用面元法预报吊舱推进器水动力性能

面元法的各个环节有不同的数值处理方法，不同的方法计算结果有所不同，通过时各种方法的计算结果进行比较，得到适合螺旋桨计算的数值处理方法。用面元法时吊舱螺旋桨的水动力性能进行了计算，得到了吊舱推进器的敞水曲线和轴承力。     

基于B样条的螺旋桨升力面设计

为了改善螺旋桨的空泡性能，从效率和空泡性能2个方面的权衡来确定环量分布．在升力面理论设计中计及桨毂的影响．桨叶几何用较少的B样条控制角点来表示．将升力面理论计算的控制点处法向速度的平方和作为优化目标函数，并通过最小化目标函数得到桨叶几何．     

螺旋桨尾流动态结构实验研究

利用2D-PIV(Particle Image Velocimetry)技术在空泡水筒中对均匀来流下螺旋桨尾流场进行了测试。分析了尾流场中轴向、径向速度分布规律。计算了尾流中轴向平均诱导速度,利用提取到的涡心坐标计算得到了梢涡涡线螺距角,分析了诱导速度、梢涡涡线螺距角在尾流中的分布、演化规律。最后利用连续随机分布的若干流场速度子样,分别对尾流场的轴向、径向湍流度做了探索性分析,得到了尾流中湍流分布特征及该特征随螺旋桨载荷的变化。试验结果与理论分析吻合,分析数据为螺旋桨的设计以及性能预报提供了参考。     

船舶螺旋桨叶片的疲劳强度研究(报告Ⅱ:螺旋桨叶片的容许缺陷尺寸和探伤范围)

为了检测船舶螺旋桨的内部缺陷,成功地发展了一种超声波检测缺陷的技术,当将这种技术应用于实际螺旋桨时,必須确定允許的缺陷尺寸,以及叶片需要加以检测的区域。本文已为达到此目的而提出了如下的分析和计算。 (1)用线弹性断裂力学和螺旋桨材料的⊿K_Φ值分析疲劳裂纹是否从桨叶的缺陷处进一步扩展。 (2)在干若种应力、桨叶厚度和纵横比条件下计算缺陷的最大容許尺寸。 (3)用不稳定升力面理论计算了螺旋桨上在尾件流区缺陷中的应力分布,根据每个螺旋桨的临界缺陷尺寸分布,确定出所要检测的区域。     

非线性隐式极限状态方程失效概率计算的组合响应面法

提出组合响应面的新方法,用以计算设计点附近非线性程度较大的隐式极限状态方程的失效概率。该方法用主响应面和多个次响应面近似对失效概率贡献较大的区域,其响应面函数形式为不含交叉的二次多项式。主响应面依据传统响应面法通过选择适当的插值点和迭代运算获得,其设计点为主设计点。延坐标轴正负方向偏移主设计点得到拟均值点。以拟均值点为基础得到一组次响应面和次设计点。通过主次响应面在各自设计点处的切平面建立组合响应面近似原隐式极限状态方程,并计算其失效概率。算例结果说明所提方法具有较高精度。     

内齿珩轮的计算机辅助几何造型

内啮合珩齿工艺是一种高效低耗的齿轮精加工工艺,所用刀具是内齿珩轮,珩轮齿面形状为渐开线螺旋面的包络面.由于该包络面形状复杂,使珩轮基体齿面形状的设计和制作成为一个难题.针对这一难题,该文提出用计算机辅助几何造型方法设计珩轮齿面.首先,根据机床要求和被加工齿轮情况确定内齿珩轮的齿数、外径、高度以及珩轮与被加工齿轮的轴距和轴交角;其次,构建内齿珩轮和被加工齿轮的运动模型,并用拉伸构型法构造齿轮的实体模型,用圆柱构造珩轮毛坯的实体模型;然后,利用几何造型技术中的集合操作,对珩轮毛坯作循环差操作,直至形成准确的珩轮模型.最后,通过沿珩轮齿宽取断面的操作,得到珩轮齿面各部位的断面形状,并标注尺寸.     

