Experimental balances for the second moments for a buoyant plume and their implication on turbulence modeling

The experimental budgets for the transport equations for heat flux and Reynolds stress are presented for a round turbulent buoyant plume. The pressure correlation terms are deduced as the closing terms and are found to constitute a substantial part of these budgets. Even though a buoyant plume is initiated by buoyancy, it is found that the production of heat flux and Reynolds stress is largely maintained by the mean flow gradients, because the buoyancy production terms are not as large. The results are used to assess the local equilibrium assumption, which implies that the production and destruction terms of the transport equations balance each other. The results are also used to investigate why the mechanical to thermal time scale ratio for a buoyant plume is different than the commonly used value. Finally, some simpler models for the pressure correlation terms, which appear in the heat flux and the Reynolds stress equations, are assessed against those deduced from the experiment.