A ventilated filling box flow in the transitional regime has been investigated over a wide range of Reynolds number, Re, and Prandtl number, Pr, to examine their effects, using direct numerical simulation. It has been shown that the parameter Re-2/Pr is an important control parameter, representing a measure of the strength of the plume source. A mapping of parameter-dependent behaviours was obtained, and the instability mechanism has been qualitatively analysed. The balance of kinetic energy equation terms demonstrated that the contribution of the shear production towards the total production reduces and the buoyancy production dominance increases with increasing Pr. Turbulence intensity in the upper buoyant layer was found to become increasingly Pr dependent with reducing Re-2/Pr. It has also been observed that with increasing Re-2/Pr the Pr dependence of the location of the interface, that forms between lower ambient and upper buoyant fluids, is more predominantly determined by that of the ventilation rate, while with reducing Re-2/Pr the Pr dependence of the location of the interface becomes affected also by the Pr dependent variation in the plume volume flux. (C) 2012 Elsevier Ltd. All rights reserved.