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基于同心旋转圆柱间Oldroyd-B型流体的六维动力系统,探讨了小间隙大扰动条件下高分子添加剂对滑动轴承间油膜非线性稳定性的影响。结果表明,弱弹性流体的失稳结构与牛顿流体相似,随着转速的增加,流体以同宿轨道分岔失稳,与牛顿流体相比,少量的高分子添加剂具有推迟流体层流的稳的作用。 相似文献
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基于修正的Darcy模型, 介绍了多孔介质内黏弹性流体热对流稳定性研究的现状和主要进展. 通过线性稳定性理论, 分析计算多孔介质几何形状(水平多孔介质层、多孔圆柱以及多孔方腔)、热边界条件(底部等温加热、底部等热流加热、底部对流换热以及顶部自由开口边界)、黏弹性流体的流动模型(Darcy-Jeffrey, Darcy-Brinkman-Oldroyd以及Darcy-Brinkman -Maxwell模型)、局部热非平衡效应以及旋转效应对黏弹性流体热对流失稳的临界Rayleigh数的影响. 利用弱非线性分析方法, 揭示失稳临界点附近热对流流动的分叉情况, 以及失稳临界点附近黏弹性流体换热Nusselt数的解析表达式. 采用数值模拟方法, 研究高Rayleigh数下黏弹性流体换热Nusselt数和流场的演化规律,分析各参数对黏弹性流体热对流失稳和对流换热速率的影响.主要结果:(1)流体的黏弹性能够促进振荡对流的发生;(2)旋转效应、流体与多孔介质间的传热能够抑制黏弹性流体的热对流失稳;(3)在临界Rayleigh数附近,静态对流分叉解是超临界稳定的, 而振荡对流分叉可能是超临界或者亚临界的,主要取决于流体的黏弹性参数、Prandtl数以及Darcy数;(4)随着Rayleigh数的增加,热对流的流场从单个涡胞逐渐演化为多个不规则单元涡胞, 最后发展为混沌状态. 相似文献
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《力学学报》2018,(6)
基于修正的Darcy模型,介绍了多孔介质内黏弹性流体热对流稳定性研究的现状和主要进展.通过线性稳定性理论,分析计算多孔介质几何形状(水平多孔介质层、多孔圆柱以及多孔方腔)、热边界条件(底部等温加热、底部等热流加热、底部对流换热以及顶部自由开口边界)、黏弹性流体的流动模型(Darcy-Jeffrey, DarcyBrinkman-Oldroyd以及Darcy-Brinkman-Maxwell模型)、局部热非平衡效应以及旋转效应对黏弹性流体热对流失稳的临界Rayleigh数的影响.利用弱非线性分析方法,揭示失稳临界点附近热对流流动的分叉情况,以及失稳临界点附近黏弹性流体换热Nusselt数的解析表达式.采用数值模拟方法,研究高Rayleigh数下黏弹性流体换热Nusselt数和流场的演化规律,分析各参数对黏弹性流体热对流失稳和对流换热速率的影响.主要结果:(1)流体的黏弹性能够促进振荡对流的发生;(2)旋转效应、流体与多孔介质间的传热能够抑制黏弹性流体的热对流失稳;(3)在临界Rayleigh数附近,静态对流分叉解是超临界稳定的,而振荡对流分叉可能是超临界或者亚临界的,主要取决于流体的黏弹性参数、Prandtl数以及Darcy数;(4)随着Rayleigh数的增加,热对流的流场从单个涡胞逐渐演化为多个不规则单元涡胞,最后发展为混沌状态. 相似文献
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研究了4自由度不平衡弹性转子在非线性油膜力、非线性内阻力和非线性弹性力联合作用下的动力学特性。结果表明,当只有非线性油膜力作用时,转子只存在由于油膜失稳而导致的倍周期分岔。而当非线性油膜力与非线性内阻力共同作用时,在油膜失稳后,转子产生低频振动。转速继续增加,还会诱发内阻失稳,产生概周期运动。在倍周期分岔中,存在分岔激变现象。本文发现的由于油膜涡动而导致的内阻失稳(概周期运动)是一种未见报道的转子失稳模式(组合失稳),它与油膜失稳(倍周期运动)一起可作为转子故障诊断的典型失稳模式。 相似文献
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表面活性剂分子溶于水自组装形成的蠕虫状胶束溶液由于其高表面活性和高黏弹性,具有独特的力学响应特性,呈现出与牛顿流体迥异的流动特征。采用考虑溶液中胶束可逆断裂和重构效应的双组分VCM本构方程,研究了溶液流变学特性,发现溶液拉伸应力随应变率呈现出拉伸硬化-稀化-硬化的强非线性特征。研究了蠕虫状胶束溶液绕圆柱流动的特性,在低雷诺数(Re~0.01)下,当维森贝格数Wi>1时,流场出现较强的不稳定性,绕流尾迹区流场呈现随时间波动的特征,通过功率谱分析发现流场波动具有拟周期特征,当胶束断裂引起流体拉伸应力对弹性响应的非线性增强时,流场波动的不规则性增强。圆柱后缘附近出现一对周期性变化的对称涡,表明流体弹性会促进流动分离的发生,流体拉伸应力对弹性变化的强非线性响应是造成低雷诺数绕流流动不稳定性的根本原因,揭示了蠕虫状胶束溶液在低雷诺数高黏弹性时绕流流动不稳定性的产生机理。 相似文献
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在低雷诺数下,忽略惯性影响的黏弹性流体中存在的非线性应力可以引发湍流现象.在流体力学的教学过程中,为了让学生对此有直观认识,设计制作了黏弹性流体实验台.采用高速摄影的方法,对流体由弹性失稳而引起的流态变化过程进行观测.在教学实践中发现,该实验台操作简单,能够非常直观地对流体弹性引起的流动失稳过程进行显示.通过学生自己动手实践,极大地加深了对流体弹性效应引起的流动失稳过程的理解. 相似文献
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曲率对机翼边界层二次失稳影响 总被引:1,自引:0,他引:1
采用非线性抛物化扰动方程(NPSE)计算了层流机翼NLF0415(2)在几个工况下横流涡的非线性发展,应用Floquet理论分析了横流涡的二次失稳.较系统地分析了后掠机翼在多种参数下表面曲率对其流动稳定性的影响.Haynes证明曲率对于横流在线性稳定性计算(LST)和非线性抛物化扰动方程计算中(NPSE)都起着非常明显的稳定作用;然而,该文计算结果表明,曲率在二次失稳计算中影响不大. 相似文献
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以非局部弹性理论为基础,采用欧拉-伯努利梁模型,考虑碳纳米管的小尺度效应,应用哈密顿原理获得了温度场作用下的输流悬臂单层碳纳米管(SWCNT)的振动控制方程以及边界条件,依靠微分变换法(DTM法)对此高阶偏微分方程进行求解,通过数值计算研究了温度场中悬臂单层输流碳纳米管的振动与颤振失稳问题。结果表明:管内流体流速、温度场中温度变化情况与小尺度参数都会对系统振动频率以及颤振失稳临界流速产生影响。其中,小尺度效应将会降低悬臂输流系统的稳定性,使系统更为柔软;而高温场与低温场对系统动态失稳的影响不同,低温场中随温度变化值的增加,系统的稳定性提高;高温场这一作用效果恰好与之相反。 相似文献
