Numerical Simulation Of Flows For Vehicle Flying In The Transitional Regime

According to the characteristic length of an aircraft and the degree of rarefaction of a gas, the atmosphere can be divided into three regimes: continuum regime at low altitude where the density of ambient gas is high, free-molecule flow regime at very high altitude where the gas is very rarefied, and transitional regime between the above ones. The vehicle flying in the transitional regime is called as transitional-regime aircraft. With the development of aerospace science, more and more new aircrafts appear in the transitional regime. Most or part of the flows around them has obviously rarefaction effect. Sometimes, there are hybrid continuum/rarefied flows. In order to follow the development of these transitional-regime aircraft, we need to study these complicated flows.This thesis presents a study of some special flows for vehicle flying in the transitional regime by our own numerically computational programs. We also discuss some flow mechanism and analyze the accurate of engineering methods.Firstly, a hybrid CFD/DSMC program is completed, and firstly used to simulate the high temperature hybrid continuum/rarefied flow around a laser thruster, which is flying in the transitional regime at higher altitude and operating in a rocket mode. We also perform numerical computation for the continuum flow under air-breathing mode at lower altitude for comparison with the above hybrid flow. For the rocket mode flow, the rarefied-gas effect increases the shock thickness obviously. The closing of the inlet speeds up shock movement and advances thrust production. We obtained a specific impulse of about 800s. Moreover, high temperature effect delays the shock speed, thus delays the thrust production, and reduces impulse coupling coefficient and specific impulse.Secondly, DSMC approach is used to simulate the shock reflection phenomenon in the transitional regime where the ambient gas is rarefied. We also analyze the rarefaction effects on the flow-Mach-number-variation-induced hysteresis and the height of Mach stem. We observe a critical Knudsen number at the given condition in this thesis. If the freestream Knudsen number is lower than that value, the transition from the regular to Mach reflection appears. Otherwise, the transition phenomenon disappears. Moreover, with the Knudsen number increasing, the Mach stem height decreases linearly.At last, we analyze the accurate of sine-squared and erf-log bridging relations for calculating the supersonic aerodynamics in the transitional regime. It appears that with the Mach number decreasing, the accurate of sine-squared bridging decreases, but erf-log bridging prove adequate. Thus, the erf-log bridging method is recommended to predict the low supersonic aerodynamic coefficients of the sub-orbital spacecrafts.