Simulation and analysis of puffing instability in the near field of pure thermal planar plumes

The near-field unsteady behaviour of a buoyant plume is characterised by the puffing instability, which is a periodic formation of vortical structures (puffs) along the plume axis. In this study, a plume is generated from a finite-width fixed temperature planar source. The formation of puffs in the near-field is shown to be a result of the bulge forming instability in the lapping flow, which develops on the heated source region on either side of the plume axis. This paper reports on the three-dimensional direct numerical simulation of these near-field behaviours. The effect of the lapping flow velocity on the bulge forming instability is further investigated by modelling the boundary layer flow in the vicinity of the plume source by use of a channel flow with a heated floor section. The channel inlet velocity is controlled to vary the lapping flow velocity. The bulge formation is found to occur above a critical Reynolds number and below a critical Froude number, where the Froude number is defined as a measure of the lapping flow velocity. It is further found that with increasing Froude number the bulge forms further downstream, and there is the transition from a quasi-periodic to single periodic oscillation mode with a reduction in the oscillation amplitude. (C) 2013 Elsevier Masson SAS. All rights reserved.