李雅普指数在轴承故障诊断中的应用研究

研究铁路货车轴承的非线性特性,针对轴承的不同状态,对测量的时间序列计算其最大李雅普诺夫指数,结果表明:不同故障状态下的最大李雅普诺夫指数不同,因此可以把最大李雅普诺夫指数作为判别轴承故障的特征量。     

绕三维水翼非定常空化流动结构的数值与实验研究

为了说明非定常空化的流动机理,该文采用数值与实验相结合的方法对绕三维水翼片状和云状空化流动结构进行了研究.实验在高速水洞中进行,采用高速录像技术观测了片状和云状空化阶段的空穴形态.数值计算基于均相流模型,汽液混合区域密度由质量传输方程调节.利用商业软件二次开发技术引入准确描述空化流场非定常特性的FBM 湍流模型,进行绕三维水翼的数值模拟,获得了随时间变化的空穴形态、压力和速度分布等流场结构.与实验结果对比发现,数值计算结果与实验基本一致.在片状空化阶段,空穴稳定地附着在水翼表面,只有空穴尾部不断的有小空泡团沿着翼弦方向脱落.在云状空化阶段,清楚得描述了空穴的产生-发展-脱落-溃灭的准周期性变化,并准确地捕捉到空泡脱落时,附着在翼型前端的U 型空穴和翼展方向不同强度的反向射流,脱落的空泡由翼型中前部旋涡状脱落.     

飞机偏航角控制系统的稳定性仿真与分析

用李雅普诺夫直接法分析了具有非线性部分的飞机偏航角控制系统的绝对稳定性问题。根据系统构成与动态方程,以递增叠加法求得李雅普诺夫函数及其导数,由此得出系统绝对稳定的条件。采用波波夫谐波线性化方法,确定非线性部分的取值范围。然后在仿真计算中采用波波夫稳定判据,证明在改变系统参数的情况下,系统稳定性将随之改变,表明李雅普诺夫直接法所得的系统绝对稳定性条件能够对系统稳定判断起到较好的指导效果。     

绕回转体初生空化流场特性的实验及数值研究

该文采用实验与数值模型相结合的方法对绕回转体的初生空化流场进行了分析,研究了头型对初生空化流场特性的影响,数值模型中,为了精确捕捉由于分离流动而产生的漩涡结构,湍流模型采用了一种基于空间尺度修正的滤波器模型(FBM),实验中,采用高速录像技术观察了初生空化形态,并应用粒子测速系统(PIV)测量了相应工况下,初生空化流场的速度及涡量分布,研究结果表明:头型对绕回转体的初生空化流场具有显著的影响,不同回转体的初生空化数随着肩部曲率突变增大而逐渐增大,在初生空化工况下,平头和锥头回转体肩部的高剪切流动区出现了不规则的漩涡分离结构,初生空化首先在该分离区域内产生,而不是发生在回转体的物面上或在物体邻近处,此时,初生空化流场体现出明显的漩涡脉动特性,流场中的低速高脉动区域对应于空化核心区,涡量主要亦集中在该漩涡分离区域内,对于圆头回转体,其初生空化流场比较稳定,“指状”的片状空泡附着在回转体表面上。     

基于Lyapunov稳定性理论的模型参考自适应控制

主要研究模型参考自适应控制设计方法,主要介绍采用Lyapunov(李雅普诺夫)稳定性理论设计的自适应控制器,并假设可以获得对象的状态变量,利用这些状态变量构成自适应控制率。在利用MATLAB语言,通过仿真分析基于Lyapunov稳定性理论的自适应控制器的性能。     

基于Lyapunov稳定性理论的模型参考自适应控制

主要研究模型参考自适应控制设计方法,主要介绍采用Lyapunov(李雅普诺夫)稳定性理论设计的自适应控制器,并假设可以获得对象的状态变量,利用这些状态变量构成自适应控制率。在利用MATLAB语言,通过仿真分析基于Lyapunov稳定性理论的自适应控制器的性能。     

液压节流阀内非定常空化特性的数值分析

基于修正的RNG k-ε湍流模型并结合Schnerr-Sauer空化模型及多相流模型对液压节流阀内部非定常空化流动进行了数值计算,分析了节流阀内空化形态的周期性变化过程及其对应的内部流场的压力脉动特性,讨论了非定常空化形态演变与压力脉动之间的关系,同时研究了不同空化阶段对节流阀内速度场的影响差异。结果表明:节流阀内空化的发展是一种非定常的周期性过程,主要包括空化的产生、脱落以及溃灭;在空化初生时,不同位置截面在轴向速度分布上均未出现反向射流,但在空化溃灭阶段,不同位置截面在靠近壁面处均存在一个宽度大约1 mm的反向射流区,且不同截面位置所对应的反向射流的强度不同;阀口下游不同监测点处压力脉动的主频与空化结构演化的周期有着良好的一致性,此外还存在一个次级频率,对应为小尺度空化脱落、溃灭的频率。     

