Turbulent convection from isolated sources

Laboratory experiments were conducted to investigate the evolution of a den se turbulent plume, specified by its buoyancy flux B-o and source diameter D, issuing into a homogeneous, nonrotating, environment. This study was mot ivated by the desire to delineate velocity and buoyancy scaling for convect ion from isolated buoyancy sources of finite extent. Such flow configuratio ns have relevance to geophysical (e.g., deep convection), environmental (e. g., urban heat island effect) and engineering (e.g., plume stacks) flows. S pecial attention was given to study the evolution of the plume following it s initiation and the flow near the source when the influence of confining b oundaries is insignificant. It was found that, for times t < 1.2(D-2/B-o)(1 /3), the descent of the plume front can be treated as one-dimensional with negligible lateral (entrainment) mean flow, the plume growth mechanism bein g the encroachment of underlying nonturbulent fluid. At larger times, the f low achieved a quasi-steady state, in which the plume width first decreases up to a distance 0.28 D (region I) and then increases (region II). The qua si-steady state velocity and buoyancy measurements in region I showed that they are strongly influenced by the lateral entrainment flow (and hence, by D), and thus classical free convection scaling is inapplicable. On the oth er hand, at large z/D (in region II), the velocity and buoyancy scaling ten d to be independent of D, indicating the fading influence of source diamete r effects. The results establish basic scaling for convection from isolated , but distributed, sources and provide a baseline with which future work th at incorporates background rotation and stratification can be compared. (C) 1999 Elsevier Science B.V. All rights reserved.