高维不确定性条件下飞行器级间分离可靠性评估

针对高维不确定性条件下飞行器级间分离精细化分析需求,建立参数化级间分离动力学仿真模型,综合考虑级间分离过程中高维不确定性因素影响,识别典型分离失效模式,构建分离可靠性模型。提出采用活跃子空间方法实现高维不确定性参数的空间降维,结合极大极小距离序贯采样方法,利用含交叉项的高阶多项式回归模型对降维空间进行近似,完成了可靠性分析。结果表明,提出的方法能够准确描述高维不确定性因素对分离可靠性的影响特征,在保证高维不确定性计算精度条件下,大幅提升分离可靠性定量分析效率,此外,不同阈值对应分离可靠性曲线与常规打靶仿真结果吻合良好,为高维不确定性条件下分离方案精细化设计提供支撑。     

微纳卫星星座的Kuhn-Munkres匹配部署优化方法

针对将半长轴、升交点赤经、纬度辐角均不同的低轨微纳卫星群部署到同一轨道面不同目标相位的星座部署问题,提出一种基于Kuhn-Munkres(KM)匹配的星座部署优化方法。通过KM算法实现卫星和目标纬度辐角的优化匹配,充分利用J₂摄动,使升交点赤经借助半长轴和纬度辐角的部署而得到同步修正,从而节约燃料。仿真结果表明,相比于传统部署方法,在相同约束下,优化后的部署方法使各星平均燃耗减少,各星燃耗量均衡性提高。弥补了传统同轨星座部署中将各星初始位置简化为空间一点且忽略部署过程中的升交点赤经漂移的不足。采用有限常值推力实现轨道机动,适用于携带微推力推进系统的微纳卫星。     

An improved prediction model for corner stall in axial compressors with dihedral effect

Corner stall predictions are important and difficult in axial compressors. However, all of the prediction models have proved to be ineffective for advanced compressor blades, which tend to use the combined sweep and dihedral. As for the prediction parameter DL, although it effectively modeled the effects of the adverse pressure gradient and secondary flow, it failed to predict the corner stall of curved blades because the model failed to consider the intersection of the boundary layer at the corner region. In this paper, the shape factor gradient Ψ of the boundary layer at the corner region was investigated by numerically studying specially shaped expansion pipes under different adverse pressure gradients. The improved prediction parameter DJ was presented based on the model of Ψ and the circumferential pressure gradient ξ. A comparison of the critical range of the prediction parameters DL and DJ was investigated using the NACA65 cascade database, which was established by a numerical method. Then, the stall criterion was validated according to the experimental results of various test facilities with different blade geometries and experimental conditions. The results show that the improved prediction parameter is able to predict the corner separation/stall flows and is in good agreement with the experimental results for axial compressors with three-dimensional designed blades.     

A methodology for simulating 2D shock-induced dynamic stall at flight test-based fluctuating freestream

A comprehensive methodology for simulating 2 D dynamic stall at fluctuating freestream is proposed in this paper.2 D CFD simulation of a SC1095 airfoil exposed to a fluctuating freestream of Mach number 0.537 ± 0.205 and Reynolds number 6.1 × 10~6(based on the mean Mach number) and undergoing a 10° ± 10° pitch oscillation with a frequency of 4.25 Hz was conducted.These conditions were selected to be representative of the flow experienced by a helicopter rotor airfoil section in a real-life fast forward flight.Both constant freestream dynamic stall as well as fluctuating freestream dynamic stall simulations were conducted and compared.The methodology was carefully validated with experimental data for both transonic flow and dynamic stall under fluctuating freestream.Overall, the results suggest that the fluctuating freestream alters the dynamic stall mechanism documented for constant freestream in a major way, emphasizing that inclusion of this effect in the prediction of dynamic stall related rotor loads is imperative for rotor performance analysis and blades design.     

Impact of reduced frequency on the time lag in pressure distribution over a supercritical airfoil in a pitch-pause-return motion

Effects of reduced frequency, stop angle, and pause duration have been studied on a thin supercritical airfoil undergoing a pitch-pause-return motion, which is one of the classic maneuvers introduced by the AIAA Fluid Dynamics Technical Committee. Experiments were conducted in a low-speed wind tunnel at both a constant mean angle of attack and an oscillation amplitude with a reduced frequency ranging from 0.01 to 0.12. The desired stop angles of the airfoil were set to occur during the upstroke motion. The unsteady pressure distribution on the airfoil was measured for below, near, and beyond static stall conditions. Results showed that the reduced frequency and stop angle were the dominant contributors to the time lag in the flowfield. For stop angles in both below- and post-stall regions, the time for the flowfield to reach its steady state conditions, known as the time lag, decreased as the reduced frequency was increased. However, in the static-stall region and for a certain value of reduced frequency, a resonance phenomenon was observed, and a minimum time lag was achieved. The pressure distribution in this condition was shown to be highly influenced by this phenomenon.     

