Study On Transient Characteristics Of Laser-induced Double Cavitation Bubbles On Solid Wall Surface

Severe damage to hydraulic machinery caused by cavitation usually reduces the service life and reliability of the equipment,so it is important to reveal the cavitation process and mechanism.There are many ways to study cavitation.Controllable cavitation property is the cornerstone to study cavitation characteristics and mechanism.Laser-induced cavitation is an effective method to achieve this goal.Studies have shown that the laser can be used as the power source to generate controllable cavitation bubbles by controlling its parameters.Therefore,the use of laser-induced cavitation has become a key method for the study of cavitation characteristics and mechanism at this stage.In this paper,a combination of computer-aided simulation and experimental researches was used to study the transient characteristics of double cavitation bubbles and the interaction between them in the "V" shape and horizontal structure.Besides,the effects of laser energy,cavitation location and other factors on the oscillation characteristics of laser-induced double cavitation bubbles were analyzed in this paper.The main research contents and results are as follows:(1)The formation of laser-induced bubble was analyzed theoretically.The oscillation characteristics of laser-induced bubble was explored based on the cavitation dynamics and the theoretical analysis.The influence of different factors on the single bubble oscillation was studied.Simultaneously,the oscillation characteristic equations of single and double bubbles were compared and analyzed.The results show that liquid viscosity,surface tension and gas content can all have a certain effect on cavitation bubble.The distance between double cavitation is an important parameter that distinguishes the oscillation behavior of single and double cavitation.(2)The Fluent software was used to simulate the transient oscillation characteristics of laser-induced double cavitation on the "V" shape and horizontal structure in different environments.The study focuses on the trend of flow field around the double bubbles in the oscillation process.The results show that double bubbles have obvious tendency of collapse and rebound,and the interaction between bubbles causes abnormal oscillation.At the end of oscillation,double bubbles will form a rotating velocity field centered on the area between them,indicating that double bubbles have the conditions to merge into one bubble.(3)A laser-induced cavitation experiment platform was built.Double bubbles oscillation behaviors in different environments were obtained by using high-speed camera,and the pulsating sequence diagram of double bubbles was extracted by the corresponding processing software in order to further analyze the transient diameter,the pulsating velocity of the wall of cavitation bubble and oscillation characteristic of the formed neonatal bubble.(4)The laser induced double oscillation characteristics in different environments was summarized.The results show that the maximum diameter and oscillation time of double laser-induced cavitation bubbles increase with the increase of laser energy.When the double cavitation induced by the same laser energy is located in the "V" structure,the bubble closer to the tip of the "V" structure will be compressed.However,the oscillation state of the cavitation bubble is basically the same at the initial position and shows a mirror image distribution.Under certain conditions,the double laserinduced bubbles will cause a hydraulic cavitation bubble on the "V" tip during the oscillation process.This bubble changes slowly and eventually merges with double laser-induced bubbles.When the double bubbles are on a horizontal structure,the bubble induced by the larger laser energy will draw another bubble into it at the later stage of pulsation,and they merge into a new bubble called neonatal bubble.The oscillation characteristics of neonatal bubble are similar to those of cavitation bubbles on the "V" shape and horizontal structure.Eventually,the neonatal bubble is gradually formed into a cavitation group and disappears after a slow oscillation.The whole process generally lasts a few milliseconds.