The Research On Mechanism Of Drag Reduction By Base Bleed On Projectile

The principle of base bleed is to append a base bleed unit at the bottom of a conventional projectile.In the early fight stage,the base bleed projectile charge low moment,high energy gas to increase the base pressure,resulting in reducing the pressure difference and achieving the purpose of extended range.In this paper,based on the potential enormous and advantgaes of computational fluid dynamics,the numerical calculation of two-dimensional/three-dimensional multiblock structure grid chemical nonequilibrium flow is developed,the flow and combustion model of base bleed projectile is established,and the reducing drag characteristic and some key issues of base base projectile under the condition of the real base bleed gas is analyzed.It can provide an important reference for the technology and theory of base bleed,the project design of base bleed projectile.The main work is as follows:Firstly,for the chemical reacting flow,based on the multi-component Naiver-Stokes equations,an efficient parallel method on the structure grid is developed,using higher-order upwind schemes,finite rate chemical reaction model and OpenMP parallel algorithm.Reconstruction of physical quantities on the grid interface use three methods including NND,WENO,MUSCL.Some schemes including Steger-Warming,Van Leer,HLLC,AUSMDV and AUSMPW+ can be used for the inviscid flux vetor calculation.Turbulence model equation use one turbulence model and two equation turbulence,and the turbulence flow model is decouplely solved with the flow equations.The chemical reaction source terms use the time operator splitting algorrithm to avoid the rigidity of chemical reaction.Using OpenMP parallel to improve numerical simulation efficiency.Numerical verification show that the numerical method developed in this paper have high reliability,and the precision meet the need for engineer.It can provide a reliable numerical tools for the research on the chemical reaction flow field under the comdition of supersonic flow.Secondly,using the developed numerical methods in this paper,a reasonable and reliable flow and combustion model of base bleed projectile are established.The effect of bleeding the hot air/real gas and Mach number on the structure of the base flow field and the bottom pressure are researched.The major region and combustion process are analyzed where the hot air/real gas on the bottom appeared secondary combsution.The effect law and mechanism of the exhaust parameters,the exhaust area,and total exhaust temperature on the structure of the bottom flow field and the drag redcution characteristics and mechanism are researched.The effect of attack angle and boattail angle on the bottom of the flow field structure and the aerodynamic characteristics of entire projectile are researched under the conditions of bleeding the real gas on the bottom,then the variation laws of the structure of the bottom flow field and the aerodynamic characteristics following the change of attack angle and boattail angle are got.Thirdly,In order to research the law about the effect of type of base bleed to flow structures and drag reduction in supersonic flow,edge bleed type is proposed,and the reduction drag performance of center bleed type and edge bleed type are carried out.Calculation results show that:within the applicable range,the drag recution performance of edge bleed type is superior to the center bleed type;when the base bleed parameters is large,the drag recution performace of center bleed type is poor,and the drag recution performance of edge bleed type is good.Fourthly,the dynamic overlapped grids approach was applied to deal with the problems of a high-speed moving base bleed projectile,the nuerical simulation of muzlle flow field containing the motion bleed projectile,and this present numerical study confirms that complicated transient phenomena exist in the early launch stages after the base bleed projectile moves from the muzzle of cannon to the free-flight stage.The results show that:under the condition of launching velocity of 900m/s,launching pressure of 60MPa,within 1.0ms time,the depressured rate in base bleed unit is up to 104MPa/s;after the time 3.0ms,the base bleed projectile through the muzzle blast shock,and go into the free flight stage.Fifthly,a calucation model is established coupling particle trajectory,flow simulation of base bleed projectile and aerodynamic characteristics of the projectile through combining present numerical method with the particle trajectory model.And the calculation model can be used to reliably compute flight process of the projectile,as well as time history of operating parameters,working state and the projectile flow field under different flight conditions.Corresponding results indicate that the combustion time,flight time and firing range increase as altitude of launch site is enhanced with a gradual decrease of the rate of extended rang.Both the firing range and the rate of extended range will be narrowed if the time of combustion and reduction stage are shorten by improving burning rate of the grain.Therefore,for the sake of a better drag reduction in engineering application,two matches should be taken into consideration.One is the match of projectile mass rate and its time variation with flight conditions,and the other is placed between the value of burning rate as well as burning area and their change rules.