基于降维算法和等效杆长的可展结构精度分析

考虑铰链间隙和杆件尺寸误差的不确定性并通过概率统计方法对其进行研究,提出了一种基于单变量降维算法(UDRM)和等效杆长模型的可展结构精度分析方法。利用UDRM将可展结构的精度性能函数解耦为多个杆件尺寸误差的独立作用形式,建立精度分析模型。引入等效杆长模型,等效杆件替代原杆件进行精度计算。将铰链间隙与原始杆件尺寸误差合并到等效杆件的尺寸误差中,同时证明了等效杆长尺寸误差近似服从正态分布。以某卫星可展开天线为算例,结合高斯求积公式求解展开状态下精度指标的分布期望和方差。通过与蒙特卡罗模拟(MCS)和一次二阶矩(FOSM)法计算结果的对比验证了本文精度分析方法的正确性和高效性。     

永磁力矩电机三相绕组不对称性改进方法研究

由于力矩电机多作为驱动电机使用,且电机对低速平稳性要求很高,所以选用了一种特殊的极槽配合方式。12极39槽的选择使得电机齿槽转矩较小的同时导致电机三相绕组不对称。降低绕组不对称对电机低速平稳性的影响,对提高绕组的对称性的方法研究有着重要的意义。以一台采用12极39槽的力矩电机为例,提出了一种绕组不对称性的改进方法,通过改变绕组的排列方式提高了绕组的对称性,且保证了电机的低速平稳性。最终通过理论研究与仿真分析验证了不对称性改进方法的有效性。     

Coupled Lagrangian impingement spray model for doublet impinging injectors under liquid rocket engine operating conditions

To predict the effect of the liquid rocket engine combustion chamber conditions on the impingement spray, the conventional uncoupled spray model for impinging injectors is extended by considering the coupling of the jet impingement process and the ambient gas field. The new coupled model consists of the plain-orifice sub-model, the jet-jet impingement sub-model and the droplet collision sub-model. The parameters of the child droplet are determined with the jet-jet impingement sub-model using correlations about the liquid jet parameters and the chamber conditions.The overall model is benchmarked under various impingement angles, jet momentum and offcenter ratios. Agreement with the published experimental data validates the ability of the model to predict the key spray characteristics, such as the mass flux and mixture ratio distributions in quiescent air. Besides, impinging sprays under changing ambient pressure and non-uniform gas flow are investigated to explore the effect of liquid rocket engine chamber conditions. First, a transient impingement spray during engine start-up phase is simulated with prescribed pressure profile. The minimum average droplet diameter is achieved when the orifices work in cavitation state, and is about 30% smaller than the steady single phase state. Second, the effect of non-uniform gas flow produces off-center impingement and the rotated spray fan by 38°. The proposed model suggests more reasonable impingement spray characteristics than the uncoupled one and can be used as the first step in the complex simulation of coupling impingement spray and combustion in liquid rocket engines.     

A fast and automatic full-potential finite volume solver on Cartesian grids for unconventional configurations

To meet the requirements of fast and automatic computation of subsonic and transonic aerodynamics in aircraft conceptual design,a novel finite volume solver for full potential flows on adaptive Cartesian grids is developed in this paper.Cartesian grids with geometric adaptation are firstly generated automatically with boundary cells processed by cell-cutting and cell-merging algo rithms.The nonlinear full potential equation is discretized by a finite volume scheme on these Cartesian grids and iteratively solved in an implicit fashion with a generalized minimum residual (GMRES) algorithm.During computation,solution-based mesh adaptation is also applied so as to capture flow features more accurately.An improved ghost-cell method is proposed to implement the non-penetration wall boundary condition where the velocity-potential of a ghost cell is modified by an analytic method instead.According to the characteristics of the Cartesian grids,the Kutta condition is applied by specially computing the gradients on Kutta-faces without directly assigning the potential jump to cells adjacent wake faces,which can significantly improve the solution con verging speed.The feasibility and accuracy of the proposed method are validated by several typical cases of sub/transonic flows around an ONERA M6 wing,a DLR-F4 wing-body,and an unconventional figuration of a blended wing body (BWB).The validation cases demonstrate a fast convergence with fully automatic grid treatment and computation,and the results suggest its capacity in application for aircraft conceptual design.     