多电机协调的参考模型自适应控制

本文基于李雅普诺夫稳定理论．利用参考模型自适应控制系统设计方法去解决多电机协调问题。为了加快协调和改善自适应过程的动态品质．本文采用了比例积分型自适应律。     

李雅普诺夫稳定性理论在目标识别效果评估中的应用

针对目标识别效果的稳定性评估，建立了目标识别系统的动态模型，随后基于李雅普诺夫稳定性理论，给出了目标识别系统识别效果的动态稳定性分析并完成初步仿真。     

基于传输方程空化模型的定常自然空化流场数值计算

采用基于传输方程的空化模型，应用自主开发的软件对水翼和水下回转体定常自然空化流场进行了数值计算。采用基于压力修正和多块结构化网格的有限体积方法数值求解Favre平均的NS方程，运用k-ε两方程加壁函数的湍流模式封闭雷诺应力。计算结果与已有实验结果符合较好。     

Detection and characterization of transport barriers in complex flows via ridge extraction of the finite time Lyapunov exponent field

This paper proposes an algorithm to detect and characterize ridges in the finite time Lyapunov exponent (FTLE) field obtained from a continuous dynamical system or flow. These ridges represent time‐dependent separatrices of the flow and are also called Lagrangian coherent structures (LCS). LCS have been demonstrated to be an effective way to analyze realistic time‐chaotic flows, although they can be quite complex. Therefore, in order to exploit the information that LCS can provide it is important to locate and characterize these structures in a systematic way. This can be accomplished by interpreting the FTLE as a height field and detecting the LCS as ridges of this graph. Methodologies developed in the image processing framework are integrated with dynamical system inspired approaches in order to characterize ridge strength and location. The main novel contribution of the proposed algorithm is a scheme to connect sets of points into curves or surfaces (rather than distributions of points around a ridge axis) and classify these curves or surfaces using a dynamical systems measure of strength. This approach provides the capability to track ranked LCS in space and time. The results are presented for a simple analytical model and noisy LCS from realistic three‐dimensional geophysical fluid data. Copyright © 2010 John Wiley & Sons, Ltd.     

Numerical analysis of unsteady behavior of cloud cavitation around a NACA0015 foil

Three-dimensional unsteady cavitating flow around a NACA0015 hydrofoil fixed between the sidewalls was simulated and the mechanism of U-shaped cloud cavity formation was clarified. A local homogeneous model was used for the modeling of the vapor–liquid two-phase medium. The compressible two-phase Navier–Stokes equations as the governing equations were solved. To describe the phase change between water and vapor, the mass transfer model based on the theory of evaporation/condensation on a plane interface was introduced. The cell-centered finite volume method was employed to discretize the governing equations. Assuming turbulent flow, the turbulent eddy viscosity coefficient was computed by using the Baldwin–Lomax model with the Degani–Schiff modification. As a result, even in the case of cavitating flow without sidewalls, the shed cloud cavities has slightly 3D structure, which was not so much large as extending across the whole spanwise direction. On the other hand, in the case of cavitating flow with sidewalls, the end of sheet cavities bows in the spanwise direction because of the development of boundary layer near both sidewalls. After that, due to the occurring of the reentrant jet towards the mid-span region, the sheet cavities breaks off from mid-span region near the leading edge of the hydrofoil, and became the vortical cloud cavities, which have the large-scale U-shaped structure.     

Numerical study of liquid nitrogen cavitating flow through nozzles of various shapes

The cavitating flow of cryogenic liquid through a spray nozzle is influenced by many factors, such as unique thermophysical properties of cryogenic liquid, the inflow temperature and the complicated geometrical structure of the spray nozzle. The geometrical parameters of liquid nitrogen spray nozzles have a profound impact on cavitating flow which in turn affects spray atomization characteristics and cooling performance. In present study, CFD simulations are performed to investigate influence of the nozzle geometry on the liquid nitrogen cavitating flow. The mixture model is used to describe the liquid-vapor two phase flow, and both the cavitation and evaporation are considered for the phase change. The predictions of mass flow of liquid nitrogen spray are validated against experimental results. The effects of geometric parameters, including the outlet orifice diameter and the length of nozzle, the inlet edge angle of orifice, the inlet corner radius of orifice, the orifice shape and different positions of swirl vanes, are investigated under a wide range of pressure difference and inflow temperature. The results show that the effects of geometric parameters on cavitating flow show different trends under subcooled conditions compared with saturated temperature conditions. The flow characteristics are more affected by the changes of the inlet edge angle, the inlet corner radius, and the orifice shape. The insert of swirl vanes has an effect on the distribution of the cavitated vapor within the orifice, but it has little influence on flow characteristics. The results could enrich our knowledge of liquid nitrogen cavitating flow in spray nozzles of various shapes.     

