Predicting turbulent mixing and transport remains a critical problem in industrial flows (combustion chambers, mixers, ventillation systems etc.) and in the environment (smoke plumes etc.). The mixing and transport processes are often a strong function of Reynolds number (Re) and yet there is a paucity of information on their Re dependence. Here we propose experiments of passive scalar mixing in isotropic grid turbulence whereby the Taylor Reynolds number (R{lambda}) will be varied from 30 to over 400 (60 < R{sub l} < 10,000). We will achieve the high R{lambda} by means of an active grid, which consists of grid bars with small wings that rotate and flap in a random way. We propose to study basic statistics (pdf, spectra etc). of a homogeneous passive scalar (linear mean profile), as well as of an inhomogeneous scalar (passive line source) as a function of Re. There are many problems concerning the nature of the fine scale structure of a scalar (e.g., the existence of derivative skewness, the relation of the scalar spectrum to the velocity spectrum, and the rate of spreading (dispersion) of a contaminant plume), placing the similarity theory developed over the past 40 years in doubt, yet there is nomore » information concerning its Reynolds number dependence in isotropic turbulence. The passive scalar will be temperature, although some experiments will be done using helium (which has a Schmidt number of 0.23). Particular emphasis will be placed on higher order statistics of both the signal and its derivative. Our experiments will be related to theory and modelling and to recent advances in Direct Numerical Simulations. We will also do further work on mixing in a jet (also as a function of Re) and will relate this work to the (shearless) grid turbulence. The duration of the proposed research is three years.« less