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 r(0) 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 r(0), 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 r(0). 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.