Study On The Flow Pattern And Cavity Shedding Characteristics Of Ventilated Cavitating Flows

Due to its high precision,long range and good concealment,the submarine weapons attract more and more attentions in the military power.In recent years,in order to reduce the water-exiting load of the submarine missile,a new technology,ventilated cavitation,is used to improve the mechanical environment of the projectile in the launching process at home and abroad.However,with the entry of non-condensable gas,a variety of flow pat-terns have been occurred in the ventilated cavitating flows,which is crucial to the success-ful launching of the projectiles.While in the ventilated cavitating flow,the strong interac-tion between the ventilated air,vapour and liquid makes the investigation of multiphase flow characteristics even more complex.Therefore,to study the flow characteristics in the ventilated cavitating flows has great significance in theory and practical value in engineer-ing.In order to address the critical technical issue related to the engineering development,the flow pattern and cavity shedding characteristics of ventilated cavitation flows around an axisymmetric body have been investigated via combined physical and numerical studies.The main contents and finding in this work are as follows:The ventilated cavitating flow patterns around an axisymmetric body at different an-gles of attack are mapped with the flow conditions.Four types of the relatively stable ven-tilated cavity,which are continuous transparent and asymmetric cavity(CTAC),bubbly cav-ity(BC),continuous transparent and bubbly cavity(CTBC)and super cavity(SC),and three unstable shedding cavity flow patterns,which are foamy cavity(FC),continuous transparent and foamy cavity(CTFC)and hybrid continuous and foamy cavity(CFC),are defined.The effects of flow parameters and ventilated parameters on the flow pattern are analyzed.The Fround number and the gas entrainment coefficient play a significant role in the transfor-mation of the flow patterns.When Froude number is relatively low,CTAC occurs with the twin-vortex mode of cavity closure.With the increasing of the Fr number,the gravity ef-fects on the cavity shape reduce,and the cavity quickly transitions from the twin-vortex mode to the re-entrant jet mode to cause the flow pattern to change to CTBC while the gas entrainment coefficient.When Fr>9.1,the flow pattern migrates to unstable shedding cavi-ty flow pattern from the relatively stable ventilated cavity.With the increasing of the value of the gas entrainment coefficient,the length of the cavity has a significant increase to cause the flow pattern to change.In the evolution of the relatively stable ventilated cavity,the flow pattern migrates to BC and then to SC with the increasing of C_Q.As for the unsta-ble shedding cavity,the flow pattern migrates to CFC and then to CTFC.The ventilated cloud cavitating flow evolution and the cavity shedding behaviors around an axisymmetric body have been investigated.The ventilated cloud cavitating flow evolution around an axisymmetric body can be divided into three parts:fast growth,growth with slightly fluctuation and quasi-periodic shedding.During the third stage of the cavity,the periodical shedding of the large cloud cavities caused by the re-entrant jet has strong impacts on the unsteady flow.The shedding cavities move downstream without intense collapse.The ventilated cloud cavity structures along the circumference is always axym-metric,resulting to the obvious three-dimensional effect of the cavity.The irregular break-off of the ventilated cloud cavity and the shedding of the large-scale U-type cavity at the middle position of the cavity are caused by the three-dimensional effect and different velocities of the re-entrant jet flow.With the increasing of the angle of attack,the deflection angle of the cavity decreases and the momentum of the re-entrant jet reduces gradually,re-sulting to the cavity break-off position moving downstream.In addition,with the increasing of the angle of attack,the thickness of the cavity approaches that of the re-entrant jet gradu-ally.The interaction between the re-entrant jet and the gas inside the cavity becomes violent,which causes the frequent shedding of the cavities.A non-dimensional criterion is derived for the ventilated cloud cavitiy shedding around an axisymmetric body is derived during the vertical launching process.The control parameters of cavity stability,non-dimensional,gas entrainment coefficient,cavitation number(Euler number)and Froude number,are derived out in the vertical launching pro-cess.Under the effect of gas entrainment coefficient,Euler number and Froude number,three kinds of cavity shedding scale,which are local small-scale shedding,large-scale vor-tex shedding,hybrid local small-scale and large-scale vortex shedding,are defined.With the increasing of Froude number,the ventilated cavity shedding pattern develops to large-scale vortex shedding from local small-scale shedding.Under the same Froude num-ber and gas entrainment coefficient,with the increase of Euler number,the cavity length increases and the cavity shedding scale decreases effectively.In addition,based on the di-mensional analysis and CFD simulations,the formula and the specific values of the formula parameters are determined.Different ventilated cavitating flow patterns under various lauching conditions are discussed.With the increasing of cavitation number and launch depth,and the decreasing of Froude number,the cavity shedding process is easier to be triggered.The cavity length increases effectively with the increase of gas entrainment coef-ficient,which improves cavity stability.