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Molecular diffusion and viscous effects on concentration statistics in grid turbulence

Published online by Cambridge University Press:  26 April 2006

Michael S. Borgas  and
Brian L. Sawford
Michael S. Borgas
Affiliation:
CSIRO Division of Atmospheric Research, Aspendale, Victoria 3195, Australia
Brian L. Sawford
Affiliation:
CSIRO Division of Atmospheric Research, Aspendale, Victoria 3195, Australia
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Abstract

Two-particle Lagrangian models in turbulence are used to consider dispersion in decaying isotropic homogeneous turbulence (i.e. approximating grid-generated wind-tunnel flows). Thomson's formulation is used, and his model is extended to incorporate molecular diffusivity and viscosity, i.e. the range of scales modelled includes the dissipation subranges as well as the intertial subrange. New terms are proposed which consistently provide well-mixed models and the impact on Saffman's well-known small-time results is considered. The new model is ideal for comparison 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 effects are far better explained when we include the molecular effects. A surprising results is the persistence in time of both source-geometry and molecular effects upon concentration fluctuations. Both of the effects are negligible at large times for mean concentrations, but persist for significant times for the fluctuations, indicating an important role for small-scale dynamics.

Type
Research Article
Information
Journal of Fluid Mechanics , Volume 324 , 10 October 1996 , pp. 25 - 54
Copyright
© 1996 Cambridge University Press

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