大涡模拟预报螺旋桨辐射噪声的三种声学方法

螺旋桨噪声是船舶的三大噪声源之一,研究螺旋桨的噪声现象具有很大的现实意义。首先划分了螺旋桨的结构化网格,进行了网格收敛性分析,然后采用大涡模拟方法计算螺旋桨水动力。在噪声计算中采用了FW-H方程、结合Virtual lab的旋转偶极子辐射、涡声方程三种方法进行预报,并与换算到自由场的试验结果进行对比分析。结果表明,采用大涡模拟可以预报螺旋桨的辐射噪声,三种方法均满足工程应用需求。     

面导纳及其振动特性

本文研究反映结构之间接触表面上能量传递情况的面导纳及其振动特性。讨论了均布力和均布速度激励状态下矩形面导纳的计算方法，分析了受接触面积的大小和形状以及激励频率影响下的面导纳的动态特性及其与传统的点导纳的区别，提出了减少结构间接触面积上振动能量传递、提高隔振效果的有效方法。     

用偏移尾座法车削圆锥面的加工设计

本文介绍了在普通机床上用偏移尾座法加工外圆锥面的工艺规程设计，以及几种偏移尾座上层的方法。以介绍方法为宗旨，详细介绍了用偏移尾座法加工外圆锥面工艺规程主要内容、方法和步骤。     

受侧壁面限制的涡对融合

针对受壁面限制的涡对融合问题,以多段翼型的二元风洞试验为例,用雷诺平均的NS方程进行了模拟。结合侧壁面的表面摩擦力线、不同x向横截面流线的拓扑结构分析及总压分布云图,刻画了多段翼型的涡流流场结构,进而分析了受侧壁面干扰的涡对融合过程。并与无侧壁面干扰的涡对融合过程进行了比较。结果发现,壁面对涡对融合过程的影响,包括了对旋涡发展速度的延迟、旋涡诱导速度的限制及壁面摩擦引起的能量损失等方面,明显延长了涡对融合所需的时间。但是侧壁面干扰的影响范围有限。根据以上结果,建议二元风洞试验中,试验段宽度应考虑侧壁面影响范围;对展弦比较大的半模,应对机身加上垫块或者采用镜像法来消除侧壁面对襟翼涡等的干扰;对于展弦比较小的模型,则应采用全模进行试验。     

Boundary element simulation of inviscid flow around an oscillatory foil with vortex sheet

The dynamic performance of a rigid foil with harmonic vertical and rotational motions in fluid flow has been studied through velocity potential theory. A boundary element based time stepping scheme is introduced to simulate the flow around the foil and the vortex wake. The body surface condition is satisfied on the exact foil surface and the motion and deformation of the wake sheet shed at the trailing edge is tracked. Kelvin condition is satisfied and a Kutta condition for the unsteady motion is proposed to circumvent the singularity at the trailing edge. Point vortex, which is reduced from wake vortex dipole, is introduced to approximate the vorticity. The performance of foil NACA0012 with harmonic vertical and rotational motions are studied extensively; the propulsion/swimming mode, energy harvesting mode and the flying mode are analysed in detail.     

矩形柱二维尾流的稳定性研究

采用实验和数值模拟两种方法对长宽比为3∶1的矩形柱尾流进行了稳定性研究.实验发现当雷诺数接近临界值时,未加扰动的矩形柱尾流呈二维定常状态.当在柱体上游一定位置和下游靠近柱体的位置上沿垂直于来流方向施加一个宽度很小的短时脉冲射流扰动以后,扰动随时间放大,并出现旋涡脱落现象,并且这种扰动激发的旋涡脱落可以持续很长时间,不会衰减.而在下游较远处施加同样的扰动时,扰动将会衰减,不能激发出持续的旋涡脱落.数值模拟再现了上述实验结果,并且表明,当扰动强度(脉冲射流出口速度)较小时,不会出现持续的规则旋涡脱落,只有当扰动强度达到一定阈值时,旋涡脱落才能被激发.     

