TURBULENT STATISTICS OF NEARLY ISOTROPIC TURBULENCE GENERATED BY FOUR ROTATING GRIDS AND THE MEAN FALLING VELOCITY OF PARTICLE THROUGH TURBULENCE

摘要

Box air turbulence is experimentally generated in a rectan-gular box by using four counter-rotating grids installed inside. Turbulence statistics are obtained from one-point measurement of LDA. Nearly isotropic turbulence with zero-mean velocity is realised in the midst of four rotating grids. The dissipation rate is estimated from the Taylor time microscale of velocity autocor-relation obtained from LDA measurement, since Taylor's frozen turbulence hypothesis is not applicable. From this estimation, the Reynolds number based on the Taylor length microscale becomes about 200 at maximum in the present experimental apparatus. The mean falling velocity of small particle in turbulent flow is measured in the box turbulence. It is found that the mean falling velocity of the inertia particle could be smaller or larger than the terminal velocity, depending on the particle property, if the ratios of particle response time to turbulence time scale are the same.