Unsteady development of a pure thermal plume over locally heated horizontal surface: Flow bifurcation

Pure thermal plumes have been extensively studied in recent decades. It has been established that for the flow that formed over horizontal surfaces with local heating areas of various shapes, a change in flow regimes is observed: the stationary regime of an axisymmetric plume is replaced by a regime with periodic destruction of the near-wall layer, and the emergence of toroidal vortex structures. The main interest for the authors of the published studies is apparently in the fully developed flow regimes (both stationary and periodic). In contrast, the causes of the destruction of the near-wall layer, leading to the emergence of the puffing regime, remain explored poorly. This paper will discuss the changes in the flow patterns of the plume formed above a locally heated horizontal surface. The main study method was axially symmetric numerical simulation in a wide range of Rayleigh numbers. Numerical data are supplemented by physical experiment results obtained in similar conditions. The flow structure close to the disk's surface is given special consideration, along with the circumstances leading to a change in flow regimes. The nature of the evolution of thermal disturbance propagating in the air above the disk surface at different values of the Rayleigh number is considered separately.