Investigation Into Transient Characteristics Of Two-dimensional Hydrofoil In Cavitation Inception

Cavitation is a physical phenomenon of unsteady complex flow occurs in liquid,involving the aspects of multiphase flow,compressible,interphase exchange,etc,and has significant influences on fields such as hydraulic engineering,irrigation engineering,marine engineering,weapon in water engineering and nuclei engineering.In order to predict and assess cavitation performance more accurately,it is necessary to do the research on mechanism of transient characteristics in incipient cavitation condition,and analyze the transient characteristics in incipient cavitation.This manuscript uses liquid argon and water as microcosmic research object to study the change of thermodynamic parameters in the process of single cavitation nuclei here,then NACA4412 is selected as macroscopic research object,the microcosmic study of cavitation nuclei is applied to the cavitation transient characteristics of macroscopic two-dimensional hydrofoil.Numerical simulation is carried out,and the experimental method is verified to the simulation result of two-dimensional hydrofoil transient characteristics.The main contents are as follows:1.After referring data,theoretical basis knowledge of cavitation,bubble nucleition,etc are introduced in this manuscript.What's more,the study method of MD,the theoretical basis knowledge of phase transition and gas state equation are also presented here.The molecular force field,boundary condition and simulation process of MD are described,then the definition in various ensemble under calculation process and the applicable type are elaborated in this manuscript.The choice and application of molecular dynamics software are introduced in detail.In the third and fourth chapters of this manuscript,LAMMPS software is used to calculated the results,and the L-J(12-6)force field and periodic boundary conditions are chosed here.2.For the study of cavitation nuclei transient characteristics in cavitation inception,we apply molecular dynamics(MD)simulation to study Lennard-Jones(L-J)fluid with different initial cavitation nuclei under the NVT-constant ensemble.The results shows that in cavitation inception,due to the decrease of liquid local pressure,the liquid molecules would enter the cavitation nuclei,which contributs to the growth of cavitation nuclei.By using molecular potential energy,it is found that the molecular potential energy is higher in cavitation nuclei part,while the liquid molecular potential energy changes greatly at the beginning of the cavitation nuclei growth.The density of the liquid and the surface layer changes more obvious,but density of vapor in the bubble changes inconspicuously.With the growth of cavitation nuclei,the RDF peak intensity increases,the peak width narrows and the first valley moved inner.With the decrease of the initial cavitation nuclei,the system pressure and total energy achieves a balance longer,and correspondingly,they are smaller.In addition,at the beginning of the cavitation nuclei growth,the total energy and system pressure change greatly.3.In order to study the cavitation nuclei transient characteristics in cavitation inception,calculated domain of water molecules with cavitation nuclei is carried out under the NPT ensemble by using the molecular dynamics simulation,analyzing the evolution of cavitation nuclei,the system pressure,system energy,molecular radial distribution function and such thermodynamic parameters.The results show that as the system initial pressure is constant,the cavitation nuclei size had a critical value.When the initial size of the cavitation nucleus is less than the critical value,even if the liquid was affected by the negative pressure,due to the intermolecular force and the hydrogen bond force,cavitation nuclei could not grow,so cavitation did not occur.Cavitation in the micro-level is not just the pressure and the change of cavitation nuclei,mainly caused by the increasing energy of the system and structural instability due to the initial system pressure,resulting in cavitation nuclei changes.As the internal cavitation nuclei size is constant,there is a critical value of external pressure.When the external pressure value is less than the critical value,the corresponding negative pressure would not be generated inside the calculated domain,or the negative pressure was large,not enough to make the system instability,the cavitationnuclei could not grow under the intermolecular force and the hydrogen bond force,so cavitation do not occur.With the growth of cavitation nuclei,the system energy and potential energy tend to increase,and the increasing trend affected the stability of the internal structure of the calculated domain,and prompts the cavitation nuclei to increase rapidly.4.The Z-G-B cavitation model was optimized by the process of nanometer scale cavitation growth in chapter 4 and the micron scale cavitation growth process at present,based on this,the RNG k-? turbulence model and the optimized cavitation model are used to simulate the incipient process of two-dimensional hydrofoil cavitation.In incipient cavitation,the pressure field produced by different attack angles is slightly different,therefore,the possibility of cavitation instability around cavitation nuclei is not the same,which causes a different probability of cavitation growth and development on the surface of the hydrofoil.The larger the attack angle is,the smaller the extreme low pressure becomes,and the negative pressure of the liquid around the cavitation nucleus gradually increases,which leads the potential energy increases and the cavitation nuclei grow quickly.Via the research of micro-cavitation nuclei,it is found that the probability of cavitation growth is different under different pressure fields.Therefore,under the influence of different attack angles,the development scale and intensity of the bubble is different.In the latter period of cavitation,the interaction between low pressure and high pressure directly causes the collapse and development of obscission cavitation bubbles,thus affecting the distribution of flow and the generation of vortices.Cavitation incipient is only affected by low pressure,while cavitation regeneration is influenced by double effect,the low pressure and turbulent flow field.