摘要
圆柱绕流在自然界是一种非常普遍的现象。在流体力学研究中,圆柱绕流也是使用最为普遍的模型之一。因为这一简单模型包含了许多流体流动中出现的复杂过程,如流动的分离、脱涡、湍流等。当流体流过圆柱体时,旋涡从圆柱体两侧交替脱落,并在尾流中形成卡门涡街。交替的脱涡导致圆柱体受到交替的作用力,使得圆柱产生振动。这种涡致振动不仅会对流动的脱涡过程产生影响,而且可能对固体结构造成损坏。尤其是当脱涡的频率与结构固有频率一致时,这种破坏更为严重。
为了研究涡致振动带来的影响并减小涡致振动带来的危害,许多研究者对涡致振动与涡致振动控制展开研究。涡致振动控制的方法很多,一般可以分为被动控制法与主动控制法。但这些方法大多通过大范围改变流场或者需要消耗比较大的能量来实现,这限制了它们在工程领域的应用范围。在本文研究中,我们提出了一种新的主动控制方法,对单圆柱与并列双圆柱绕流的涡致振动进行了控制。该方法通过流动控制的方法用微小致动器在流动关键区域进行扰动,影响流固系统之间的耦合关系,来控制圆柱的涡致振动。
我们知道,在分离点附近流动的失稳可以迅速被放大,所以在这些区域进行流动控制是最有效的。而通过控制流动分离来控制涡致振动也是最经济最有效的手段。流场中很小的扰动也有可能导致流动发生变化,这为通过小尺寸扰动控制大尺寸结构的涡致振动提供可能。本课题研究中,我们将在圆柱绕流的分离区采用周期性激励来影响流体流动,进而抑制涡致振动。
为了方便实验的设计,我们首先建立了典型圆柱涡致振动系统的振动模型,并推导出振动微分方程与振动固有频率。然后我们分析了涡致振动控制所需要的理论振幅。
实验首先采用了最简便的声激励作为致动器来控制圆柱涡致振动。声激励实验控制装置加工简单,安装方便,因此被用来检验控制的可行性,并初步确立控制各参数。当用来导出内置声激励的狭缝尺寸为0.2mm×2mm时,声激励对涡致振动控制没有太大的效果,而当狭缝长度增加到10mm时,圆柱涡致振动开始下降。这说明扰动的范围必须超过一定尺度,致动器对流动控制才能影响涡致振动。实验表明,有效激励位置在分离区附近,有效激励频率在剪应区不稳定频率及其倍频附近的一个很广的范围内。当激励频率在不稳定频率倍频时,控制效果达到最优。
当流体流过一对并列的圆柱,流体流动方式与声激励的控制效果与二者之间的相对间距(中心距与直径比值)有很大关系。在相对间距为1.2时,声激励同时在对称的分离区进行扰动。结果显示控制效果相对单圆柱单激励更加优秀。随着相对间距增加,控制效果减弱。
尽管声激励作为实验扰动源十分方便,但是声激励同时会引入额外噪声,并且能量损失很大,不适合实际的应用。因此,我们采用一种压电陶瓷材料的悬臂梁作为新的致动器,并根据模型计算设计了弹性支撑的实验装置。
在圆柱处于共振状态下对其进行控制的实验表明,当压电致动器各参数按照声激励实验进行参数优化配置时,单圆柱的涡致振动也可以被抑制。并且这种控制在相对间距为1.2的并列双圆柱控制中更加有效。证明在流动对称分离区两侧的同时扰动可以更加有效减小涡致振动。当相对间距为1.8时,由于两圆柱的双频脱涡产生,使得控制效果不明显。当相对间距为3.0时,两个圆柱的相互影响减弱,涡致振动控制与单圆柱控制类似。
对圆柱振动速度与尾流中流体波动速度测量发现致动器影响了二者之间的相位关系。当没有致动器扰动时,两信号之间同相波动;当有致动器扰动时,两信号在同相与反相之间跳动,涡致振动的幅值受到抑制。对流场速度矢量分布的测量显示流动脱涡强度有所减小。这显示压电致动器的微小扰动影响了流固之间的耦合关系,削弱了脱涡强度,从而抑制了圆柱的涡致振动。致动器的振幅与壁面粘性单元尺度相当,但输入能量比所节省的能量至少小一个数量级。
在实验中,我们还发现了一种涡致振动的被动控制方法。通过细微调整双圆柱的空间位置,使得两圆柱在小间距条件下产生一定的异面角度,流动的脱涡与圆柱的涡致振动都可以被大幅度抑制。实验测量了这种被动控制方式的有效异面角度、有效雷诺数范围以及控制效果。对两刚性圆柱的实验也证实了相似的结论。
The cylinders in a cross flow is one of most basic and revealing cases in the general subject of fluid-structure interaction problems. When the fluids flow around cylinders in the modern Reynolds number, the vortex streets shed from the cylinder. The shedding vortices would generate oscillating forces, which in turn cause the structures to vibrate. The resultant vibration can influence the flow field and the structures, especially when the shedding frequency is at or near the structural natural frequency. The vortex-induced vibration (VIV) is of practical importance because of its potential destructive effect to structures, such as bridges, stacks, towers, offshore pipelines, and heat exchangers.
