跨流域高超声速绕流环境Boltzmann模型方程统一算法研究

如何准确可靠地模拟从外层空间高稀薄流到近地面连续流的航天器高超声速绕流环境与复杂流动变化机理是流体物理的前沿基础科学问题. 基于对Boltzmann方程碰撞积分的物理分析与可计算建模, 确立了可描述自由分子流到连续流区各流域不同马赫数复杂流动输运现象统一的Boltzmann模型速度分布函数方程, 发展了适于高、低不同马赫数绕流问题的离散速度坐标法和直接求解分子速度分布函数演化更新的气体动理论数值格式, 建立了模拟复杂飞行器跨流域高超声速飞行热环境绕流问题的气体动理论统一算法. 对稀薄流到连续流不同Knudsen数0.002 ≤Kn_∞ ≤1.618、不同马赫数下可重复使用卫星体再入过程(110–70 km)中高超声速绕流问题进行算法验证分析, 计算结果与典型文献的Monte Carlo直接模拟值及相关理论分析符合得较好. 研究揭示了飞行器跨流域不同高度高超声速复杂流动机理、绕流现象与气动力/热变化规律, 提出了一个通过数值求解介观Boltzmann模型方程, 可靠模拟高稀薄自由分子流到连续流跨流域高超声速气动力/热绕流特性统一算法.     

高超声速飞行器前缘疏导式热防护结构的实验研究

针对高超声速飞行器前缘疏导式防热结构的特点,设计前缘内嵌高导热率材料结构和一体化层板热管结构两类对比实验,用于验证前缘疏导式防热结构的可行性.利用球形短弧氙灯作为辐射热源模拟气动加热,分别对钢质前缘、内嵌铜材料的钢质前缘和一体化层板式热管前缘进行加热,测量前缘驻点区域和尾部翼面区域的温度变化.实验结果表明:内嵌高导热率材料的前缘疏导结构能够降低头部驻点区的温度,提高尾部低温区的温度,实现对前缘结构的热防护;以蒸馏水作为工质一体化层板式热管前缘结构,在较低热流条件下也能够实现对前缘驻点区的疏导式热防护,但在较高热流条件下,由于水蒸气压力过大使得层板式前缘结构发生破坏,体现出热管内部工作介质对结构防热效果和应用范围都起到的关键作用.     

尖化前缘的稀薄气体化学非平衡流动和气动加热相似律研究

以近空间尖前缘高超声速巡航飞行器的研制为背景, 作者在前一阶段采用模型理论分析方法, 陆续研究了沿微钝前缘驻点线的化学非平衡流动和气动加热相似律, 文章是上述研究的综合回顾和深化讨论.稀薄条件下, 驻点附近流动和传热出现一系列与连续流动模型不同的新特征, 超出了经典气动热预测理论的适用范围.作者建立了一个沿驻点线能量传递和转化的广义模型, 并分别推导了具有实际物理意义的边界层外离解非平衡流动判据和边界层内复合非平衡流动判据.基于这些判据构建了预测非平衡流动驻点气动加热的桥函数, 并讨论了稀薄非平衡真实气体流动和气动加热的相似律, 发现新型近空间尖前缘飞行器遭遇的气动热环境不同于传统大钝头航天器再入问题, 传统的天地换算相似准则将会失效.这些理论分析结果可为稀薄非平衡化学反应流及气动加热的实验和计算提供一个标模检验的手段.      

稀薄气体流动非线性耦合本构关系研究进展

临近空间高超声速飞行器流场蕴含着复杂的非线性流动机理与丰富的热化学非平衡流动现象, 基于Newton摩擦定律和Fourier热传导定律的Navier-Stokes(N-S)方程不足以描述高超声速飞行器从连续流到稀薄流的多尺度非平衡现象。非线性耦合本构关系(nonlinear coupled constitutive relations, NCCR)作为一种全新的本构方程体系, 在严格满足热力学熵条件的基础上, 巧妙地构建了应力与热流的非线性表达形式。然而, NCCR方程的强非线性耦合特性是求解过程的一大难题。为了克服这一技术瓶颈, 提出了混合迭代算法, 为实现NCCR方程的高效稳定求解提供了坚实的理论基础。在该理论研究的基础上, 考虑到原始NCCR方程对热通量演化方程的简化处理, 降低了方程的计算精度, 提出了改进的NCCR+方程。该方程在强激波压缩区域和膨胀区域表现出比传统NCCR方程更高的计算精度与更强的非平衡流动模拟能力。同时, 为了解决临近空间高超声速空气动力学的多尺度与多物理效应耦合难题, 提出了NCCR与转动非平衡的耦合计算模型, 拓展了NCCR方程在双原子气体中的模拟能力。为了揭示稀薄气体效应与真实气体效应的耦合作用机理, 进一步建立了NCCR与热化学反应的耦合计算方法。大量研究结果表明, 考虑多物理效应的NCCR方程在低Kn下能够恢复到与N-S方程一致的解。随着Kn的增加, 流场的非平衡程度逐渐增强, 其结果与N-S方程差异显著, 而与DSMC方法计算结果和实验数据具有更好的一致性。      

