低雷诺数下方柱—分隔板涡致振动数值研究

发布时间：2018-08-16 08:06

【摘要】：钝体绕流在自然界中是一种非常普遍的现象，涉及流动分离、旋涡脱落及振动等诸多问题，尾迹流场极为复杂，蕴涵着十分复杂的物理机理。脱涡过程在钝体两侧交替发生，使得钝体四周应力产生周期性的波动，进而使钝体受到周期性的作用力，如果钝体允许发现弹性应变或为弹性支撑，钝体会产生振动，钝体的振动进一步影响尾迹脱涡模式，这种流体与结构之间相互作用称为涡致振动(Vortex Induced Vibration，VIV)。涡致振动对结构体的稳定性和安全性会产生重大影响，因此研究如何防止结构发生强烈的涡振来提高结构体的安全性能具有较强的实际意义。另一方面，近年来利用钝体涡致振动进行能量收集在新能源领域中也被广泛关注。因此对钝体绕流及涡致振动的机理研究具有十分重要的意义。在涡致振动被动控制技术中，安装于钝体底部顺主流方向上的分隔板是改变旋涡脱落和尾迹特性的最有效的装置。本文采用数值模拟的方法，对低雷诺数下方柱及方柱-分隔板绕流特性进行研究，柱体弹性支撑，运动方向垂直于来流方向，揭示低雷诺数下方柱涡致振动特性及其尾流特征，探索方柱VIV的锁振和驰振产生的机理，并详细讨论产生和影响这些现象的参数，包括方柱的阻力系数和升力系数、振动频率比和无量纲脱涡频率、振幅及其幅值谱图、“拍”和“相位开关”以及尾迹中旋涡的脱落模式等，更进一步研究了在驰振阶段时，分隔板对方柱涡致振动的影响。通过分隔板被动控制，得到分隔板对方柱涡致振动的影响规律。本文得到以下结论。首先，针对横向振动的方柱系统进行数值计算，在60≤Re≤250范围内，可以清晰地观察到方柱涡致振动的无量纲脱涡频率曲线出现几个不同的阶段，分别是VIV初支、锁振区、过渡分支和驰振区。当方柱振动位于锁振和驰振区时，涡致振动换能系统具有最优能量收集效率。振动频率比接近1时，振幅愈大；最大位移幅值并不是出现在f*=1处，而是出现在靠近频率锁定段的左端，发生“失谐”现象。在振动频率比f*≈1时，升力系数Cl达到最小，此位置为“相位开关”。在驰振阶段，方柱位移和升力系数时程曲线出现了“拍”现象。方柱涡致振动在初支、锁振区和过渡分支，尾流脱涡形态为典型的2S模式。在170≤Re≤231（11.81≤U*≤16.05）时，方柱涡致振动处于驰振阶段，尾涡形态仍为2S模式。而当驰振充分发展时，即Re=232，即方柱振动振幅达到最大值时，尾涡形态由2S模式转为P+S模式。其次，在驰振范围内，针对横向振动的方柱-分隔板系统进行数值研究，讨论了方柱和分隔板的受力、振幅、脱涡频率、位移和升力相位差及自由剪切层的变化趋势。结果表明在分隔板对方柱的尾流结构产生明显影响，抑制尾流区剪切层之间的相互作用；随着分隔板的长度L变化(L=0D～7D)，流动特性呈现出三个不同阶段。在流动第一阶段，方柱-分隔板系统的振幅和振动频率都很高，有利于涡致振动的能量收集。在第二阶段初始部分，即1.5≤L/D≤4.1，振幅虽大于方柱驰振时的振幅，，但振动频率较低，既不利于能量收集，也不利于被动控制。在第二阶段后部分，即4.1≤L/D≤5.7，振幅逐渐低于方柱驰振时的振幅，同时振动频率较低，有利于被动控制。第三阶段，方柱-分隔板的振动频率虽然很高，但仍低于固定绕流，有利于被动控制。
[Abstract]:Flow around a bluff body is a very common phenomenon in nature, which involves many problems such as flow separation, vortex shedding and vibration. The wake flow field is extremely complex and contains very complex physical mechanism. The vortex shedding process occurs alternately on both sides of the bluff body, which causes periodic stress fluctuations around the bluff body, and consequently makes the bluff body subject to periodic fluctuations. The interaction between the fluid and the structure is called Vortex Induced Vibration (VIV). Vortex induced vibration has a significant impact on the stability and safety of the structure. On the other hand, in recent years, the use of bluff-body vortex-induced vibration for energy collection has been widely concerned in the field of new energy. Therefore, it is very important to study the mechanism of bluff-body flow and vortex-induced vibration. In the passive vibration control technology, the separator mounted on the bottom of the bluff body along the mainstream direction is the most effective device to change the vortex shedding and wake characteristics.
In this paper, the flow around a cylinder and a square cylinder-separator under low Reynolds number is studied by numerical simulation. The elastic support of the cylinder is perpendicular to the flow direction. The vortex-induced vibration characteristics and wake characteristics of the cylinder under low Reynolds number are revealed. The mechanism of vibration locking and galloping of the square cylinder VIV is explored. The generation and influence of the vortex-induced vibration are discussed in detail. The parameters of these phenomena, including drag coefficient and lift coefficient of square cylinder, vibration frequency ratio and dimensionless vortex shedding frequency, amplitude and amplitude spectrum, beat and phase switch, and shedding mode of vortex in wake, are further studied. The effect of separator on vortex-induced vibration of square cylinder in galloping stage is also studied. Dynamic control is used to get the law of vortex induced vibration of the separation column.
Firstly, numerical calculation is carried out for a square cylinder system with transverse vibration. Within the range of 60 < Re < 250, several different stages of the dimensionless de-vortex frequency curve of the square cylinder vortex-induced vibration can be clearly observed, namely, the initial support of VIV, the vibration locking region, the transition branch and the galloping region. When the vibration frequency ratio is close to 1, the amplitude increases; the maximum displacement amplitude does not appear at f * = 1, but near the left end of the frequency locking section, resulting in "detuning" phenomenon. The vortex-induced vibration of the square cylinder is in the galloping stage and the wake vortex shape is in the 2S mode when the galloping stage is 170 < Re < 231 (11.81 < U * < 16.05). When the vibration amplitude of the square cylinder reaches the maximum value, the vortex pattern changes from 2S mode to P+S mode.
Secondly, in the galloping range, the force, amplitude, frequency of vortex shedding, phase difference of displacement and lift, and the variation trend of free shear layer are discussed. The results show that the separation plate has a significant effect on the wake structure of square columns and restrains the shear layer in the wake region. The interaction between the separators and the flow characteristics exhibit three different stages as the length of the separator varies from L=0D to 7D.
In the first stage of the flow, the amplitude and vibration frequency of the square column-separator system are very high, which is beneficial to the energy collection of vortex-induced vibration. In the third stage, although the vibration frequency of the square column-separator is very high, it is still lower than that of the fixed flow.
【学位授予单位】：重庆大学
【学位级别】：硕士
【学位授予年份】：2014
【分类号】：TB535

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