When a particle moves in a fluid, the fluid is disturbed by the particle and the fluid mass is mixed. This phenomenon is commonly observed in particle-laden flows and dispersed bubble flows. This mass mixing can be composed of two mechanisms. One is the mass transfer by convective flows that are induced by the reaction of the particle drag and the other is the turbulent mass mixing in the turbulent wake of the particle. The effect of the former one can be evaluated using the previous studies on the particle drag. However, the effect of the turbulent mixing is little understood. The turbulent mixing caused by a particle wake is investigated by visualization and noninvasive concentration measurement using a photochromic dye. A sphere brass particle of 5mm in diameter is dropped in kerosene filled in a vertical pipe and the mixing of dye is visualized. The photochromic dye, which is activated by an ultraviolet light, keeps its color in a few minutes after the activation. A part of the fluid is activated without disturbances and is subjected to the mixing by the particle wake. The visualized dye patterns indicate the dye is mixed isotropically by large-scale vortex motions when the particle sheds the vortices. Furthermore, the photochromic concentration measuring (PCM) technique is developed to obtain the concentration of the mixed dye. This PCM technique is based on the Lambert-Beer’s law for light adsorption and provides the average dye concentration along the light path. The measured concentration distribution shows rather isotropic mixing in longitudinal direction. The turbulent diffusion coefficient (TDC) is calculated from the temporal changes in the measured concentration distributions. The evaluated TDC shows strong time-dependency, which is attributed to the change in scale and strength of wake vortices. The TDC is about 104 times larger than the molecular diffusion coefficient at its maximum. The effect of particle Reynolds number on the turbulent mixing is also investigated for the Reynolds number range from 263 to 3290. The observed mixing patterns show a drastic change at the critical Reynolds number on the vortex shedding from the particle. The Reynolds number dependency on the non-dimensional TDC and mixing time are discussed.

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