301 大気乱流拡散に関する風洞実験 : 乱流レイノルズ数Re_λによる平均濃度特性の変化(流体工学I)(オーガナイズドセッション(a)材料力学・材料強度に関する新展開)

Abstract

Wind tunnel experiments on particle diffusion were conducted to investigate the mechanism of turbulent diffusion for a wide range of Re_λ. The value of Re_λ was systematically changed from 36 to 410 by controlling turbulent intensity and vortex scale using an active turbulent generator. Measurements on particle concentration were made by a laser system and the mean concentration properties were examined for different Re_λ.The mean concentration profiles were almost satisfactory Gaussian and its mean width grew larger with Re_λ. Stream wise growth rate of it's variance became larger as Re_λ increased and was correctly described by the Taylor's diffusion theory. Taking Lagrangian time scale T_L and virtual origin T_0 determined from it's streamwise growth of the variance into account, only relative diffusion corresponding to the long-time diffusion occurred for the grid turbulence of Re_λ ≒36. On the other hand in the excited turbulence, the diffusion field shifted toward the short-time diffusion as Re_λ increased from 241 to 410. For Re_λ≒410, the diffusion field had the characteristics of short-time diffusion in the whole measured area, in which the diffusivity of about 110cm^2/sec was attained. There, the meandering motion of plume dominated the diffusion mechanism.