Behavior of naturally unstable and periodically forced axisymmetric buoyant plumes of helium and helium–air mixtures

Abstract

Effects of forcing on the behavior of naturally unstable buoyant plumes of helium and helium/air mixtures are investigated experimentally. Axisymmetric buoyant plumes originating from a 10 cm diam nozzle are perturbed in a periodic manner by an upstream loudspeaker introducing a sinusoidal streamwise velocity oscillation with a peak-to-peak velocity magnitude of about 3% of the mean velocity at the nozzle exit. The experimental conditions in this study corresponded to Richardson number, Ri=[(ρ∞−ρp)gd]/ρ∞V02, of 42. Video images of the naturally unstable plumes as well as the forced plumes were analyzed to study the features of the resulting flow oscillations and the vortical structures. It was found that the plume responds readily to the imposed oscillations with the toroidal vortices forming at the forcing frequency. Plume images indicate a more complicated and turbulent state of the flow within the large-scale toroidal vortices as they convect downstream. These vortical structures have lateral dimensions of the order of the nozzle diameter at low oscillation frequencies, but the vortex length scales become smaller at higher forcing frequencies. Additionally, mushroom-shaped smaller-scale vortex pairs appear in the very early part of the forced plumes that are absent in the unforced case. Frequency spectrum of the plume centerline velocity fluctuations, detected by two total pressure probes located at one-half and two diameter heights above the nozzle exit, show that the plume predominantly responds to the imposed flow excitation. However, as the flow evolves downstream, frequency spectra exhibit a broader range of frequencies some of which do not coincide with the imposed one and its harmonics. The streamwise convection velocities of the large-scale vortical structures are not influenced by the imposed oscillation frequency. As the excitation frequency approaches that of the natural frequency of the flow, a more chaotic behavior of the large-scale vortices is observed in terms of their convection velocity. This is in contrast to the behavior of momentum dominated shear flows (jets and mixing layers), where a large degree of spatial and temporal coherence of the flow is attained when the flow is excited at its natural frequency. To complement these experiments and probe into the nature of the axisymmetric plume instabilities, we also visualized the transitional behavior of these buoyant plumes from their initially non-oscillatory state to the final periodic state. This is important as far as identifying the nature of the instability as either absolute or convective. Our findings show that the experiments exhibit features that point toward a convective-type instability.