圆柱尾迹涡的三维演化及结构特征

应用无粘涡丝运动学理论和局部诱导近似(LIA)方法,以Lagrange观点跟踪涡丝在背景流场中运动,用数值方法研究了中等Re数(≈10~3)下圆柱分离尾迹中Kármán涡和涡辫区涡丝的三维演化的机制和动力学过程,及其结构特征。背景流场考虑为尾迹时间平均速度流场和Kármán涡街流场。初始展向小扰动为指数形式和谐波形式。结果指出:Kármán涡和涡辫区中的涡丝具有展向不稳定性,形成流向涡量。在尾迹的初期输运过程中,表现出有序的大、小尺度涡结构。并进一步分析了其产生的机理。     

二次涡与卡门涡形成过程

本文用离散涡位流理论与边界层理论相结合的方法,同时考虑到边界层及后剪切层分离的离散涡模型,研究并计算了圆柱非定常运动过程中二次涡的发生、发展与影响。研究了对称主涡发生运动不稳定性的直接原因和条件,分析卡门涡的形成过程。本文提出,在二次涡的影响下对称主涡从有涡量补充的非自由涡变为没有涡量补充的自由涡,这是对称主涡发生运动不稳定性的条件,在小扰动影响下可诱发成卡门涡。 本文考虑不可压、大雷诺数、层流情况。所得结果与流场显示结果十分相似。     

三个共轴涡环运动混沌特性的研究

本言语利用离散涡环方法对三个共轴涡环运动进行了数值模拟，在此基础上，采用非线性动力学中混沌特征量的计算方法，得到了涡环呈规则运动和混沌运动的结论。结果表明，三个共轴涡环在运动时，总是会产主规划与混沌的现象。而这些现象的产生取决于三个涡环的初一文析得出对于利用共轴涡环模拟轴对称射流场涡结构的演变具有重要的参考价值。     

旋翼尾流与地面干扰时地面涡现象的研究

用Ｎ－Ｓ方程对近地飞行时旋翼尾流与地面干扰时产生的地面涡现象进行了数值计算旋翼对流场的作用由分布在特定区域内的动量源项模拟结果表明，旋翼尾流撞到地面后的卷起和轴向流动的拉伸作用是形成地面涡的原因；地面边界层形成的二次分离涡向地面涡内输入（与尾流所携带的涡量）相反的涡量，而使地面涡保持平衡；地面涡的存在和运动使旋翼附近流场大大改变     

非对称槽道中涡旋波的特性研究

利用PIV流场显示技术，对振荡流体在非对称槽道中涡旋波的产生、发展和消失的规律进 行了实验研究和分析，测得了涡旋波流场的速度矢量图，阐明了涡旋波流场周期性变化的特 点. 结合涡动力学方程，深入分析并揭示了做周期性运动的流体能在槽道中产生波的特性这 一规律，从中发现：流体周期变化的非定常性和不对称的槽道结构是形成涡旋波流动的主要 因素. 本文对涡旋波流场中各个旋涡的速度分布和涡量进行了测量和计算，分析了涡旋波 强化传质的机理，并研究了Re数对涡旋波流动的影响     

《涡运动理论》评介

涡动力学,即涡量和旋涡的动力学,是整个现代流体力学的一个主要支柱．狭义地讲,涡旋动力学讨论集中涡的运动和相互作用;广义地讲,涡动力学涵盖了整个有旋流体的动力学及其对无旋流的作用（如涡声）．著名理论家Ｓａｆｍａｎ（ＪＦｌｕｉｄＭｅｃｈ,１９８１,１０６...      

Are vortices twisted by shear stress?

涡旋运动是流体运动的基本运动形式,黏性流体的运动几乎是处处有旋的.这就容易造成误解：黏性是产生涡量的条件,也是产生涡量的机制.实际上,黏性流体中的剪切力不能在流体内部产生涡量.而是在剪切变形的不连续处产生涡量,再由黏性扩散到流体的其他区域.准确地讲,黏性流动的涡量不是由剪切应力"搓"出来的,而是在剪切变形的不连续处"裂"出来的.     

涡是搓出来的吗?

涡旋运动是流体运动的基本运动形式,黏性流体的运动几乎是处处有旋的.这就容易造成误解:黏性是产生涡量的条件,也是产生涡量的机制.实际上,黏性流体中的剪切力不能在流体内部产生涡量.而是在剪切变形的不连续处产生涡量,再由黏性扩散到流体的其他区域.准确地讲,黏性流动的涡量不是由剪切应力"搓"出来的,而是在剪切变形的不连续处"裂"出来的.     