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Yujie We 《Acta Mechanica Sinica》2014,(6):846-848
The construction and operation of high-speed rail(HSR)grid within the past two decades in China,in terms of the scale,may be possibly comparable to any national-wide construction in the history of China,even the Great Wall.By counting railways with commercial train service at the speed of200 km/h,China has the world’s longest HSR network with over 19 369.8 km of track in service today and this number 相似文献
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R. J. Sobey 《国际流体数值方法杂志》1982,2(3):277-297
Space-time finite element solutions of the convection–dispersion equation using higher-order nodal continuity and Hermitian polynomial shape functions are described. Five separate elements ranging from a complete linear element with C0,0 nodal continuity to a complete first-order Hermitian element with C1,1 nodal continuity are subjected to detailed analysis. Wave deformation analyses identify the source of leading or trailing edge oscillations, trailing edge oscillations being the major source of difficulty. These observations are confirmed by numerical experiments which further demonstrate the potential of higher-order nodal continuity. The performance of the complete first-order Hermitian element is quite satisfactory and measurably superior to the linear element. 相似文献
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Cécile Lemaitre Md. Mahmud Alam Pascal Hémon Emmanuel de Langre Yu Zhou 《Comptes Rendus Mecanique》2006,334(3):158-163
Rainwater rivulets appear on inclined cables of cable-stayed bridges when wind and rain occur simultaneously. In a restricted range of parameters this is known to cause vibrations of high amplitudes on the cable. The mechanism underlying this effect is still under debate but the role of rainwater rivulets is certain. We use a standard lubrication model to analyse the dynamics of a water film on a cylinder under the effect of gravity and wind load. A simple criterion is then proposed for the appearance and position of rivulets, where the Froude number is the control parameter. Experiments with several geometries of cylinder covered with water in a wind tunnel show the evolution of the rivulets with the Froude number. Comparison of the prediction by the model with these experimental data shows that the main mechanism of rivulet formation and positioning is captured. To cite this article: C. Lemaitre et al., C. R. Mecanique 334 (2006). 相似文献
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We establish a theoretical model to explain the nucleation of a crystal of helium by an acoustic over-pressure. We explain the interfacial laws for this ultra-fast cristallization, close to the sound speed. Assuming spherical symmetry and taking into account the experimental data, we recover the dynamics of the growth and melting during an over-pressure impulse. To cite this article: M. Ben Amar et al., C. R. Mecanique 331 (2003). 相似文献
