【作者】 赵海洋；

【导师】 任兵； 刘伟；

【作者基本信息】 国防科学技术大学 ， 流体力学， 2007， 博士

【摘要】 再入返回舱为了获得较大的气动阻力并减少气动加热,通常采用钝倒锥外形,然而在跨声速区这种外形经常是动态不稳定的,受到外界扰动后容易进入表现为极限环形式的自激振动状态。目前国内外对返回舱动态失稳的物理机理及其控制方法十分关注,但仍有许多问题尚未解决。本文主要针对返回舱跨声速区的动态不稳定现象,构建适用于计算气固耦合的非定常复杂流场的时、空高阶精度格式,采用数值模拟方法探讨返回舱动态不稳定特性的物理机理;运用机理分析的结果,研究返回舱动态不稳定特性的被动控制方法;最后采用计算空气动力学、飞行力学和控制技术相结合的多学科融合的研究方法,对返回舱动态不稳定问题的主动控制方法进行初步的探索。首先,基于返回舱非定常流场多模态特性对格式精度和高分辨率的要求,引入Power限制器,对NND格式和WNND格式(包括通量型和守恒变量型)进行改进,构建了PNND格式和PWNND格式(包括通量型和守恒变量型)。在此基础上,针对不同物理问题特点,建立了时间导数离散方法不同的显式和隐式PNND格式、PWNND格式,并通过求解线性方程、一维Euler方程、三维定常/非定常Navier-Stokes方程,对PNND格式和PWNND格式进行了数值分析和代码验证。其次,采用时、空高精度的PWNND数值计算方法,分别对返回舱定常流场、强迫振动流场和自由振动流场进行了数值模拟,分析返回舱跨声速区动态不稳定机理。①定常流动计算结果表明:跨声速区,后体在整个气动力贡献中所占的比例较大;攻角较小时,后体涡核呈闭合环状;攻角增大时,后体涡环逐渐断裂为两部分,其中处于下方的马蹄涡占主导地位,控制整个回流区内的流动。②返回舱强迫振动/自由振动流场的俯仰力矩迟滞曲线基本呈双“8”字形。对前、后体俯仰力矩曲线作出分析的结果显示:返回舱的动态不稳定性主要由其后体俯仰力矩系数的滞后引起,而前体存在相位前移,起到增强返回舱动态稳定性的作用。③分析了反映返回舱动态特性的特征尺度的选取方法;根据流场与返回舱之间的能量交换关系,提出了减少流场对后体做功(减少外界能量输入)的被动控制方法,即可通过修改返回舱的后体外形来改善返回舱的动态稳定性,为返回舱动态不稳定的被动控制提供了理论依据。再次,基于返回舱动态不稳定性的机理分析,分别从修改后体外形、移动质心位置以及两者组合的方法出发,研究了各种被动控制方法对返回舱俯仰动态不稳定的控制效果。①对六种不同后体外形的返回舱自由振动流场的计算结果表明:其中五种外形都通过减少振动过程中流场对返回舱后体做功,有效地增强了返回舱的动态稳定性(与原始外形相比极限环振幅减小),另外一种外形由于底部面积增加(造成外界能量输入增加)而没有达到控制目的。②对三种不同质心位置的返回舱自由振动流场的计算结果表明:质心位置越靠近返回舱大头顶点,振动周期越小,一个振动周期内前体对流场做的功就越多,其动态稳定性也就越好;相反,质心位置距离返回舱大头顶点越远,其动稳定性越差。③对三种修改外形与移动质心位置的组合被动控制方法的返回舱自由振动流场的计算结果表明:组合控制方法比单独一种方法能够更有效地改进返回舱的动态稳定性。最后,采用计算空气动力学、飞行力学和控制技术相结合的多学科研究方法对返回舱俯仰动态稳定性主动控制问题进行了初步的探索。利用简化的双姿控发动机组成的控制系统,采用时、空高精度数值计算方法及多种控制技术,对带横向喷流的流固耦合流场进行了数值模拟,通过计算结果分析比较了不同控制方法的控制效果。①研究了带横向喷流飞行器的强迫振动流场,结果表明:强迫振动喷流流场的气动特性与稳态喷流状态有明显的区别,飞行器的振动对横向喷流流场产生了非定常效应,使得推力放大因子随俯仰角周期变化,与固定姿态下得到的推力放大因子存在差异。②第一种经典控制方法在加大阻尼比后,能够将返回舱改出极限环振动状态,但当俯仰角速度最终稳定在零附近做小振幅振荡时,姿控发动机的控制力矩很小(控制力矩仅与俯仰角速度相关),不能够把返回舱控制在平衡位置,偏差较大。③与第一种方法相比,第二种经典控制方法在控制量中考虑了俯仰角的贡献,阻尼比较小时,返回舱仍然很难改出极限环振动状态,提高阻尼比后,返回舱能够很快改出极限环振动状态;随着自振角频率的增加,返回舱能够在更短的时间内达到基本稳定状态,并且俯仰角收敛至更加靠近平衡攻角的位置。针对第二种经典控制方法设计了一种分段自振角频率控制策略,使得返回舱更加逼近平衡位置,控制效果得到改善。④智能化的模糊控制方法避免了经典控制方法对运动数学模型过于依赖的问题,数值模拟结果显示:除了第一种经典的模糊控制方法存在调节死区外,其它四个模糊控制算例都能够在不需要控制系统精确数学模型的基础上,对返回舱的动态不稳定特性进行有效的控制。更多还原

