柔性隔板对固定圆柱绕流结构影响的实验研究

Antisymmetric vortices shed periodical from the shear layers of a bluff body and lead to a fluctuating lift force which could cause a structure to vibrate (vortex-induced vibration). Many methods have been applied to control this flow phenomenon and the passive control technique as a splitter plate behind a bluff body is an easy and effective one. In this experiment, a flexible film which is attached to a stationary cylinder is induced to vibrate by the unsteady vortices. Hydrogen bubble visualization technique and Particle Imaging Velocimetry (PIV) are utilized to investigate the flow structures of the cylinder affected by the length of flexible film and Reynolds number. Different data processing techniques, such as proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD), will be adopted to analyze the mechanisms. The drag of the stationary cylinder with a flexilbe film is measured and then the relationships between the flow structures and aerodynamic characteristics will been explained. These investigations not only clarify the mechanisms of the flow structure and drag reduction for the cylinder with a flexible film, but also shed light on the significance of flexible wing for flapping motions.

对于钝体绕流周期性脱落的反对称涡会诱发涡致振动这一现象，人们采用了多种方法对其进行控制，加装后缘隔板是其中一种简单有效的手段。本课题提出在静止圆柱的后缘安装柔性隔板，采用氢气泡流动显示和PIV速度测量技术，利用相位本征正交分解(POD)、动力学模态分解（DMD)等方法，探讨柔性隔板长度、来流雷诺数等对圆柱绕流结构和特性的影响规律；分析柔性隔板对静止圆柱的减阻效果及抑制振动的机理。研究工作不仅有利于揭示柔性隔板对静止圆柱绕流及涡致振动的影响机制，而且在一定程度上将解释柔性翼面在昆虫和鱼类运动过程中的重要性。

在利用柔性隔板对均匀流场中固定圆柱的控制实验中，当雷诺数增大或隔板长度增加时，柔性隔板会受来流影响向一侧发生偏转，这在一定程度上增大了圆柱的阻力系数，削弱了隔板的控制效果。而相比之下，刚性板工况在较高雷诺数下的控制效果更好，并且其自身流场统计量随雷诺数的变化表现出较强的规律性。当把隔板长度设定为三倍的圆柱直径、取不同雷诺数进行实验时，刚性板各工况的回流区长度随雷诺数的增加呈现出一种先增大后减小的趋势。此时，圆柱阻力系数也表现为随雷诺数的增长不断下降。当雷诺数达到1860以后，其阻力系数便接近维持恒定不变，据此我们推测剪切层不稳定性在Re=1860处开始显现。最后，固定雷诺数，通过改变分隔板的长度，发现当分隔板长度为两倍圆柱直径时，流动控制效果最好。对于柔性特性在压气机叶片模型的流动控制实验中，叶片模型的自激振动不仅改善了近尾迹区的尾迹偏移现象，而且显著削弱了近尾迹区时均流向速度的亏损，但是叶片的振动也导致流场脉动量的急剧增大。这些现象的产生是由于叶片的自激振动导致了绕流结构的改变：当叶片攻角较小时，脱落的前缘涡在近尾迹区形成顺时针单涡涡街，直到攻角AOA≈8°时流动分离；当叶片自激振动现象出现时(AOA=10°)，近尾迹区内不仅产生了新的双涡涡街结构，而且首次发现了翼尖涡表面环绕其上的细小多涡结构。