Abstract
The characteristics of turbulent particle diffusion were investigated in a strong turbulence generated by an active turbulence generator installed to in a small wind tunnel. Instantaneous visualization image analysis was conducted on the plume's meandering motion using a digital camera for grid turbulence(R_λ=52) and excited turbulence(R_λ=526). To clarify the characteristics of the meandering plume, relative diffusion, meandering diffusion and total diffusion, as described in the Taylor's diffusion theory, were obtained by extracting the brightness values from a large number of instantaneous plume image pictures. In the excited turbulence accompanied by a wide inertial subrange, the time dependence on relative diffusion spread σ_r(T) showed almost the same tendency to the Batchelor's suggestions, i.e. 'σ_r∝T in thenear-field', '∝T^<3/2> in the intermediate-range' and '∝T^<1/2> in the far-field'. Meandering parameter σ_m was enhanced near the point source and then gradually increased as ∝ T^<1/2> downstream. Transverse relative diffusion was observed to have non-Gaussian profiles with high kurtosis value of 3.1〜9.6. The 'intermediate-field' was not clearly observed for the grid turbulence where only comparatively small scale vortexes acted, where σm increased only slightly near the source and become to be almost constant in the downstream region. Estimated based on the Gifford's model, the contribution rate of the relative diffusion and the meandering diffusion (σ_r/σ_m 19 to the total diffusion strongly depends on the turbulence Reynolds number of these turbulence fields.
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