Investigation on thermal performance of a high-temperature heat-pipe thermal protection structure

Sharp leading edges with a millimeter-scale radius are required for hypersonic vehicles from aerodynamic reasons. However, with the leading edges being so sharp, stagnation regions at wing and tail leading edges suffer a hostile thermal environment. Therefore, a high-temperature heat pipe is considered to be integrated into the structure of the leading edge to reduce the temperature of the stagnation point. In this paper, a superalloy-refractory composite-container-“wall” combined with the wick and working fluid structure is proposed, which is proved to be a feasible design of a heat pipe for the semi-passive thermal protection system (TPS). The effects of different material of the exterior surface on the temperature distributions are investigated. The effect of the half wedge angle, the design length and porosity of the wick is also investigated to find the effect of the geometry of the structure of the leading edge on the operation of the heat pipe.     

具有局部稀薄气体效应的高超声速尖锥气动加热特征研究

以微钝尖锥为飞行器前缘模型,采用基于分子运动论的DSMC方法模拟不同前缘曲率半径的尖锥在高超声速来流下的气动热环境,计算驻点热流率,并与Fay-Riddell公式和其他修正理论作对比,研究具有局部稀薄气体效应的高超声速尖锥气动加热特征及其变化规律.发现修正的Cheng参数适合作为工程上判断驻点区域稀薄气体效应影响大小的判据.     

前缘一体化高温热管结构防热效果的实验研究

为了研究复杂构型前缘一体化高温热管结构在高热流密度状态下的防热效果, 设计了飞行器气动加热轨道, 实现了高温热管低状态完全启动、高状态极限考核。然后采用超声速电弧风洞驻点自由射流结合轨道模拟技术, 模拟乘波体飞行器的前缘疏导构件的气动加热环境, 开展了前缘一体化高温热管结构防热效果研究。实验结果表明, 一体化高温热管结构能够多次使用, 低状态下高温热管的启动时间约为115 s, 在高状态下结构依然有效, 降温系数达到24.5%, 验证了前缘疏导式防热结构的防热效果, 可为未来新型高超声速飞行器非烧蚀热防护系统的设计提供指导。      

Structure and parameters of the shock layer formed when a supersonic underexpanded jet interacts with a counterpropagating hypersonic flow in the transition regime

The method of molecular dynamics (the Bird system) has been used to mathematically model a planar, strongly underexpanded supersonic jet that encounters a hypersonic flow of rarefied gas. Particular attention is paid to the structure and parameters of the shock layer close to the plane of symmetry. The results of calculations are presented for currents of a monatomic gas, simulating argon, with a Mach number of the external flow of M_∞=5.48, a Mach number at the nozzle edge of M_a=1, a ratio of the density at the nozzle edge to the density of the unperturbed flow equal to 130, and various stagnation temperatures of the external flow and of the jet. The evolution of the structure and the parameters of the shock layer as the Knudsen number Kn_∞ varies from 0.02 to 0.35 is considered. The results are compared with the data calculated for the shock layer when argon flows around thermally insulated cylinders. The main features and regularities of the relaxation of the translational degrees of freedom of the gas for external and jet flows are considered. Data are presented on the form of the distribution function over velocities and its evolution as gas moves through the shock layers. Zh. Tekh. Fiz. 68, 13–18 (July 1998)     

高超声速分离-再附流动峰值热流的模型理论

具有大尺度分离区的高超声速分离-再附流动中会在再附点后产生极大的热流峰值.通过分析再附点后局部区域的流动和传热特征,提出了斜驻点-三层结构模型理论框架.继而将此理论框架应用于中低Reynolds数的高超声速压缩拐角流动,提出了其再附点局域流动的外缘参数的工程求解方案,给出了对应的峰值热流值,并与数值计算结果做了对比验证...     

