Plume statistics in thermal turbulence: mixing of an active scalar

Statistical properties of the temperature field in turbulent convection are studied experimentally. We show that the skewness of the plus and minus temperature increments can be used to quantitatively characterize the mixing zone in the convective flow and the result reveals how the mixing zone evolves with the Rayleigh number. We also present evidence for the saturation of the temperature structure function exponent and that the saturation is related to thermal plumes. A more direct study of the thermal plumes suggests that their sizes have a distribution that is approximately log-normal.