Influence of SPS casing treatment on axial flow compressor subjected to radial pressure distortion

The present work is about the stall margin enhancement ability of a kind of stall precursor-suppressed (SPS) casing treatment when fan/compressor suffers from a radial total pres-sure inlet distortion. Experimental researches are conducted on a low-speed compressor with and without SPS casing treatment under radial distorted inlet flow of different levels as well as uniform inlet flow. The distorted flow fields of different levels are generated by annular distortion flow gen-erators of different heights. The characteristic curves under these conditions are measured and ana-lyzed. The results show that the radial inlet distortion could cause a stall margin loss from 2% to 30% under different distorted levels. The SPS casing treatment could remedy this stall margin loss under small distortion level and only partly make up the stall margin loss caused by distortion in large level without leading to perceptible additional efficiency loss and obvious change of charac-teristic curves. The pre-stall behavior of the compressor is investigated to reveal the mechanism of this stall margin improvement ability of the SPS casing treatment. The results do show that this casing treatment delays the occurrence of rotating stall by weakening the pressure perturbations and suppressing the nonlinear amplification of the stall precursor waves in the compression system.     

用于粒子分离器的砂粒反弹特性实验研究

针对砂粒反弹特性所面临的基础问题及其复杂特点,采用实验与理论分析的方法研究砂粒反弹机理。以砂粒与板面碰撞反弹为起点,自行设计了测量砂粒反弹系数的实验系统,并用高速摄影仪捕捉砂粒的运动轨迹,通过图像处理获得不同粒径的砂粒与不同材料的板面碰撞前后的速度矢量。以实验测量数据为基础,理论分析砂粒运动特性,总结砂粒反弹系数与入射角度之间的关系,建立不同材料的砂粒反弹系数的拟合公式,为进一步提高粒子分离器的分离效率提供研究基础与理论支撑。     

内埋武器投放分离相容性的风洞投放试验预测与评估

采用基于运动动力学相似的风洞投放试验对先进战斗机内埋武器投放分离相容性进行预测与评估，给出载机在不同飞行马赫数、攻角、弹舱长深比及舱内武器剩余数量、不同弹射力、折叠翼是否展开下，内埋导弹从载机弹舱投放分离后的运动轨迹和俯仰姿态角变化规律，研究这些因素对内埋导弹投放分离相容性的影响。结果表明：处于超声速飞行状态下（马赫数为1.5）的载机，攻角处于0°、2°、3°时投放内埋导弹后弹体俯仰角处于低头状态，利于攻击载机前下方敌方目标；在给定的初始分离条件下，对于两种不同的弹舱长深比，内埋导弹均能安全分离，但对内埋导弹俯仰方向运动影响较为显著；弹舱内武器剩余数量对内埋导弹分离特性影响较小，导弹能快速地远离载机干扰流场，投放分离后弹体俯仰角一直处于低头状态；随着内埋导弹初始分离速度增大，可使弹体快速地穿过载机的下洗流场，有利于内埋导弹与载机的安全分离；导弹的不同气动布局对内埋导弹分离相容性有一定的影响。     

三角翼大攻角绕流数值模拟研究

采用Harten-Yee的二阶精度隐式TVD格式对N-S方程进行数值模拟,研究了后掠角为65°和70°的三角翼绕流的流场特性。计算结果表明：TVD格式中限制器函数的选取对于三角翼绕流结构的计算结果具有较重要的影响。结合张涵信的旋涡定性分析理论对旋涡的发展过程进行研究,此外,还对二次涡破裂现象和网格分布影响等问题进行了探讨。     

Effects of sweep angles on turbulent separation behaviors induced by blunt fin

An experimental study was conducted on turbulent separation behaviors induced by blunt fins with different sweep angles at Mach number 6.0. The Nano-particle based Planar Laser Scattering technique(NPLS) was applied to visualize the flowfield, complemented by pressure tests.Sweep angles of the fins were 10°, 20°,...,60°, with the same leading edge diameter of 10 mm. Fine structures of the interference flowfield induced by blunt fins have been obtained, including the shock systems and vortexes. I...