辐射平衡壁温下有限催化模型数值模拟

文章发展了高超声速飞行器辐射平衡壁温下有限催化的数值方法,将数值模拟结果与基于返回舱外形的风洞实验数据进行了对比,并进一步针对典型高超声速飞行器钝双锥研究了辐射平衡壁温下有限催化对气动热环境的影响规律.针对返回舱外形的数值实验表明,完全催化与完全非催化边界条件下壁面热流密度均与风洞实验结果偏差较大,而采用合适的有限催化模型获得的壁面热流密度与风洞实验结果符合良好.针对典型高超声速飞行器钝双锥的研究表明,在辐射平衡温度边界条件下驻点附近氧原子的催化复合系数约为0.17,氮原子的催化复合系数约为0.026,大面积区则分别降为0.005 3和0.01.在驻点热流密度方面,完全催化的壁面热流峰值比有限催化高约21%,而完全非催化的壁面热流峰值比有限催化低约29%.      

Determination of heat losses and their influence on the performance characteristics of high-enthalpy hot-shot tubes

An analysis of the losses of heat into the walls of settling chamber in a hypersonic hot-shot tube has been performed. Tests without diaphragm rupture showed that the fall of settling-chamber pressure during the operating flow regime in the tube was the consequence of the transfer of heat from working body to wall; this has allowed us to evaluate the heat-transfer coefficient α and the inner-surface temperature of the wall T _w. An empirical formula relating the coefficient α with the pressure and working-body temperature in the settling chamber in the range of pressures and temperatures 160 to 540 bar and 700 to 3400 K was obtained. Using the gained dependences of α and T _w on pressure and temperature, we have developed a physical model for calculating the working-body characteristics in the tube with allowance for enthalpy losses. We found that by the hundredth millisecond of the operating regime the disregard, in such calculations, of the wall heat flux in the first settling chamber resulted in overestimation of the stagnation temperature in the test section in comparison with similar calculations made without allowance for the heat losses by 6–18 % in terms of the full-scale temperature for aircraft flight in Mach number range 5 to 8. The developed calculation procedure has been tested in experiments without diaphragm rupture.     

复杂构型前缘疏导热防护技术

针对前缘驻点区热流密度高、热流梯度大的特点，基于疏导式热防护思想，研制了一体化疏导式热防护前缘结构.根据力热承载要求，设计了一体化薄壁结构方案，内部充装碱金属工质，在气动加热条件下形成工质相变及输运的热管式闭式循环，实现驻点区热量的快速疏散.通过结构封装、热管充装等工艺流程，实现结构样件，最终通过了飞行试验验证.结果表明，疏导结构启动时间约为94.5 s，疏导效率达到23.4%.      

Saturated flow boiling characteristics in single rectangular minichannels: effect of aspect ratio

In this study, saturated flow boiling characteristics of deionized water in single rectangular minichannels are investigated experimentally. A special attention is paid to the effect of aspect ratio (channel width to depth, W_ch/H_ch) on the heat transfer and total pressure drop. Experiments are conducted for various values of the mass flux and the wall heat flux. Flow visualization is used as a complementary technique for a deeper physical understanding of flow phenomena. The results show that the channel aspect ratio has a significant effect on both the local two-phase heat transfer coefficient and the total pressure drop. In general manner, the aspect ratio of 1 presents the highest heat transfer coefficients, while the aspect ratio of 0.25 demonstrates the lowest ones. On the other hand, the lowest values of the pressure drop are obtained at the extreme values of the aspect ratio (0.25 and 4).     

A hybrid particle approach for continuum and rarefied flow simulation

A hybrid particle scheme is presented for the simulation of compressible gas flows involving both continuum regions and rarefied regions with strong translational nonequilibrium. The direct simulation Monte Carlo (DSMC) method is applied in rarefied regions, while remaining portions of the flowfield are simulated using a DSMC-based low diffusion particle method for inviscid flow simulation. The hybrid scheme is suitable for either steady state or unsteady flow problems, and can simulate gas mixtures comprising an arbitrary number of species. Numerical procedures are described for strongly coupled two-way information transfer between continuum and rarefied regions, and additional procedures are outlined for the determination of continuum breakdown. The hybrid scheme is evaluated through a comparison with DSMC simulation results for a Mach 6 flow of N₂ over a cylinder, and good overall agreement is observed. Large potential efficiency gains (over three orders of magnitude) are estimated for the hybrid algorithm relative to DSMC in a simple example involving a rarefied expansion flow through a small nozzle into a vacuum chamber.     

