Dynamic Mechanisms And Applications Of Large Free-surface Deformation

Large free-surface deformation is a challenge to the study of modern fluid dynamics.For the experiments,there are great difficulties to observe the evolution of free surface due to the high requirement on the spatial and temporal resolution.As to numerical simulation,Euler method is not able to meet accuracy in describing the free surface,and Lagrange method would encounter huge mesh distortion to failure.However,the large free-surface deformation is a key factor to determine the process accuracy and material quality in modern precision manufacturing and material preparation,e.g.,additive manufacturing,integrated circuit printing,directional deposition/controlled phase transformation,etc.Therefore,the mechanisms and laws of large free-surface deformation have been the research focus of the scientific community and received much attention.It is necessary to develop appropriate numerical models and experimental measurements to disclose the characteristics of large free-surface deformation,and to explain the dynamics mechanisms based on the hydrodynamic theories.Smoothed particle hydrodynamics(SPH),a new generation of the numerical method,which combines the advantages of meshless and Lagrange property,has overcome the shortcomings of traditional numerical methods.Good applicability is shown to the study of large free-surface deformation dynamics.Furthermore,the application of high-speed imaging provides the possibility for observing and recording the morphologies of large free-surface deformation.In our study,a numerical tool appropriated to large free-surface deformation is developed based on the physical models of SPH discrete particles.The calculation ability has been improved in thermal radiation by combination with the mesh method of finite elements.The typical processes of large free-surface deformation in the droplet dynamics and phase change kinetics are predicted by SPH numerical models,which are verified by a high-speed imaging platform.According to the above guidelines,the thesis has been supported by both theoretical analysis and numerical prediction with experimental verification.Besides,exploratory research has been conducted to disclose the characteristics and dynamics mechanisms of large free-surface deformation connected with process parameters,which are summarized as follows.(1)The mechanisms are put forward to explain the formation and pinching-off of the central jet during the liquid film receding on the smoothed surface,as well as the ejection of secondary micro-droplet from the central jet.The inertia scope of droplet ejection is determined by analyzing the free-surface deformation dynamics by varying the Weber number.Additionally,the characteristics of large free-surface deformation and tendencies are summarized for the micro-droplet ejection.(2)The pinning of contact lines is explained by the agglomeration and adsorption of nanoparticles under the capillary force,based on the observation of droplet impact dynamics and nanoparticle distribution.The effects of impact velocity,the volume fraction of nanoparticle and heating temperature of the surface on the droplet wetting are analyzed,which suggested that the maximum normalized wetting radius meets the empirical relation of_max=0.457(2(20)0)^0.190)0)^0.13.(3)The oscillating deformation of the free surface in the micro-channel is numerically analyzed to predict the penetration patterns along with droplet spreading.And then the pattern change is illustrated by the regime map under the effect of impact inertia,microchannel size and substrate wettability.On this basis,the combination of permeation patterns is revealed with the different impact inertia and porosity during droplet spreading on porous surfaces.(4)The melting process of quartz glass under radiation heating is studied numerically based on SPH-FEM combining framework.The temperature and phase distributions inside the deformed quartz ingot are analyzed.Then the scheme of"optimizing thermal parameters+bottom heater"is proposed to eliminate the solid inclusion in molten quartz.Two phase change modes of contacting evaporation and rebounding evaporation are predicted for droplet wetting on high-temperature substrate numerically and experimentally.The analysis on evaporation dynamics points out that the peak of the evaporation rate is in the stage of droplet receding.The above conclusions develop numerical analysis and experimental verification methods suitable for the dynamic study of large free-surface deformation.Besides,different processes are involved in our research with the focus on large free-surface deformation,which provides a more comprehensive theoretical guidance for the process optimization of3D printing,thermal spraying,and the thermal design of industrial furnaces.