Effects of Velocity Fluctuations of Upper Flow on Turbulent Structure of Atmospheric Boundary Layer

Abstract

The effects of various velocity fluctuations of upper flow on turbulent structure of atmospheric boundary layer were experimentally investigated. The atmospheric boundary layer was reproduced in a wind tunnel. In order to produce various velocity fluctuations in the upper flow, three kinds of turbulence grids were set to the entrance of the test section. The streamwise and spanwise velocities were simultaneously measured by using hot-wires anemometer. Turbulence statistics were estimated. The vertical profiles of Reynolds shear stress, Ruw and Ruv for M25 and M50 decrease as z/L increase, on the other hands, those for M75 increase. The ratio of the integral length scale, _x,g/_x,a, for M75 were higher than those for M25, M50 at z/L=0.67, on the other hands, _y,g/_y,a, _z,g/_z,a, for M75 were lower than those for M25, M50 at 0.50≦z/L≦0.67. The joint probability density functions of u and w for M75 show the strong negative correlation in the upper region of the atmospheric boundary layer. It is suggested that the effects on the turbulent structure of the atmospheric boundary layer depends on the scale of velocity fluctuations of the upper flow.