气固两相混合层二维涡配对的数值研究

采用双向耦合模型中的速度耦合模型,数值模拟了气固两相混合层中涡的配对、合并过程,文中采用拟谱方法对流场进行了直接数值模拟,用Lagrange模型跟踪固粒,结果发现,在双向耦合过程中Stokes数仍然是重要的参数,但不是唯一影响流场的参数,流场的发展还与固粒的体积浓度、固粒的相对密度以及固粒大小等因素有关。固粒抑制流场的发展,阻碍涡的配对、合并,加快了涡量的扩散;小St数的固粒仍然跟随流体运动,大St数的固粒趋向于低涡量区的趋势减弱。     

深海多立柱浮式平台涡激运动特性研究

基于不可压缩纳维-斯托克斯(Navier-Stokes)方程和改进的延迟脱体涡模拟方法(improved delayed detached edd ysimulation method,IDDES),数值研究了深海多立柱浮式平台在海流作用下的涡激运动响应特性.以张力腿平台为对象,计算了该平台在0°,22.5°和45°流向角下的横向和艏摇涡激运动响应,分析了涡激运动响应幅度、频率比随约化速度的变化规律,研究了涡激运动响应能量的分布趋势.数值预报结果与模型实验数据吻合良好,证实了数值模型的有效性;研究发现,当约化速度介于7.0和14.0之间时,横向运动发生锁频,运动幅值稳定在0.2D~0.4D(D为立柱宽)之间,而艏摇涡激运动和约化速度呈线性递增关系;在横向运动锁频区间内,由于艏摇激振力矩主要受升力主导,艏摇频率与横向运动频率相同;相对于0°来流,22.5°和45°流向下的涡激运动频率更高,但艏摇运动能量仅为0°流向角下的10%.基于计算结果,进一步分析了多立柱平台涡激运动中的三维流场结构.      

风力机叶片的三维非定常气动特性估算

结合动量-叶素理论、非定常空气动力和动态失速模型来计算风力机叶片的二维非定常气动特性,并在此基础上经过适当修正后考虑三维旋转效应的非定常气动特性。分析比较二维和三维两种计算结果,给出更为合理的计算叶片非定常气动特性的方法。计算结果表明,风力机叶片的三维非定常气动特性计算结果与二维时的计算结果相比有较大改善。     

塑性理论在桩端盖机设计中的应用

桩端盖机设计和生产必须解决塑性极限弯矩、配套电动机功率、工件下料尺寸等计算问题．把桩端盖机的工件加工过程视为一个塑性折弯过程，根据问题的特点，提出基于Tresca屈服准则的一种简化计算方法，解决了工件在塑性折弯过程中的受力分析、应力和变形分析等问题，得到相应的数学表达式．最后结合某厂生产实际给出了一个计算实例，为桩端盖机设计提供了理论依据．     

Analytical solutions for a single blade in vertical axis turbine motion in two-dimensions

An analytical model for a time dependent two dimensional flow around a moving profile is developed. The model is suitable for fast aerodynamic and aeroelastic coupling calculations. It determines the inviscid pressure distribution in the vicinity of one blade and the force on the blade in arbitrary two dimensional motion. The method is more flexible than previous analysis: it can represent any profile, pitching motion and blade attachment position. The method is based on conformal mapping techniques and Laurent's series decomposition and is faster and more accurate than standard panel methods. A main idea is to directly treat the singularities of the flow in a mapped plane where any geometrical plane is simplified to a circle. The vorticity is assumed to be shed in the form of a continuous vortex sheet near the trailing edge.     

Large aerodynamic forces on a sweeping wing at low Reynolds number

The aerodynamic forces and flow structure of a model insect wing is studied by solving the Navier-Stokes equations numerically. After an initial start from rest, the wing is made to execute an azimuthal rotation (sweeping) at a large angle of attack and constant angular velocity. The Reynolds number (Re) considered in the present note is 480 (Re is based on the mean chord length of the wing and the speed at 60% wing length from the wing root). During the constant-speed sweeping motion, the stall is absent and large and approximately constant lift and drag coefficients can be maintained. The mechanism for the absence of the stall or the maintenance of large aerodynamic force coefficients is as follows. Soon after the initial start, a vortex ring, which consists of the leading-edge vortex (LEV), the starting vortex, and the two wing-tip vortices, is formed in the wake of the wing. During the subsequent motion of the wing, a base-to-tip spanwise flow converts the vorticity in the LEV to the wing tip and the LEV keeps an approximately constant strength. This prevents the LEV from shedding. As a result, the size of the vortex ring increases approximately linearly with time, resulting in an approximately constant time rate of the first moment of vorticity, or approximately constant lift and drag coefficients. The variation of the relative velocity along the wing span causes a pressure gradient along the wingspan. The base-to-tip spanwise flow is mainly maintained by the pressure-gradient force. The project supported by the National Natural Science Foundation of China (10232010)     

ON THE FORCE DECOMPOSITIONS OF LIGHTHILL AND MORISON

Lighthill's assertion that the viscous drag force and the inviscid inertia force acting on a bluff body immersed in a time-dependent flow operate independently is not in conformity with the existing exact solutions and experimental facts. The two force components are interdependent as well as dependent on the parameters characterizing the phenomenon: the rate of diffusion of vorticity, relative amplitude of the oscillation, and the surface roughness.     

