Plume Dynamics in Natural Convection in a Horizontal Cylindrical Annulus

Measurements of the unsteady temperature fluctuations in the plume region between differentially heated horizontal concentric cylinders are reported. In particular, power spectral density estimates of the temperature fluctuations within the plume show the development and breakdown of the oscillatory plume structure at high Rayleigh number, Ra_d, by two relatively independent processes: (1) the development of harmonic oscillations related to the dominant plume oscillation frequency, and (2) interactions between the oscillating plume and the adjacent relatively stagnant core flow (shear and entrainment). The harmonic oscillations are shown to be the dominant energy transfer mode at moderate Ra_d (up to Ra_d = 10⁸), acting to disperse the plume energy without generating a broadband spectrum. The spectral density estimates show that while a distinct plume oscillation is still present near the inner cylinder at Ra_d = 10⁹, the plume becomes increasingly turbulent as the outer cylinder is approached. A new correlation for the plume oscillation frequency, which is found to be proportional to $Rad0.5$⁠, is also presented.