| The study of cavitation can effectively solve the key scientific problems of cavitation erosion.It is of great significance to improve the cavitation resistance of materials in the fields of hydraulic machinery,aerospace and so on.On the other hand,cavitation force can be transformed into processing force,which can be applied in biomedical,cavitation cleaning and micro-fabrication of material surface structure and other fields.Cavitation mechanism is complex,mainly by describing the transient characteristics of cavitation bubble,such as the initiation,pulsation and collapse microjet.The law of evolution and multiphase coupling of cavitation bubble were studied,and the cavitation mechanism and its influence on material properties were revealed,involving optics,mechanics,material science and other disciplines.Laser-induced cavitation refers to the phenomenon that micro-explosion occurs inside the liquid to form plasma cavitation bubbles by laser breakdown of liquid medium.It has the advantages of good spherical symmetry,accurate positioning and strong controllability.In this paper,the mechanism of laser-induced cavitation under complex conditions was studied.The effects of various characteristics of cavitation bubbles were revealed from the aspects of microparticle conditions and microstructure conditions.The distribution of the light field enhancement effect of the particles was simulated.The transient characteristics of bubbles in the microparticle solution were detected,and the jet velocity of collapse of near-wall bubbles was calculated.Finally,the cavitation gain phenomenon caused by the microparticles was found.The effect of wall shape on the transient characteristics of cavitation bubble was studied by experiment,and the asymmetric pulsation and displacement of bubble were found.Specific research contents are as follows:(1)The mechanism of laser breakdown of liquid was analyzed based on the theory of cavitation dynamics.We established a model of the motion of the surrounding particles in the process of bubble pulsation in liquid.And we deduced the equations of micro-jet and shock wave of collapse of near-wall bubble,which laid a theoretical foundation for subsequent research.(2)It was found that the size and position distribution of the enhancement field are related to the particle size,spacing and spatial distribution of the particles by analyzing the field enhancement phenomenon around the particles in the liquid.It was found that under the same laser parameters,single bubble was formed in distilled water,while multi-bubble was formed in micro-particle solution by comparing with the transient characteristics of laser cavitation bubble in distilled water and micro-particle solution.The radius and the first pulsation period of the dominant bubble vary with the change of particle concentration,and the jet velocity produced by the collapse of the dominant near-wall bubble in micro-particle solution is higher.The field enhancement effect of micro-particles is an important factor leading to cavitation gain.(3)The effect of wall structure on the transient characteristics of laser-induced cavitation bubble was studied.We found the phenomenon of displacement of near-wall bubble when it collapse,and cavitation bubble begin to deform during the pulsation process.The collapse time of the bubble is related to the wall structure.The larger the closed area is,the smalle the free space of the bubble is,and the longer the collapse time is.In the stepped structure,a single bubble will be "cut" into a double by the structure,and the size ratio of the double bubbles is related to the height of the inclined plane.(4)It was found that there was a fan-shaped cavitation erosion area near the moving position of the bubble by analyzing the surface morphology of the target after cavitation erosion.The radius of the fan-shaped cavitation erosion area increased with the increase of the laser energy.The maximum radius of the an-shaped cavitation erosion area was consistent with the distance traveled by the bubble. |
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