Utilization Of DSMC Method And Bridging Relation For Transitional Regime Aerodynamics Of Long-range Shells And Rockets

On the basis of the Knudsen number(Kn)and the degree of rarefaction of atmosphere,the flight altitudes of a high-altitude aerocraft can be divided into three different regions,i.e.continuum flow regime at low altitude in the dense atmosphere,free-molecule flow regime at very high altitude in very rarefied atmosphere,and transitional flow regime in between.Freemolecular flow regime and transition flow regime are collectively called rarefied regime.The calculation of aerodynamic force coefficients in continuum flow regime is based on the continuous medium hypothesis.But the hypothesis is no longer valid in rarefied flow regime,the N-S equation is also invalid.At present,The flight altitude of the long-range shells and rockets can reach 80 km or even more,it can fly in the transitional region for quite a long time.Therefore,establishing a model,which can calculate the aerodynamics coefficients in transitional regime,has great significance on the study of long-range shells and rockets.And the combination of DSMC and bridging relations is one well-established method to annalyze aerodynamics characteristics in transitional regime.Firstly,the mathenatical models which will be applied in the DSMC simulation method has been studied,including the collision models between two molecules,the expression of post collision velocities,the reflection of molecules on the surface of the object.And the implementation and application of them in the DSMC procedures is established.Then,the DSMC program is employed for simple shaped bodies in rarefied flow under different Mach and Knudsen numbers.Secndly,in order to quickly attain the aerodynamics coefficients in the transitional regime at the initial design stage of the long-range shells and rockets,an engineering calculation method-bridging relation,is proposed.Based on the results of DSMC simulations,sinesquared and erf-log bridging relations are constructed and their applicability and accuratcy in calculating the aerodynamics coefficient in transitional flow regime are analyzed.It appers that the two bridging relations can both calculate aerodynamic coefficients well.However,the undetermined parameters in the erf-log bridge relation depend on the results of DSMC simulation,which cause the sine-square bridge function is not fast enough and convenient to calculate the aerodynamic coefficients of long-range shells and rockets in the transitional regime.Hence,the sine-squared bridging method is chosen to calculate the aerodynamic coefficients of the long-range shells and rockets in the transitional regime.Finally,based on sine-squared bridging method,an integrated model of Bridging-RelationCalculation,which is suitable to the engineering design,is forward here.And the feasibility of the model is verified by the DSMC simulation results of three-demension blunt-nosed Cone.Then the aerodynamic coefficients of a long-range shell and rocket in the transitional region are calculated and analyzed by using the model.The results show that the rarefied gas effect has a great influence on the aerodynamic characteristics in the transitional regime.With the enhencement of rarefied gas effect,the lift,drag and pitch moment coefficients of the Longrange shells and rockets increase obviously,while the lift-drag ratio decreases and the pressure center moves forwards.