On Nonlinear-Evolution of C-type Instability in Nonparallel Boundary Layers

The process of evolution, especially that of nonlinear evolution, of C-type instability of laminar-turbulent flow transition in nonparallel boundary layers are studied by means of a newly developed method called parabolic stability equations （PSE）. Initial conditions, which are very important for the nonlinear problem, are investigated by computing initial solution of the harmonic waves, modifying the mean-flow-distortion, and giving initial value of TS wave and its subharmonic waves at initial station by solving linear PSE. A numerical method with high-order accuracy are developed in the text, the key normalization conditions in the PSE are satisfied, and nonlinear PSE are solved efficiently and implemented stably by the spatial marching. It has been shown that the computed process of nonlinear evolution of C-type instability in Blasius boundary layer is in good agreement with the experimental results.     

多重网格法在混合网格中的应用

设计了一种高效的混合网格聚合方法,在此基础上应用多重网格技术来加速Navier-Stokes方程的收敛过程.方程求解采用Jamson的中心格式有限体积法.对NACA0012翼型和ONERA M6机翼的粘性绕流进行了数值模拟,算例结果与试验结果符合良好,加速收敛效果显著.     

微型旋转翼与拍动翼气动力特性的比较

 用拟压缩性法求解三维非定常不可压N-S方程,研究了旋转翼及拍动翼的气动力和流场。当昆虫正常悬停时,如果翅膀旋转,产生的平均升力以及气动效率并不比拍动时的差,甚至还略好,因此,厘米级微型飞行器可以采用旋转翼,这比拍动模式容易实现。另外,小迎角旋转时升阻比可达3.1（在15°迎角,雷诺数4 000情况下）,气动效率较高而且升力系数不小,多个桨叶可以产生很大的升力。因此,在悬停时,多个桨叶小迎角旋转是厘米级微型飞行器的一个很好的方案。     

跨声速涡轮叶栅三维流场的数值模拟

以具有试验测量数据的一个高压涡轮动叶中截面平面叶栅和一个高压涡轮导向器环形叶栅为对象,采用计算流体动力学(CFD)软件CFX-TASCflow,对两个涡轮叶栅的跨声速三维粘性流场进行了数值计算,验证了CFX-TASCflow软件在航空燃气涡轮设计计算中的可靠性,并对所研究的跨声速涡轮叶栅粘性流场进行了分析.      

基于应变量耦合的低成本Ti-Al-V-Fe合金本构关系

为了获取新型低成本Ti-Al-V-Fe合金热成形工艺窗口,研究了热加工参数为变形温度875~1100℃、应变速率0.001~1 s^-1、变形量70%的低成本Ti-Al-V-Fe合金热变形行为。结果表明:流变应力与变形温度成反比,与应变速率成正比,合金为典型负温度、正应变敏感材料。以热模拟实验数据为依据,运用多元线性回归方法,确定了材料常数与应变的函数关系,建立了基于应变量耦合的α+β两相区及β单相区Arrhennius本构方程,其耦合系数为0.98,表明建立的模型在给定任意应变量时可准确预测流变应力。根据热激活能,判别合金在不同相区软化机制,单相区为动态回复,两相区为动态再结晶。     

共轴刚性旋翼桨毂阻力特性及流动机理

共轴刚性旋翼直升机在高速飞行时,桨毂流动复杂、分离强、阻力大。为明晰其阻力特性和流动机理,采用CFD方法针对已完成风洞试验的共轴桨毂组合模型进行数值模拟研究,获得了桨毂组合模型各单独部件的阻力、表面流动和空间流场特征,阐明了产生阻力最大的部件和影响阻力的主要因素,揭示了中间轴整流罩和塔座设计参数的减阻机制。分析结果表明:上、下旋翼桨毂是产生阻力的主要部件;中间轴和塔座的分离尾流对桨毂表面流动产生较大的干扰作用,使桨毂整流罩表面受干扰区域产生气流分离;具有较缓和逆压梯度的中间轴整流罩和塔座能有效减小分离尾流对桨毂整流罩的干扰,从而降低整个共轴桨毂系统的阻力。     

A transition prediction method for flow over airfoils based on high-order dynamic mode decomposition

This article presents a novel approach for predicting transition locations over airfoils, which are used to activate turbulence model in a Reynolds-averaged Navier-Stokes flow solver. This approach combines Dynamic Mode Decomposition (DMD) with e^N criterion. The core idea is to use a spatial DMD analysis to extract the modes of unstable perturbations from a steady flowfield and substitute the local Linear Stability Theory (LST) analysis to quantify the spatial growth of Tollmien–Schlichting (TS) waves. Transition is assumed to take place at the stream-wise location where the most amplified mode’s N-factor reaches a prescribed threshold and a turbulence model is activated thereafter. To improve robustness, the high-order version of DMD technique (known as HODMD) is employed. A theoretical derivation is conducted to interpret how a spatial high-order DMD analysis can extract the growth rate of the unsteady perturbations. The new method is validated by transition predictions of flows over a low-speed Natural-Laminar-Flow (NLF) airfoil NLF0416 at various angles of attack and a transonic NLF airfoil NPU-LSC-72613. The transition locations predicted by our HODMD/e^N method agree well with experimental data and compare favorably to those obtained by some existing methods (LST/e^N or $\gamma -\stackrel{￣}{R{e}_{\theta \text{t}}}$). It is shown that the proposed method is able to predict transition locations for flows over different types of airfoils and offers the potential for application to 3D wings as well as more complex configurations.     

2024T4铝合金蠕变本构关系研究

设计了一种简易单向拉伸蠕变试验装置,对2024T4铝合金在175℃、多个应力水平下的单向拉伸蠕变进行了试验。根据试验数据的特点,将2024T4铝合金的蠕变本构关系分解为由一个双指数函数和一个一次函数组成的两分量表达式,并利用试验数据通过最小二乘拟合确定出本构关系中的待定参数,得到2024T4铝合金175℃时的本构关系。通过对拟合数据与试验数据的对比分析表明,所建立的蠕变本构关系能够有效表征2024T4铝合金175℃时在不同应力水平下的蠕变行为。     

数值模拟有外挂物的空腔流动

数值模拟了有外挂物存在的开式空腔的超音速流动。数值计算采用全隐式无分裂方法,湍流模型采用修正的B-L代数模型。研究了有外挂物存在的空腔流动的剪切层自持性振荡,并与无外挂物存在时的现象进行类比。压强与时间的变化曲线、空腔底部流向的静压系数分布以及空腔后壁面法向的静压系数分布表明,空腔内的压力脉动与外挂物所处位置是有关的。对于Rossiter振荡模态,数值计算结果能够与实验结果相吻合。     

Eckhaus方程的有限差分格式

本文主要研究了Eckhaus方程的初始边界值问题的一个有限差分法,提出了有条件的能量守恒的有限差分格式,证明了差分解的收敛性与稳定性.