轴对称体空化水动力脉动特性的实验研究

为了研究轴对称体非定常空化的脉动特性,采用高速全流场显示技术和动态测力系统实验研究了绕半球型和平头轴对称体的非定常空化流场及其动力特性。实验在闭式水洞中进行,采用高速摄影的方法观察了在不同空化数下绕半球型和平头轴对称体的空穴形态,总结了在不同空化数下空泡形态的脉动特性;测量了轴对称体受到的阻力,并对阻力信号进行了时频分析,得到了轴对称体阻力在非定常空化阶段的时频特征。结果表明:空泡形态及其对应的动力特征随着空化数的变化存在明显的非定常特性,空化流场形态与动力特征频率存在高度的相关性。并且不同头型轴对称体的脉动特性存在明显的差异,半球型轴对称体空泡流动的脉动主要是空泡尾部的高频小脱落引起的,而平头轴对称体的空泡流脉动成分主要是大尺度的漩涡空泡团的周期性脱落,空化流场的低频特征频率与空泡的大断裂相对应。     

Preliminary results on acoustic modelling of cavitating propellers

The numerical prediction of the acoustic pressure field induced by cavitating marine propellers is addressed. A hydrodynamic model for transient sheet cavitation on propellers in non–uniform inviscid flow is coupled with a hydroacoustic model based on the Ffowcs Williams–Hawkings equation. The proposed hydroacoustic approach, novel to marine applications, allows to split the noise signature into thickness and loading term contributions. Both hydrodynamic and hydroacoustic model equations are solved via boundary integral formulations. Numerical predictions of the propeller noise by using the Ffowcs Williams–Hawkings equation are compared to those obtained by a classical Bernoulli equation approach. The influence of cavitation on the noise waveforms is discussed by comparing non–cavitating and cavitating propeller flow results. The authors wish to thank Prof. S.A. Kinnas for providing a detailed documentation of the experiment used as the test case in the present analysis. The present work was supported by the Ministero dei Trasporti e della Navigazione in the frame of INSEAN Research Program 2000–02.     

附着型空穴断裂及脱落机制的实验研究

为深入研究附着型空穴断裂及空泡脱落机理,该文采用实验方法对收缩-扩张流道内的云状空化流动现象进行了研究。实验在空化水洞中进行,采用同步测量技术,通过高速摄像机和压力传感器同步获取了云状空化阶段的附着型空穴形态和壁面压力。研究结果表明,附着型空穴断裂及空泡脱落存在两种不同的机制:回射流机制和间断面推进机制。其中,回射流于附着型空穴尾部形成,紧贴壁面并持续向前推进,其厚度远小于附着型空穴厚度,在回射流向上游推进过程中,其覆盖区域壁面压力波动较小,当回射流运动到附着型空穴前缘,"剪断"附着型空穴,造成空穴断裂及脱落;附着型空穴内部形成的向上游推进的间断面是造成空穴断裂及脱落的另一种机制,间断面厚度为当地空穴厚度,间断面前后含气率差异较大,间断面前为高含气率的纯汽相区,间断面后为低含气率的水汽混合区。间断面向上游推进过程中,间断面位置处会出现压力尖峰,当间断面推进到喉口位置,带来喉口部位压力升高,降低了喉口位置空泡生成率,使得新生成的空泡与向下游运动的脱落型空泡分开,导致空穴断裂及脱落。     

Semianalytical solution of unsteady quasi-one-dimensional cavitating nozzle flows

Unsteady quasi-one-dimensional bubbly cavitating nozzle flows are considered by employing a homogeneous bubbly liquid flow model, where the nonlinear dynamics of cavitating bubbles is described by a modified Rayleigh–Plesset equation. The model equations are uncoupled by scale separation leading to two evolution equations, one for the flow speed and the other for the bubble radius. The initial-boundary value problem of the evolution equations is then formulated and a semianalytical solution is constructed. The solution for the mixture pressure, the mixture density, and the void fraction are then explicitly related to the solution of the evolution equations. In particular, a relation independent of flow dimensionality is established between the mixture pressure, the void fraction, and the flow dilation for unsteady bubbly cavitating flows in the model considered. The steady-state compressible and incompressible limits of the solution are also discussed. The solution algorithm is first validated against the numerical solution of Preston et al. [Phys Fluids 14:300–311, 2002] for an essentially quasi-one-dimensional nozzle. Results obtained for a two-dimensional nozzle seem to be in good agreement with the mean pressure measurements at the nozzle wall for attached cavitation sheets despite the observed two-dimensional cavitation structures.     

轴流泵叶顶泄漏涡空化的数值模拟与可视化实验研究

该文以某一等比例缩放模型泵为研究对象,采用修正的SST k-ω湍流模型和空化模型,对额定工况下轴流泵叶顶泄漏涡空化流进行了数值模拟,并与高速摄影结果进行了对比分析。探讨了叶顶区域泄漏涡空化流场结构,揭示了不同空化数下空化发生位置和空泡形态演变过程。研究结果表明,改进的数值模拟方法准确计算了叶顶区域空化流场的流动结构;轴流泵的初生空化为叶顶间隙空化和叶顶泄漏涡空化,随着空化数σ降低,叶顶泄漏涡卷吸区也出现了剪切层空化;在空化数较小工况下,沿着叶片吸力面在轴向形成空泡云,并在叶片尾缘存在周期性的空泡脱落和爆破过程,破坏了流动稳定性,并诱导产生空化噪声。