Experimental study on the anisotropy in von Kármán swirling flow system

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.