In order to reduce or eliminate the potential disaster by VIV, many methods have been developed to control the VIV. However, these methods mostly control the VIV by modifying a wide range flow filed or by exciting at high power compared to the saving power, which limit their application in realistic engineering. In this thesis, a new VIV control method was proposed to control the vibration of single cylinder and two side-by-side cylinders. The VIV was controlled by micro actuators through influencing the interaction between the cylinder and the fluids.
As we known, flow control is most effective when the control is near the critical regions, because the flow instabilities could be amplified quickly in these regions. Therefore the micro actuators periodically excited near the separation regions during VIV control. It was also reported that little disturbance could perturb the flow separation. However, no researcher reported how wide the exciting range and how large the exciting amplitude are required to influence the vortex-induced vibration.
In this thesis, in order to control the VIV of big-size structure by micro actuator, we firstly gave the dynamic model of the vibration system. Two typical supported systems were modeled with vibration differential equation presented. The actuator's acting size and amplitude in flow control were then analyzed.
Acoustic excitation was initially used in the VIV control due to its convenience. The internal acoustic excitation emitted from a slit with size 0.2mm×10mm on the cylinder. When the exciting position is near one separation region and the exciting frequency is matching to the instability frequency and its harmonic frequency, the VIV could be reduced by acoustic excitation.
With another identical cylinder presented beside this one, the control effects are different with the spacing ratio between the two side-by-side cylinders changed. At the spacing ratio of 1.2, the VIV was controlled more greatly than single cylinder when two acoustic excitations symmetrically perturbed near separation regions. With the increment of the spacing ratio, the control effect decreased.
While the slit's length is only 2mm, acoustic excitation cannot reduce the VIV even using same controlling characteristics. It implies that enough exciting range is needed to be able to control the VIV.
As a type of micro actuator, acoustic excitation successfully reduced the VIV. But acoustic excitation can induce additional noise during control, and the wasting energy is also considerable. Therefore a new micro actuator made up of Piezoelectric Ceramic (PZT) cantilever was used in the VIV control. New experiment system was designed according to the model calculation, and the cylinder is spring-supported.
The carried out experiment demonstrated that the VIV of single cylinder could also be reduced by micro PZT actuator. The control is most effective when the excitation is near separation region and when the exciting frequency is matching to the instability frequency and its harmonic frequency. For the control of two side-by-side cylinders, the VIV decreased most when two actuators perturbed simultaneously near separation regions at spacing ratio of 1.2. Because at this ratio, the two cylinders acted as one big bluff body, and perturbations at both separation regions can reduce the VIV more. When the spacing ratio was 1.8, there were two bistable vortices and the control had no effect. While at spacing ratio of 3.0, the control effect was same as that of single cylinder.
The measurements of the cylinder's vibration velocity and flow fluctuations showed PZT actuators changed the phase between these two signals, and the vorticity was also reduced by the actuators. It shows that the actuators controlled the VIV by influencing the interaction between the cylinder and the fluid. The critical amplitude of the actu- ator for reducing VIV was also explored, which is of the same order of magnitude as viscous length-scale, while the input energy is at least one order less than the saving energy.
During the experiments, a novel passive control method was discovered to effectively suppress the vortex-induced vibration. When the two side-by-side cylinders were arranged slightly non-coplanar as small spacing ratio of 1.2, the single vortex street was significantly suppressed and therefore the vortex-induced vibration was controlled.
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