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Based on the use of two different preparation procedures for reconstituting triaxial samples of sand, i.e. wet tamping and dry pluviation, significant differences in associated mechanical behaviour are observed on a reference sand with respect to the phenomenon of ‘static’ liquefaction. Wet tamping favours the initiation of liquefaction instability, whereas dry pluviation favours a more stable behaviour, less susceptible to liquefaction. Microscopic observation of corresponding sand specimens allows us to identify two well differentiated structures, i.e., for wet tamping, an irregular structure with predominance of aggregates (aggregated grains) and macropores, very contractant and unstable and, for dry pluviation, a more regular structure, without macropores, more dilatant and more stable. These observations show the importance of further characterization, based on the introduction of appropriate parameters, of the initial structure of sandy materials, strongly dependant upon their mode of formation (natural or artificial). To cite this article: N. Benahmed et al., C. R. Mecanique 332 (2004). 相似文献
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Sources of Complexity in Human Systems 总被引:3,自引:0,他引:3
Complex is a special attribute we can give to many kinds of systems. Although it is used often as a synonym of difficult, it has a specific epistemological meaning, which is going to be shared by the incoming science of complexity. Difficult is an object which, by means of an adequate computational power, can be deterministically or stochastically predictable. On the contrary complex is an object which can not be predictable because of logical impossibility or because its predictability would require a computational power far beyond any physical feasibility, now and forever. For complexity refers to some observing system, it is always subjective, and thus it is defined as observed irreducible complexity. Human systems are affected by several sources of complexity, belonging to three classes, in order of descending restrictivity. Systems belonging to the first class are not predictable at all, those belonging to the second class are predictable only through an infinite computational capacity, and those belonging to the third class are predictable only through a trans-computational capacity. The first class has two sources of complexity: logical complexity, directly deriving from self-reference and Gödel's incompleteness theorems, and relational complexity, resulting in a sort of indeterminacy principle occurring in social systems. The second class has three sources of complexity: gnosiological complexity, which consists of the variety of possible perceptions; semiotic complexity, which represents the infinite possible interpretations of signs and facts; and chaotic complexity, which characterizes phenomena of nonlinear dynamic systems. The third class coincides with computational complexity, which basically coincides with the mathematical concept of intractability. Artificial, natural, biological and human systems are characterized by the influence of different sources of complexity, and the latter appear to be the most complex. 相似文献