【Abstract】 Reentry capsules typically have blunt conical noses, which are designed to achieve the aerodynamic drag and decrease the aerodynamic heating. Unfortunately these blunt bodies tend to experience dynamic instability and this can lead to angular motions that frequently develop into limit cycles. The dynamic instability of the reentry capsules has been studied for long time, but many problems still remain unresolved. In order to study the mechanism of dynamic instability, it has to de done at first to simulate the transonic steady/unsteady flowfield of reentry capsules with high order accuracy schemes developed in this thesis. The passive control methods are developed on the basis of study of the mechanism of dynamic instability. With computational fluid dynamic, the intercourse of flight mechanics and control techniques, active control methods are developed elementarily to weaken or eliminate the dynamic instability of reentry capsules.Firstly, based on the demands for schemes in simulating the unsteady flowfield numerically, the NND schemes and WNND(Weighted NND) schemes are updated with the power limiter. The second-order accurate flux- or conservational variable- PNND (Power NND) schemes and the third-order accurate flux- or conservational variable-PWNND(Power WNND) schemes are designed. According to different characteristics of physical problems, different explicit or implicit time discrete schemes are constructed. Some typical numerical tests, including the linear-wave equation, 1D Euler equations and 3D Navier-Stokes equations, are presented to demonstrate the excellent capability of the PNND schemes and the PWNND schemes in reducing smearing near discontinuities and getting good resolution in smooth regions.Secondly, with the high order accurate conservational variable- PWNND schemes, the steady flowfield, the forced oscillation flowfield and the free oscillation flowfield are simulated to analyze the mechanism on the dynamic instability of reentry capsules.①The numerical results of steady flowfield show: The proportion between base aerodynamic and the total aerodynamic in transonic speeds is larger than that in supersonic speeds. The ring vortex distorts with the incident angle increasing, the horseshoe vortex becomes dominant behind the capsule.②The numerical results of forced/free oscillation flowfield show: The hysteresis loops of pitch moment coefficient become in the style of double "8" in C_m～θplane. The dynamic instability is caused by the hysteresis of the base pitch moment coefficient.③The characteristic length and velocity are analyzed for the dynamic instability of reentry capsules. According to the energy relation between the capsules and the flowfield, the passive control methods of modifying the base shape are proposed to improve the dynamic instability of reentry capsules.Thirdly, based on the analysis of the mechanism on the dynamic instability, the control effects are evaluated for several passive control methods including base shape modified, mass center moved and the combination methods.①The numerical results of free oscillation flowfield with six passive control conditions of base shapes modified show: Five shapes improve the dynamic stability(the swing of the limit cycle decreases) and one fails with the energy increasing from the flowfield.②The numerical results of free oscillation flowfield with three passive control conditions of mass center moved show: The work conducted by the front body of the reentry capsules to the flowfield increases with the mass center moving to the top point of the reentry capsules, and the dynamic stability turns better. The dynamic stability turns worse with the mass center moving to the base of the reentry capsules.③The numerical results of free oscillation flowfield with three combination passive control methods show: To improve the dynamic stability of the reentry capsules, the combination passive control methods should be considered.Lastly, with computational fluid dynamic, the intercourse of flight mechanics and control techniques, active control methods are developed elementarily to weaken or eliminate the dynamic instability of reentry capsules. The unsteady coupling flowfield with cross jet are solved in simplified attitude control system by the high order accurate PWNND schemes. The results are analyzed to evaluate the control effects.①The numerical results of forced oscillation flowfield of a space vehicle with cross jet show: During the forced oscillation, the normal force amplification factor changes with the pitching angle periodically. The oscillation of the space vehicle makes unsteady effects on the jet flowfield and the aerodynamic performance shows great difference with the steady jet flowfield.②The first classic active control method can break the limit cycle oscillation with the damp ratio increasing. The control moment turns weak when the angular velocity of the reentry capsule converges to zero. The reentry capsules can’t keep the balance place and the error of the pitch angle and the balance place is a bit large.③The second classic active control method considers the contribution of the pitch angle. This control method can also break the limit cycle oscillation with the damp ratio increasing. With the angular frequency increasing, the reentry capsules can get the steady state in shorter time. The reentry capsules can get closer to the balance place. A control strategy with subsection angular frequency is introduced to the second classic active control method. The pitch angle gets more close to the balance place and the control effect gets better.④In the fuzzy control methods, the exact oscillation mathematic model of the reentry capsule isn’t needed, which is indispensability for the above two classic active control methods. The numerical results show: The first classic fuzzy control method has a bug that the adjust dead region appears during the control process. The other four fuzzy control examples all display effective control to the dynamic instability of the reentry capsule without the exact oscillation mathematic model.更多还原