Experimental Determination of Organized Fluctuating Fluid Forces acting on the Finite Circular Cylinders with Different Aspect Ratios

本文用实验方法确定了均匀流动条件下作用于板上长径比为1 ~ 12的有限圆柱上的有组织的波动流体力。采用条件采样技术对圆柱体表面脉动压力进行处理，滤波后的脉动速度信号具有明确的频率，在此频率下，可在FFT分析中确定其清晰的组织行为。从这些结果得到了局部和全局的有组织脉动流体力，并讨论了纵横比对有组织脉动流体力细节的影响。研究结果表明：(1)长径比对有限圆柱的气动特性影响较大，但总体特性可分为两种情况。一种是高纵横比大于7或8的大纵横比圆柱体，可称为“高纵横比圆柱体”；另一种是低纵横比小于7或8的圆柱体，可称为“低纵横比圆柱体”。(2)在高纵横比圆柱体的情况下，已经证实了许多明显的有组织的波动流体力。侧壁出现三种不同频率、不同性质的有组织波动，顶部出现两种有组织波动。(3)在低纵横比圆柱体的情况下，仅在侧壁和顶部检测到一个明显的有组织的波动流体力。柱体侧壁出现与卡门涡街相对应的波动，Strouhal数在0.15 ~ 0.21之间。对于长宽比为2或3的圆柱体，也可以在顶部表面检测到与卡门涡街相对应的有组织的波动。

The organized fluctuating fluid forces acting on finite circular cylinders with aspect ratios of 1 to 12 located on the ground plate in a uniform flow have been determined experimentally. The fluctuating pressures on the cylinder surface were processed using the conditional sampling technique, for which the filtered fluctuating velocity signals with well defined frequencies at which the clear organized behaviors can be confirmed in the analysis with FFT.The local and global organized fluctuating fluid forces have been obtained from these results, and the influences of aspect ratios on the details of the organized fluctuating fluid forces have been discussed. The results obtained from the present research project can be summarized as follows :(1) The aerodynamic characteristics of the finite circular cylinders are severely affected by the aspect ratios, but the general characteristics can be classified in two cases. The first is the case for the cylinders with large aspect ratios as high as more than 7 or 8, which can be called "high aspect ratio cylinder", and the other is the case for the cylinders with low aspect ratios smaller than 7 or 8, which can be called "low aspect ratio cylinder".(2) In the case of high aspect ratio cylinders, many noticeable organized fluctuating fluid forces have been confirmed. The three organized fluctuations with different frequencies and different natures on the side wall appears as well as two organized fluctuations on the top.(3) In the case of low aspect ratio cylinders, only a single noticeable organized fluctuating fluid force have been detected on the side wall and the top. On the side wall of the cylinders a fluctuation corresponding to the Karman's vortex street appears with Strouhal numbers from 0.15 to 0.21. For the cylinders with aspect ratios of 2 or 3, a organized fluctuation corresponding to the Karman's vortex street can be also detected on the top surfaces.