UNIVERSAL ASPECT RATIO OF TURBULENCE STRUCTURES OVER VARIOUS TYPES OF SURFACE GEOMETRY

Abstract

The aim of this study is to reveal the dependency of turbulence structures on the types of surface geometry. The characteristics of the turbulence structures in the horizontal sections at the several measurement heights over the four types of surfaces, (1) Square Array, (2) Hight Variation, (3) 2D Canyon, (4) Flat surface, were investigated using Particle Image Velocimetry technique. Our attention mainly goes to the aspect ratio of turbulence structures, and they are quantitatively measured from the distributions of two-point correlation coefficients. The aspect ratios (streamwise length / spanwise width) of the structures, which were extracted using the threshold of Ruu=0.4, were found to be accurately parameterized by the shear strength at that height. The interesting result is that there is a critical value in the shear strength. The aspect ratio of turbulence structures is linearly increased with increasing shear strength until the critical value, and then the aspect ratio converges a constant value 7 : 2 regardless the magnitude of the velocity gradient.