高超声速飞行器磁控热防护系统建模分析

针对高超声速飞行器防热, 搭建了螺线管磁控热防护系统的物理模型. 采用低磁雷诺数磁流体数学模型, 分析了外加磁场强度及磁场形态对磁控热防护效果的影响. 对比了三种磁场类型(磁偶极子、螺线管、均布磁场)下磁控热防护效果的差异, 分析了螺线管几何参数对磁控热防护效果的影响. 研究表明, 磁场降低表面热流作用存在“饱和现象”; 三种磁场形态的磁控热防护能力从小到大依次为磁偶极子、螺线管、均布磁场; 相同驻点磁感应强度条件下, 增大螺线管半径有利于提高磁控热防护效果, 缩短螺线管与驻点距离不利于驻点和肩部防热, 螺线管长度对磁控热防护效果影响相对较小.     

Propagation of ultrafast pulses in the anomalous-dispersion regime of silicon-on-insulator strip nanowaveguides

The dynamic evolution of ultrafast high-intensity pulses with a 100 fs half-width at 1/e intensity point based on the silicon-on-insulator (SOI) strip nanowaveguides is considered and investigated numerically under the condition of anomalous group-velocity dispersion (GVD) regime. For ultrafast high-intensity pulses propagating in millimeter-long SOI nanowaveguides, the interplay between the dispersion and nonlinear effects such as the two-photon absorption, free-carrier absorption, free-carrier dispersion, and self-phase modulation has to be taken into account, which results in the significant optical wave breaking phenomenon that occurs near the pulse leading edge for an unchirped Gaussian pulse in the anomalous GVD regime. However, when the input Gaussian pulse with linear up-chirp is introduced, the position of the optical wave breaking shifts from the leading pulse edge to its trailing edge along the several millimeters-long SOI nanowaveguides.     

Surface optical waves at the boundary with a nonlinear Kerr medium

A rigorous solution consistent with a plane wave approximation is given to the boundary problem for Maxwell’s equations for surface optical waves at the boundary with a nonlinear Kerr medium. Exact formulas for the flux intensity (J ₀) and energy density (W ₀) of these waves are derived depending on the parameters of the adjacent media and the propagation constant (ξ). It is shown that these variables as functions of ξ have minima. Thus, J ₀ and W ₀ increase sharply as the propagation constant deviates from the minimum value ξ_min. Their values are greater, the larger the difference between the dielectric constants of the linear and nonlinear media is. An expression for the propagation velocity of a nonlinear surface wave is also obtained.     

高超声速飞行器热管冷却前缘结构一体化建模分析

针对高超声速飞行器工作时前缘恶劣的气动加热环境, 为了保证飞行器前缘的尖锐外形, 提出内嵌高温热管前缘结构. 针对热管内部复杂流动与换热情况, 对内嵌高温热管前缘结构进行一体化建模, 将模型的核心部件液态金属热管工作状况的计算与实验进行对比以验证模型的可靠性. 本文还分析了给定工况下内嵌高温热管前缘结构的热防护效果, 其中壁面最高温度下降了11.6%, 最低温度上升了8%, 高温区和低温区均被封闭在前缘外层区域, 内层温度更加均匀, 实现了热流由高温区向低温区的转移, 削弱了高温区的热负荷. 本文还分析了接触热阻对热管冷却前缘结构效果的影响. 关键词： 热管 前缘 疏导式热防护 气动热     

Classical and thermodynamic work fluctuations

We have studied the nature of classical work (W _c ) and thermodynamic work (W) fluctuations in systems driven out of equilibrium both in transient and time periodic steady state. As the observation time of a large number of trajectories increases, the fraction of trajectories which exhibit excursions away from the typical behaviour, namely, W _c < 0, W < Δ F and dissipated heat Q < 0 decreases as anticipated for macroscopic time scales. An analytical expression for such trajectories is obtained. Trajectory for which W _c < 0 may not correspond to W < Δ F or Q < 0. The applicability of steady state fluctuation theorems is discussed in our linear as well as nonlinear models.     

Rarefied Flow Computations Using Nonlinear Model Boltzmann Equations

High resolution finite difference schemes for solving the nonlinear model Boltzmann equations are presented for the computations of rarefied gas flows. The discrete ordinate method is first applied to remove the velocity space dependency of the distribution function which renders the model Boltzmann equation in phase space to a set of hyperbolic conservation laws with source terms in physical space. Then a high order essentially nonoscillatory method due to Harten et al. (J. Comput. Phys. 71, 231, 1987) is adapted and extended to solve them. Explicit methods using operator splitting and implicit methods using the lower-upper factorization are described to treat multidimensional problems. The methods are tested for both steady and unsteady rarefied gas flows to illustrate its potential use. The computed results using model Boltzmann equations are found to compare well both with those using the direct simulation Monte Carlo results in the transitional regime flows and those with the continuum Navier-Stokes calculations in near continuum regime flows.