Investigation of dynamics of discrete framed structures by a numerical wave-based method and an analytical homogenization approach

In this article, the analytical homogenization method of periodic discrete media (HPDM) and the numerical condensed wave finite element method (CWFEM) are employed to study the lon-gitudinal and transverse vibrations of framed structures. The valid frequency range of the HPDM is re-evaluated using the wave propagation feature identified by the CWFEM. The relative error of the wavenumber by the HPDM compared to that by the CWFEM is illustrated in functions of fre-quency and scale ratio. A parametric study on the thickness of the structure is carried out where the dispersion relation and the relative error are given for three different thicknesses. The dynamics of a finite structure such as natural frequency and forced response are also investigated using the HPDM and the CWFEM.     

辐射换热对平板气膜冷却性能影响

为了分析辐射换热对平板气膜冷却性能的影响,分别对不考虑辐射换热(耦合计算)与考虑辐射换热两种条件下的平板隔热屏进行了三维流热耦合数值模拟研究,并与绝热壁(不考虑隔热屏固壁导热)的计算结果进行了对比。辐射换热采用离散坐标法。得到了3种不同主/次流总温比(Rt)条件下气膜冷却效果、流场、单位面积冷流体热负荷以及流量系数的对比结果与变化规律。结果表明:气膜冷却效果沿流向逐渐降低,且随总温比的增加而降低;在展向上气膜冷却效果呈现中间高两边低的分布规律,最大与最小相对值在绝热壁条件下达到最大,为94.5%,辐射换热次之,为13.5%,不考虑辐射换热时最小,为9.8%。与不考虑辐射换热结果相比,辐射换热对气膜冷却效果的影响在流动方向逐渐变大,气膜孔附近展向温度梯度增大,冷流体热负荷增加了92.8%,二者计算的流量系数相差不大,为1.1%,与绝热壁计算结果相比,流量系数减小了13.1%。随着总温比增加流量系数减小,最大值与最小值相差不大,为1.1%。     

涡轮叶片疲劳-蠕变寿命稳健性优化方法

为了清晰地反映涡轮叶片的疲劳-蠕变交互作用,提高寿命预测结果的准确性及可靠性,并改善涡轮叶片疲劳寿命对随机变量的敏感程度,分别采用Manson-Coffin公式和Larson-Miller方程计算了涡轮叶片的低循环疲劳寿命和蠕变持久寿命,利用修正的时间-寿命分数法计算了涡轮叶片疲劳-蠕变损伤,在此基础上,将响应面法(RSM)与果蝇优化算法(FFOA)相结合,考虑载荷、材料参数、疲劳-蠕变交互程度的不确定性,对涡轮叶片疲劳寿命进行了稳健性优化设计。优化结果表明:涡轮叶片疲劳-蠕变小时寿命的概率区间减小了8.48%,验证了该优化方法的工程可行性。     

压气机稳定边界的数值预测研究

为研究不同压气机稳定性预测方法的准确性,采用最大静压升系数法与商用CFD软件,对某4级低速压气机的稳定边界进行了数值模拟,并与试验结果对比。计算结果表明:最大静压升系数法在预测压气机稳定边界上有一定的准确性。使用商用CFD软件NUMECA对某4级压气机进行模拟计算,考虑了不同的网格划分方案、使用不同的湍流模型和差分格式等因素,以出现数值发散作为压气机失稳的标准,结果表明:主流区的网格对计算结果影响较大。不同差分格式的结果显示,中心差分格式的结果最好。     

白天激光漫反射测距中微弱信号检测的单开门多触发方法

白天激光漫反射测距因存在强背景噪声,导致微弱回波信号的检测十分困难,严重限制了激光漫反射测距的广泛应用。因此,提出了采用单开门多触发方法解决白天激光漫反射测距的微弱信号检测难题。首先,介绍了一种GM-APD (Geiger-ModeAvalanchePhotoDiode,盖革模式雪崩光电二极管)多触发概率分布函数的快速求解方法。接着,取激光漫反射测距系统典型参数,理论分析了多触发情况下的回波信号检测概率,分析结果表明单开门多触发方法可提高激光漫反射测距在白天情况下的回波信号检测概率。同时,通过对触发次数限制的优选,可以增加有效检测比,减少白天观测条件下回波信号的提取代价。最终,通过系统仿真验证了理论分析结果的正确性。因此,单开门多触发方法是解决白天激光漫反射测距微弱信号检测的可行途径之一。     

预测飞机稳定尾旋的解析法和图像法

尾旋是飞机的极限飞行状态,此一状态极易造成机毁人亡。由于尾旋试飞有极大的风险性,所以在试飞前要进行足够的安全论证,包括理论分析和风洞试验,一般采用两者相结合的方式进行。介绍了解析法和图像法两类方法来研究飞机的稳定尾旋。