结构振动对湍流近尾迹的影响

EFFECT OF STRUCTURAL VIBRATIONS ON THE TURBULENT NEAR WAKE

研究了圆柱绕流中流体与结构的相互作用，侧重结构振动对湍流尾迹的影响。用激光测振仪测量圆柱在升力方向的位移；用热线和ＬＤＡ（二维）测量湍流的近尾迹。通过变化自由流的速度和圆柱体直径（特征尺寸）来变化雷诺数。用两个振动特性不同的（一个相对刚性，一个相对弹性）圆柱来产生尾迹，研究固体结构振动对湍流近尾迹的平均速度场和湍流场的影响。结果表明，结构自由振动对湍流近尾这场影响明显，该影响随雷诺数的变化不明显。

The structure free vibration cases can be seen in many engineering applications, such as tall buildings, bridges, turbomachinery blades, fuel rods in nuclear reactors, etc. The data of structure free vibration are not well documented. Therefore, the present study concentrates only on the free vibration case. It is well known that vortex shedding from a structure creates unsteady fluid excitation forces. The forces cause structural vibrations which, in turn, influence the flow field and the excitation forces. This process gives rise to a complicated fluid-structure coupling that modifies the vortex shedding frequency, the separation behavior, the magnitude and frequency contents of the unsteady forces, the wake structure and the turbulence field in the near wake and perhaps even the far wake. An experimental study of the fluid-structure interactions had been initiated, with a focus on effect of structural vibrations on the turbulent near wake. The wake velocity field investigations were carried out using two brass cylinders with diameters 3.8mm and 12.7mm. This choice was dictated by the fact that a large vibration effect on the wake was desired and by the fact that a very inelastic cylinder has to be used to simulate the rigid case. The transverse vibration displacements of cylinders were measured using a laser vibrometer, while hot-wires and laser Doppler anemometer were used to examine the near wake. The flow Reynolds numbers (Re = U∞d/v, where U∞ is free stream velocity, d is cylinder diameter and v is the fluid kinematic viscosity) were varied from 1000 to 8000 by changing the upstream velocity and by experimenting with cylinders of different diameters. The near wake structure of two cylinders, one relatively rigid and one elastic, were measured in detailed and the data were used to analyze the vibration effects on the mean and turbulence field. It is found that the vibration effects on the near wake are very significant; While the mean velocity after normaliztion using conventional length and velocity scales is not influenced by cylinder vibrations, the turbulence field is significantly affected. Notable effects include a very substantial increase in the turbulence statistics, u2 and v2. Due to enhanced mixing resulted from cylinder vibration, the double-peak behavior commonly observed in the streamwise turbulence intensity is found to disappear after x/d = 10. Symmetry of the velocity field is not destroyed by the cylinder vibrations at the locations we examined. With the increase of Reynolds number, the effect of vibration on streamwise turbulence intensity is found decrease slightly at x/d = 20. However, Reynolds number dependence is not obvious in the nearer locations examined or for other turbulence parameters.