Research On Transient Process And Microscopic Mechanism Of Two-dimensional Hydrofoil Cavitation Collapse

In order to accurately predict the influence of cavitation collapse on hydraulic mechanical performance and further explore the related mechanism of cavitation,this paper takes NACA4412 hydrofoil as the research object,through the collapse transient of the falling cavitation during the two-dimensional hydrofoil cavitation process.The process is numerically simulated and compared with the cavitation process test.The shortcomings of the existing process methods are analyzed.Then,based on the molecular dynamics method,the influencing factors of the microscopic bubble collapse process are explored.The main research contents and results of this paper are as follows:1.The cavitation collapse process is greatly affected by the angle of attack and has a certain inhibitory effect on the velocity field.The results show that most of the large bubbles are flat during the movement of the falling bubbles with the mainstream to the trailing edge of the hydrofoil,and the large cavitation collapses and collapses into smaller cavities or deformations to produce bubbles of various shapes until completely collapsed under the dual action of the reflow stream and the high pressure of the trailing edge.The high-speed area of the suction surface increases with the increase of the angle of attack,and the cavitation collapse process has a significant effect on the suppression of the inflow velocity,the high-speed region is shrinking in the process of moving from the leading edge to the downstream with the falling cavitation,the more severe the bubble collapse,the more obvious the velocity fluctuations.In addition,with the increase of the angle of attack,the time period of bubble detachment is obviously advanced,the volume of detached bubble is obviously increased,and the time period of corresponding bubble collapse is longer.2.The distance between the main collapse region of the detached bubble is positively correlated with the hydrofoil angle of attack: the bubble collapse regions of the 6°,8°,and 10° angles of the hydrofoil are mainly concentrated in the front of the hydrofoil trailing edge 0.4l and behind the trailing edge of 0.4l,behind the trailing edge of 0.4l and behind the trailing edge of 0.8l.3.The cavitation collapse process has a certain influence on the pressure field of the hydrofoil.The results show that with the increase of the angle of attack,the longer the low pressure zone of the hydrofoil suction surface,the longer the duration,and the smaller the corresponding high pressure zone,the shorter the duration,so that the bubble collapse zone gradually moves to the rear high pressure zone.The increase of the angle of attack increases the volume of the detached bubble,and the pressure peak caused by the collapse is larger,and the pressure shock generated during the collapse of the bubble acts on the pressure field of the hydrofoil,causing the pressure gradient near the collapse point more intense,thus affecting the growth of attached cavitation.4.The bubble collapse region is consistent with the vortex core and the turbulent energy fluctuation region.The study found that the development of vortex nucleus and turbulent kinetic energy and the disordered areas are mainly concentrated in the high-incidence area of vacuol collapse,and with the increase of the angle of attack,the fluctuation range of the vortex core and the turbulent kinetic energy also increases,and the velocity gradient distribution of the vortex core is also extremely obvious.In addition,the cavitation collapse process has a certain degree of influence on the hydrodynamic characteristics of the hydrofoil.5.Comparing the test with the simulation results,it can be seen that when the cavitation number is the same,as the angle of attack increases,the degree of cavitation gradually increases,the cavitation is more likely to fall off,and the volume of the cleaved cavitation increases accordingly,resulting in a longer period of collapse of the bubbles.The impact oscillation caused by the collapse of the large bubble greatly affects the surrounding flow field,and has a certain inhibitory effect on the growth and development of the hydrofoil leading edge.Due to the presence of a certain amount of non-condensable gas in the test water during the test and the processing accuracy of the test model,the volume of the detached bubble cluster is larger than that of the simulation,and the corresponding bubble collapse time period is longer.Although there are some differences between the experimental results and the numerical simulation results,the whole process tends to be consistent.6.The micro-molecular dynamics method was used to study the cavitation collapse process of liquid argon between parallel solid surfaces.It was found that different relaxation time,temperature and initial cavitation core size had a certain degree of influence on the collapse of cavitation nucleus.The results show that when the relaxation time is short,the time of complete collapse of the vacuolar nucleus is shorter;when the relaxation time is longer,the longer the bubble collapses,but as the relaxation time continues to increase,the space is empty.There is no significant change in the time of bubble collapse.In a certain temperature range,as the temperature increases,the time for the bubble to reach the collapse point is shorter,and when the temperature reaches a certain value,as the temperature continues to increase,the time at which the bubble core reaches the collapse point is basically maintained.constant.In addition,there is a critical value for the initial size of the cavitation nucleus within a certain range.When the initial size of the cavitation nucleus is smaller than the critical size,the cavitation is more likely to collapse;When the initial size of the cavitation nucleus is larger than the critical size,the cavitation nucleus is relatively stable and not easily collapsed.