Experimental study of three-dimensional vortex structures in translating and rotating plates

Motivated by the unsteady force generation of flying animals, vortex formation and vorticity transport processes around small aspect-ratio translating and rotating plates with a high angle of attack are investigated. Defocusing Digital Particle Image Velocimetry was employed to explore the structure and dynamics of the vortex generated by the plates. For both translating and rotating cases, we observe the presence of a spanwise flow over the plate and the consequent effect of vorticity transport due to the tilting of the leading-edge vortex. While the spanwise flow is confined inside the leading-edge vortex for the translating case, it is widely present over the plate and the wake region of the rotating case. The distribution of the spanwise flow is a prominent distinction between the vortex structures of these two cases. As the Reynolds number decreases, due to the increase in viscosity, the leading-edge and tip vortices tend to spread inside the area swept by the rotating plate. The different vorticity distributions of the low and high Reynolds number cases are consistent with the difference in measured lift forces, which is confirmed using the vorticity moment theory.     

融入压力分布信息的气动力建模方法

气动外形优化设计与飞行器性能分析中, 直接运用数值模拟或风洞实验获取气动力的成本高, 构建代理模型是提高外形优化和性能分析效率的重要途径. 然而, 构建模型的过程中, 研究者只关注积分后的气动力和力矩信息. 本文通过充分利用采样过程中所产生的压力分布信息, 来提高建模的精度和泛化性, 进而降低样本获取的成本. 提出了一种小样本框架下融入压力分布信息的气动力建模方法, 首先通过数值模拟或风洞试验获得不同流动参数状态下翼型表面的压力分布信息和气动系数, 其次通过本征正交分解技术对压力分布信息进行特征提取, 获取不同输入参数状态下压力分布信息对应的POD系数, 之后结合输入参数通过Kriging算法对压力分布信息进行建模, 将压力分布信息积分得到低精度气动系数的预测模型, 最后低精度气动系数结合输入参数通过Kriging算法构造高精度的气动系数预测模型. 通过同状态变翼型算例以及CAS350翼型变状态算例进行验证, 该方法相比于传统的克里金模型直接预测气动力, 有效提高了气动力的预测精度和模型的鲁棒性, 同时缩小了学习样本的数据量.      

模型昆虫翼作非定常i运动时的气动力特性

基于Navier-Stokes方程的数值解,研究了一模型昆虫翼在小雷诺数(Re=100)下作非定常运动时的气动力特性.这些运动包括翼启动后的常速转动,快速加、减速转动,常速转动中快速上仰(模拟昆虫翼的上挥或下拍、翻转等运动).有如下结果在小雷诺数下,模型昆虫翼以大攻角(α=35°)作常速转动运动时,由于失速涡不脱落,可产生较大的升力系数.其机理是翼转动时,翼尖附近(该处线速度大)上翼面压强比翼根附近(该处线速度小)的小得多,因而存在展向压强梯度,同时存在着沿展向的离心力,此展向压强梯度和离心力导致的展向流动在失速涡的轴向方向,其可避免失速涡脱落.模型昆虫翼在快速加、减速转动和快速上仰运动中,虽然雷诺数小,但由于在短时间内产生了大涡量,也可产生十分大的气动力,例如在快速上仰运动中,升力系数可大于10.     

Numerical simulation of flapping‐wing insect hovering flight at unsteady flow

A computational fluid dynamics (CFD) analysis was conducted to study the unsteady aerodynamics of a virtual flying bumblebee during hovering flight. The integrated geometry of bumblebee was established to define the shape of a three‐dimensional virtual bumblebee model with beating its wings, accurately mimicking the three‐dimensional movements of wings during hovering flight. The kinematics data of wings documented from the measurement to the bumblebee in normal hovering flight aided by the high‐speed video. The Navier–Stokes equations are solved numerically. The solution provides the flow and pressure fields, from which the aerodynamic forces and vorticity wake structure are obtained. Insights into the unsteady aerodynamic force generation process are gained from the force and flow‐structure information. The CFD analysis has established an overall understanding of the viscous and unsteady flow around the virtual flying bumblebee and of the time course of instantaneous force production, which reveals that hovering flight is dominated by the unsteady aerodynamics of both the instantaneous dynamics and also the past history of the wing. A coherent leading‐edge vortex with axial flow and the attached wingtip vortex and trailing edge vortex were detected. The leading edge vortex, wing tip vortex and trailing edge vortex, which caused by the pressure difference between the upper and the lower surface of wings. The axial flow, which include the spanwise flow and chordwise flow, is derived from the spanwise pressure gradient and chordwise pressure gradient, will stabilize the vortex and gives it a characteristic spiral conical shape. Copyright © 2006 John Wiley & Sons, Ltd.