基于表面涡方法的单列圆柱流体诱导振动及流弹性不稳定研究

基于表面涡方法和流固耦合模型研究了Re=2.67×104时的单列圆柱流体诱导振动问题,计算了流体力、振动响应、涡脱落频率等,并给出了涡云图。计算模拟结果很好地重现了刚性单列圆柱在T/D=1.5(小间隙比)下以宽窄尾涡交替和多频为特征的非均匀流态,以及T/D=2.0的涡脱落现象。此外,该文还研究了单列弹性圆柱在T/D=1.5时的流体诱导振动以及流体弹性不稳定问题,计算了SG=1.29时圆柱列的无量纲临界速度。     

Numerical study of bluff body flow structures

Our recent progress in numerical studies of bluff body flow structures and a new method for the numerical analysis of near wake flow field for high Reynolds number flow are introduced. The paper consists of three parts. In part one, the evolution of wake vortex structure and variation of forces on a flat plate in harmonic oscillatory flows and in in-line steady-harmonic combined flows are presented by an improved discrete vortex method, as the Keulegan-Carpenter number (KC) varies from 2 to 40 and ratios ofU _m toU ₀ are ofO(10⁻¹),O(1) andO(10), respectively. In part 2, a domain decomposition hybrid method, combining the finite-difference and vortex methods for numerical simulation of unsteady viscous separated flow around a bluff body, is introduced. By the new method, some high resolution numerical visualization on near wake evolution behind a circular cylinder at Re=10², 10³ and 3×10³ are shown. In part 3, the mechanism and the dynamic process for the three-dimensional evolution of the Kármán vortex and vortex filaments in braid regions as well as the early features of turbulent structure in the wake behind a circular cylinder are presented numerically by the vortex dynamics method. This study was supported by the National Natural Science Foundation of China and the Laboratory for Nonlinear Mechanics, Institute of Mechanics, Chinese Academy of Sciences, as well as by the National Basic Research project “Nonlinear Science”.     

Three-dimensional vortex wake structure of flapping wings in hovering flight

Flapping wings continuously create and send vortices into their wake, while imparting downward momentum into the surrounding fluid. However, experimental studies concerning the details of the three-dimensional vorticity distribution and evolution in the far wake are limited. In this study, the three-dimensional vortex wake structure in both the near and far field of a dynamically scaled flapping wing was investigated experimentally, using volumetric three-component velocimetry. A single wing, with shape and kinematics similar to those of a fruitfly, was examined. The overall result of the wing action is to create an integrated vortex structure consisting of a tip vortex (TV), trailing-edge shear layer (TESL) and leading-edge vortex. The TESL rolls up into a root vortex (RV) as it is shed from the wing, and together with the TV, contracts radially and stretches tangentially in the downstream wake. The downwash is distributed in an arc-shaped region enclosed by the stretched tangential vorticity of the TVs and the RVs. A closed vortex ring structure is not observed in the current study owing to the lack of well-established starting and stopping vortex structures that smoothly connect the TV and RV. An evaluation of the vorticity transport equation shows that both the TV and the RV undergo vortex stretching while convecting downwards: a three-dimensional phenomenon in rotating flows. It also confirms that convection and secondary tilting and stretching effects dominate the evolution of vorticity.     

Flow visualization studies of vortices

An actual vortex in the Kármán vortex street downstream of a circular cylinder has a core of finite dimension which increases downstream. The circulation of the vortex is nearly constant. The ratiob/a which is 0.281 according to the theory of Kármán, grows from 0.2 to 0.4 in the near wake. In the flow about a circular cylinder rotating in a uniform flow, a Kármán vortex street, Görtler-type vortices and Taylor vortices are generated at the same time. In the flow about a circular cylinder impulsively started with a constant velocity, the primary twin vortices behind the cylinder induce secondary twin vortices near the separation point. At the beginning of the motion, the separation does not occur even though a reverse flow is observed in the boundary layer. Mutual slip-through of a pair of vortex rings was achieved by increasing the Reynolds number. A vortex ring rebounds from a plane surface due to the separation of the flow on the surface induced by the vortex ring, and the secondary vortex ring is formed from the separated shear layer.     

