Direct and large-eddy simulation of the transition of two- and three-dimensional plane plumes in a confined enclosure

We investigate here, the free convection flow induced by a line heat source in a confined geometry. The buoyancy forcing of this flow can be character ized by a Rayleigh number, Rn, which is chosen in the range where an interm ittent spatial transition from laminar to turbulent flow takes place. The o bjective of the study is to explore this flow with help of numerical simula tions. We restrict ourselves to the case of an air flow with Ra = 10(10). F or the numerical simulation techniques, we employ Direct Numerical Simulati on (DNS) and Large-Eddy Simulation (LES), With help of DNS we consider firs t, a 2D representation of this how at a resolution of 195(2) which is found to be sufficient to represent the heat source and its resulting flow. Next , we consider the 3D case at a resolution of 195(3) The 3D simulation revea ls a symmetrical time mean recirculation which covers the domain above the heat source. This large scale circulation is driven by the small scale lami nar plume generated by the heat source and which breaks down into turbulenc e. The flow is found to be essentially 3D, especially near the top wall. No clear turbulent inertial range is present. A LES for the same flow has bee n carried out at a resolution of 45(3). The comparison of the les results w ith the DNS data has been used to investigate the performance of several su b-grid models. It turns out that simple equilibrium sub-grid models perform fairly well in estimating the statistics of the flow. (C) 2000 Elsevier Sc ience Ltd. All rights reserved.