MOLECULAR-DIFFUSION AND VISCOUS EFFECTS ON CONCENTRATION STATISTICS IN GRID TURBULENCE

Two-particle Lagrangian models in turbulence are used to consider disp ersion in decaying isotropic homogeneous turbulence (i.e. approximatin g grid-generated wind-tunnel flows). Thomson's formulation is used, an d his model is extended to incorporate molecular diffusivity and visco sity, i.e. the range of scales modelled includes the dissipation subra nges as well as the inertial subrange. New terms are proposed which co nsistently provide well-mixed models and the impact on Saffman's well- known small-time results is considered. The new model is ideal for com parison with recent concentration-fluctuation measurements in decaying wind-tunnel turbulence and the results are encouraging. In particular , the fluctuation intensities, both along and across the wind tunnel, are well described by the new model. In addition, small source-size ef fects are far better explained when we include the molecular effects. A surprising result is the persistence in time of both source-geometry and molecular effects upon concentration fluctuations. Both of the ef fects are negligible at large times for mean concentrations, but persi st for significant times for the fluctuations, indicating an important role for small-scale dynamics.