Slip boundary conditions for rough surfaces

The velocity slip and temperature jump for a two-dimensional rough plate under hypersonic conditions were analyzed using the Direct Simulation Monte Carlo (DSMC) method. Surface roughness was explicitly modeled by introducing various structures on the flat plate. The influences of relative roughness height, which involves the roughness height, roughness spacing, incoming velocity, and the degree of rarefaction, were analyzed and discussed. It is found that with the increase of the relative roughness height, the jump temperature increases, while the slip velocity decreases gradually. The effects of surface roughness on the slip coefficients can be attributed to the change of accommodation coefficients. A new slip model for rough surfaces was established in this paper, which accounts for the coupling effects of gas rarefaction and surface roughness, without the effort to model the surface roughness explicitly. The nitrogen flows in the microchannel, and flows over a blunt cone and an axisymmetric bi-conic body, were simulated under the modified and conventional slip boundary conditions, respectively. The numerical solutions were validated with experimental data. It can be safely concluded that compared with the traditional first-order slip boundary conditions, the modified slip model improves the accuracy of macroscopic properties, especially the heat transfer coefficient.     

Hypersonic shock wave and turbulent boundary layer interaction in a sharp cone/flare model

A direct numerical simulation of hypersonic Shock wave and Turbulent Boundary Layer Interaction(STBLI) at Mach 6.0 on a sharp 7° half-angle circular cone/flare configuration at zero angle of attack is performed. The flare angle is 34° and the momentum thickness Reynolds number based on the incoming turbulent boundary layer on the sharp circular cone is Re_θ = 2506. It is found that the mean flow is separated and the separation bubble occurring near the corner exhibits unsteadiness. The Reynolds analogy factor changes dramatically across the interaction, and varies between 1.06 and 1.27 in the downstream region, while the QP85 scaling factor has a nearly constant value of 0.5 across the interaction. The evolution of the reattached boundary layer is characterized in terms of the mean profiles, the Reynolds stress components, the anisotropy tensor and the turbulence kinetic energy. It is argued that the recovery is incomplete and the near-wall asymptotic behavior does not occur for the hypersonic interaction. In addition, mean skin friction decomposition in an axisymmetric turbulent boundary layer is carried out for the first time. Downstream of the interaction, the contributions of transverse curvature and body divergence are negligible, whereas the positive contribution associated with the turbulence kinetic energy production and the negative spatial-growth contribution are dominant. Based on scale decomposition, the positive contribution is further divided into terms with different spanwise length scales. The negative contribution is analyzed by comparing the convective term, the streamwise-heterogeneity term and the pressure gradient term.     

基于自适应神经模糊推理系统的导弹直/气复合控制系统设计

针对导弹直/气复合控制问题,提出了一种基于自适应神经模糊推理系统的导弹直/气复合控制系统设计方法.首先,建立了直/气复合导弹数学模型.然后,针对常规模糊控制设计严重依赖经验的问题,在模糊控制的基础上引入神经网络,通过样本数据学习建立自适应神经模糊推理系统(ANFIS),并设计了脉冲调宽调频调制器.最后,通过仿真试验验证了所提的控制系统设计方法.仿真结果表明,基于ANFIS的导弹直/气复合控制系统能够快速精确地跟踪加速度指令.     

临近空间高超声速稀薄流中梯形缝隙绕流分析

针对高超声速飞行器表面不规则缝隙的构型特征，将其简化为梯形缝隙模型；采用直接模拟蒙特卡洛(DSMC)方法对高超声速稀薄流中的缝隙绕流问题进行数值模拟；研究了不同的缝隙壁面倾角对流场结构以及壁面参数的影响，为航天器结构设计以及热防护设计提供了数值支持。结果表明：改变倾角会改变流场结构，当倾角减小时，外部流场会更深入缝隙内部，使得缝隙内流场和表面的气动参数增强，且倾角越小，影响的程度越深。同时，倾角减小也会减小缝隙内的旋涡强度，当倾角小于30°时，缝隙内的旋涡区域完全消失。