Study On Locally Rarefied Gas Effect Of Hypersonic Flow Based On Direct Simulation Monte Carlo Method

The design of high-speed near-space vehicles requires accurate prediction of the aerodynamic characteristics of the aircraft.However,during the process of passing through the atmosphere,as the flight altitude changes.The flight process will go through different gas flows from continue to rarefied,and complex nonequilibrium effect such as high temperature and rarefied gas effect often appear in the flow field.Therefore,in order to realize the maneuvering and control of the aircraft,its aerodynamic characteristics must be accurately predicted.Computational Fluid Dynamics(CFD)is one of the main methods.When the flight altitude is low,the incoming gas density is greater and the traditional CFD method based on Navier-Stokes(NS)equation can be used.However,as the flight altitude increases,the incoming flow density decreases.The method will gradually fail and need to use Direct Simulation Monte Carlo(DSMC)method.In this paper,a flat plate is used as a simplified calculation model of a near-space vehicle,and the hypersonic flat plate problem on locally rarefied gas effect and the degree of failure of traditional CFD is studied based on the direct simulation Monte Carlo method.Firstly,numerical simulations use DSMC and CFD methods are carried out on a flat plate model with a flight height of 60～80km and an incoming Mach number in the range of 10～25.The effects of flight height and incoming Mach number on continuum failure parameters,flow field macroscopic quantities and surface aerodynamic quantities are studied.Based on the rarefied characterization parameter Zh,the error variation law of surface friction and heat transfer coefficient calculated by NS relative to DSMC is quantitatively given.The study found that the rarefied areas are mainly concentrated in the front edge of the flat plate,in the boundary layer near the wall and at the location of the shock wave.Taking the DSMC results as a reference,NS underestimated the peak temperature in the boundary layer,and the calculated surface friction and heat transfer coefficients were relatively large.As the flight altitude and incoming Mach number increase,the rarefied gas effect increases,and the difference between the two calculation results gradually becomes larger.Secondly,the DSMC and CFD methods are used to simulate the experimental conditions of the hypersonic flat plate,the difference between the numerical results and the experimental results is compared and analyzed,and improved boundary conditions are proposed.The study found that the results calculated by DSMC are more consistent with the experimental results,and the results calculated by the traditional CFD method are somewhat different from the experimental results.In this paper,the improved NS method developed by the research group is used to simulate the experimental conditions,and the boundary conditions based on the rarefied characterization parameter Zh are proposed.The results show that the improved method can improve the boundary layer profiles,surface friction and heat transfer coefficient calculated by traditional NS,and the velocity profile is more consistent with the experimental results.Finally,the DSMC method is used to simulate the rough plate model,and the effect of the cavity on the flow field structure and the surface aerodynamic force and heat is studied.The study found that the cavity aspect ratio determines the flow field structure in the cavity.The cavity affects the flow field only near the cavity.Local high heat flux can generate in the cavity and the heat flux can reach more than 2 times level of the smooth plate.The cavity does not significantly change the drag of the plate,but transforms between friction and pressure load as the aspect ratio changes.The increase of the surface roughness at the micro-scale will lead to the increase of the drag coefficient.The effect of the rough plate can be equivalent by coordinating the surface adaptation coefficient of the smooth plate.