旗面周期摆动的实验研究  

Experimental Study of a Periodical Flapping Flag

作　　者：白夜[1] 贾永霞[1] 李存标[1] 朱一丁[1] 

机构地区：[1]北京大学工学院,湍流与复杂系统国家重点实验室,北京100871

出　　处：《物理学报》2016年第12期174-180,共7页Acta Physica Sinica

基　　金：国家自然科学基金(批准号:109103,010062);国家重点基础研究发展计划(批准号:2009CB724100);群体项目(批准号:10921202,11221062,11521091);国家杰出青年科学基金(批准号:10921202)资助的课题~~

摘　　要：用实验方法研究了旗面周期摆动的运动过程,采用改进的算法优化了粒子图像测速仪测量结果,定量得了水洞中摆动旗面的近壁流场信息.通过选定旗面包络上的一个拐点,将其振幅作为特征长度重新计算旗面运动的Strouhal数.多组实验结果中,新的Strouhal数均为0.21左右,这与相同Reynolds数下圆柱绕的Strouhal数结果相近.A flag flapping in the wind is a classical fluid-structure interaction problem that concerns the interaction of elastic bodies with ambient fluid. The fluid-flag interaction can give rise to three self-sustained oscillation states under certain conditions, i.e., stretched-straight state, periodic state, and chaotic state. This paper reports an experimental study of a cantilevered polydimethylsiloxane （PDMS） flag flapping in a uniform flow at a periodic state. A heavy flag is well designed, with metal strips imbedded in the PDMS sheet. Immersing the elastic but self-sustaining flag into the water flow, we use the time-resolved particle image velocimetry （PIV） and visualization techniques to obtain the whole flow field around the midspan of the flapping flag. A unique PIV image processing method is used to measure the near-wall flow velocities around the flap-ping elastic flag at the periodic state. The image processing technique adopts a radon transform technology to determine the moving interface in the particle images. The interface velocity distribution is subsequently calculated. Artificial particles of uniform size with the interface velocity are added into the flag region. Therefore, the whole velocity field over a flapping period is accurately obtained, giving the basic data to analyze the flag flapping. It is found that there exists an ＂inflection＂ point in the envelope curve of the flag flapping. Based on the analyses of the flapping states and velocity fields, a unified flapping Strouhal number （St=2Af/U） is proposed by choosing the amplitude of the＂inflection＂point as the characteristic length, which is similar to the Strouhal number of the flow around a circular cylinder over the same range of Reynolds number.

关 键 词：摆动旗面 Strouhal数 粒子图像测速仪 

分 类 号：O314[理学—一般力学与力学基础]