【作者】 徐珊姝；

【导师】 吴子牛；

【作者基本信息】 清华大学 ， 力学， 2008， 博士

【摘要】 相对于某固定尺寸的飞行器,根据气体的稀薄程度不同,可以将大气分为:低空高密度的连续流区,高空极低密度的自由分子流区和介于两者之间的过渡区。飞行于过渡区中的飞行器可以称为过渡区飞行器。随着航空航天事业的发展,越来越多的新型飞行器出现在过渡区中,其周围流场的大部分区域或局部区域存在着明显的稀薄效应,甚至可能出现连续流和稀薄流混合的复杂流场。为了适应这些过渡区飞行器的发展,需要对它们周围的流动问题进行深入研究。本文重点围绕过渡区飞行器所遇到的特殊流动问题,建立相应的数值计算程序,对其进行模拟研究,同时也讨论部分流动机理问题和工程方法的适应性问题。首先,建立了CFD/DSMC混合计算程序,首次对在高空过渡区飞行的火箭模式激光推进器周围的高温连续/稀薄混合流场进行了数值模拟研究,并与低空大气吸气模式激光推进器周围的连续流场作对比。研究发现,过渡区火箭模式流场中,激波厚度明显增加,激波传播速度加快,推力产生提前,得到的比冲大于800s;另外,高温效应则减慢了激波传播速度,延迟了推力的产生,并导致冲量耦合系数和比冲降低。之后,采用DSMC方法,对过渡区稀薄环境下的激波反射规律进行了数值模拟研究,分析了稀薄效应对马赫数变化引起的迟滞现象以及马赫杆高度的影响。研究表明,在本文给定的条件下,存在一个临界努森数,当来流努森数小于该值时,会发生正规反射到马赫反射的转捩,反之则无;另外,随着努森数的增加,马赫反射中的马赫杆高度呈近似线性降低。最后,分析了sine-squared和erf-log两种桥函数在一般超音速过渡区气动特性系数计算方面的适应性问题。研究发现,随着飞行马赫数的降低,sine-squared桥函数的精度明显下降,而erf-log桥函数始终保持很好的精度。建议采用erf-log桥函数,计算以低超音速飞行的亚轨道飞行器的过渡区气动特性系数。更多还原

【Abstract】 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.更多还原