Experimental investigation of pair dispersion with small initial separation in convective turbulent flows

We report an experimental investigation of pair dispersions in turbulent thermal convection with initial separation r₀ ranging from sub-Kolmogorov scale to scales in the inertial range. In the dissipative range of scales we observed the exponential growth of the separation between a pair of particles predicted by Batchelor and obtained a Batchelor constant 0.23±0.07. For large r₀, it is found that, for almost all time ranges, both the mean-square separation and distance neighbor function exhibit the forms predicted by Batchelor, whereas the two quantities agree with Richardson's predictions for small r₀. Moreover, the measured value of the Richardson constant g=0.10±0.07, which is smaller than those found in other turbulence systems. We also demonstrate the crossover of the mean-square separation from the exponential to the Batchelor regimes in both temporal and spatial scales.