Regimes of flow past a vortex generator

A complete parametric investigation of the development of multi-vortex regimes in a wake past simple vortex generator has been carried out. It is established that the vortex structure in the wake is much more complicated than a simple monopole tip vortex. The vortices were studied by stereoscopic particle image velocimetry (SPIV). Based on the obtained SPIV data, a map of the regimes of flow past the vortex generator has been constructed. One region with a developed stable multivortex system on this map reaches the vicinity of the optimum angle of attack of the vortex generator.     

Effects of Dipole–Dipole Interaction on Vortex Motion in Bose–Einstein Condensates

In this paper, we study the dynamics of a single vortex and a corotating vortex pair in a quasi-two-dimensional Bose–Einstein condensate. The numerical results are obtained by solving the Gross–Pitaevskii equation. It is shown that the vortex motion is strongly dependent on the dipole–dipole interaction (DDI). For a single vortex, the periodic motion occurs with increasing dipole strength. The x-coordinate and y-coordinate as a function of time close to the shape of cosine wave and triangular wave when DDI is an isotropic repulsive interaction, respectively. In addition, it is evident that strong and anisotropic DDI makes the movement of vortex become fast and the amplitude enlarges. In the case of a corotating vortex pair, the dynamics is qualitatively consistent with a single vortex for weak dipole strength. Moreover, the periodic dynamics only appears for strong and anisotropic DDI and more rapid vortex motion is indicated.

     

A BEM for the prediction of free surface effects on cavitating hydrofoils

 In this paper, a boundary element method (BEM) for cavitating hydrofoils moving steadily under a free surface is presented and its performance is assessed through systematic convergence studies, comparisons with other methods, and existing measurements. The cavitating hydrofoil part and the free surface part of the problem are solved separately, with the effects of one on the other being accounted for in an iterative manner. Both the cavitating hydrofoil surface and the free surface are modeled by a low-order potential based panel method using constant strength dipole and source panels. The induced potential by the cavitating hydrofoil on the free surface and by the free surface on the hydrofoil are determined in an iterative sense and considered on the right hand side of the discretized integral equations. The source strengths on the free surface are expressed by applying the linearized free surface conditions. In order to prevent upstream waves the source strengths from some distance in front of the hydrofoil to the end of the truncated upstream boundary are enforced to be equal to zero. No radiation condition is enforced at the downstream boundary or at the transverse boundary for the three-dimensional case. First, the BEM is validated in the case of a point vortex and some convergence studies are done. Second, the BEM is applied to 2-D hydrofoil geometry both in fully wetted and in cavitating flow conditions and the predictions are compared to those of other methods and of the measurements in the literature. The effects of Froude number, the cavitation number and the submergence depth of the hydrofoil from free surface are discussed. Then, the BEM is validated in the case of a 3-D point source. The effects of grid and of the truncated domain size on the results are investigated. Lastly, the BEM is applied to a 3-D rectangular cavitating hydrofoil and the effect of number of iterations and the effect of Froude number on the results are discussed. Received 6 November 2000     

Simulating three-dimensional vortex motion by a vortex blob method

A three-dimensional vortex blob method was applied to calculate several vortex motions: the deformation of pseudo-elliptic vortex rings, the jet issuing from the pseudo-elliptic nozzle into flow of uniform velocity, the unsteady separated flow around a circular disk with an angle of attack, and the interaction of several vortex rings which approximately reproduced the Kolmogorov spectrum. In the first three cases, the viscous diffusion of vorticity was included. The pseudo-elliptic vortex rings experienced axis switching and split into a few deformed vortex rings. Rolling-up vortices in the pseudo-elliptic jet had a symmetric arrangement in the minor-axis plane and an antisymmetric arrangement in the major-axis plane in the developing region; further downstream, the vortices were arranged antisymmetrically in both planes. The wake behind the disk normal to the main flow reproduced the spiral and columnar modes of instability. A problem in the three-dimensional vortex method is that vorticity tends to diverge at a stage of evolution of the vortex motions. An approximate method of avoiding the divergence of vorticity is proposed.