冯·卡门涡旋流动系统中各向异性的实验研究

Experimental study on the anisotropy in von Kármán swirling flow system

利用Tomo–PIV测量了冯·卡门涡旋流动系统中心区域的三维速度信息,通过2种方法计算得到的速度二阶结构函数,分析了该系统中流动的各向异性特性。实验结果表明:冯·卡门涡旋流动系统中心区域的流动均匀性较好,但均方根速度的不同分量间呈现出显著的各向异性,水平分量与竖直分量比值约为1.5。通过2种方法计算得到的速度二阶结构函数结果相近,并给出:经过各个方向分量平均后的速度二阶结构函数各向同性度较高;但其在尺度空间中的分布呈现“水平面内各向同性度高,竖直平面内存在一定各向异性”的特性,且随着尺度的减小,此各向异性会逐渐减弱并接近各向同性。该研究为认识湍流流动以及冯·卡门涡旋流动系统提供了基础理解和分析方法。

The degree of anisotropy in the Von Kármán Swirling(VKS)flow system was experimentally investigated.The three-dimensional velocity near the center of VKS was measured by tomographic PIV and two methods were adopted to calculate the second order Velocity Structure Function(VSF2)in order to study the scale-by-scale anisotropy.It is found that the fluctuation velocity is highly homogeneous.However,the Root-Mean-Square(RMS)velocity in the vertical direction is one-third times smaller than that in the horizontal direction,which characterizes the large-scale anisotropy.This large-scale anisotropy has left its fingerprint on the small scales,which is reflected by the observation that the scale-space distribution of VSF2 is isotropic in the horizontal plane while it is not in the vertical plane.Besides,this anisotropy diminishes as scale decreases,consistent with the local isotropy assumption proposed by Kolmogorov.This experimental study provides new insights into turbulent flows.