble/air-layer coexistence and transformation mixed flow characteristics are focused.Bubble/air-layer flow experiment on flat plate is carried out firstly.The flow development process and multi-scale air partition phenomena under different flow parameters are recorded.At the same time,the flow morphology of bubble/air-layer at different bow wedge heights is compared to illustrate its important influence on air coverage area.The influence mechanism is analyzed based on PIV velocity measurement results of single-phase flow.Then,the bubble/air-layer phenomena in the experiment are successfully simulated based the multi-scale coupling algorithm.The reliability of the numerical method is verified from four aspects: the development process,the influence law of air injection,the influence law of inflow velocity and the influence law of bow wedge height.The void fraction distribution,drag reduction distribution and two-phase vortex flow field,which are difficult to measure in the experiment,are extracted and analyzed from the numerical simulation.Finally,based on the experimental and simulation data of different schemes,the scaling law for the effective air layer length and the critical air flow rate is analyzed,and the suggestion of similarity criterion is given. |
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