The effects of the free-stream density ratio on the evolution of the incompressible, high Reynolds and Froude number, confined mixing layer are investigated numerically. Two-dimensional simulations of the spatially developing flow with and without external forcing are obtained using the Lagrangian transport element method. Results indicate that a nonunity density ratio alters the flow characteristics significantly. In the unforced flow, it increases the layer growth as the slow stream becomes denser, biases the speed of both the linear instability waves and the rollup eddies toward that of the denser stream, and modifies entrainment in favor of the dense fluid. These results, which are in agreement with experimental and analytical evidence, are analyzed in terms of the evolution of the vorticity field and, in particular, of the action of the mechanism of baroclinic vorticity generation. It is found that this mechanism creates vorticity of opposite signs across each eddy, which, through simple kinematical arguments, is linked to the alteration of the eddy speed and the modification of the local entrainment patterns. High-amplitude external forcing modifies the growth behavior of the layer while leaving its entrainment characteristics and the eddy speeds unaffected. In this case the layer growth is no longer monotonically varying with the free-stream density ratio. Instead, it is a strong function of the momentum ratio, reaching a minimum at a momentum ratio of unity and increasing more significantly for higher values of this parameter. Enhancement of the layer growth via forcing occurs only when the momentum ratio is substantially different from unity. It is found that the forced layer growth characteristics are related to the layer orientation, which is also a function of the momentum ratio. Using this fact and basic principles, a simple analytical model is derived to explain the numerical results. It is suggested that the unforced flow behaves differently due to its initial instability characteristics that are bypassed when forcing is present.

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Experiments on the onset of buoyant instabilities leading to periodic formation of vortical structures in planar buoyant plumes of helium and helium/air mixtures injected into quiescent air are reported for a range of nozzle widths (w=20–70 mm), plume fluid densities (pure helium to that approaching air), and velocities at the nozzle exit. First, the plume parameters corresponding to the onset of the oscillatory instability were experimentally determined by varying the nozzle exit velocity for different nozzle widths and plume fluid densities in two different nozzle configurations. These configurations corresponded to a freestanding rectangular nozzle and a rectangular nozzle surrounded by a flat plate in the plane of the nozzle exit. The observed plume behavior in the near field was characterized as nonoscillatory, transitional, or pulsatile. The onset of pulsations in the near field of these buoyant plumes (within a height of two nozzle widths) was best correlated in terms of the plume source Reynolds number and the plume fluid to ambient density ratio. It was also found that the boundary conditions surrounding the nozzle exit had an influence on the onset of plume instability in the near field. Specifically, at a given plume to the ambient density ratio, the plumes with flat plate surround were found to transition to the oscillatory state at a lower value of the threshold velocity and therefore are less stable than the plumes originating from freestanding nozzles. Subsequently, the plume oscillation frequencies were measured as a function of plume width, plume source velocity, and the density ratio for a range of these parameters. The plume oscillation frequency was found to correlate well in terms of the nondimensional parameters, Strouhal number, S=(fw)/Vp, and Richardson number, Ri=[(ρ∞−ρp)gw]/ρ∞Vp2, yielding a correlation S=0.55Ri0.45 determined for 1<Ri<102. This correlation is somewhat different from that of the axisymmetric buoyant plumes, which can be attributed to the differences in mixing rates and the strength of the local buoyancy flux in planar and axisymmetric plumes. The vortical structures formed in the unstable plumes also exhibit several distinct vortex pair modes. The centers of the formed vortex pairs, in general, do not remain colinear and distort with respect to each other when compared with the axisymmetric plume vortex rings, which are toroidal. The convection speeds of the vortex pair centers were also measured and reported in this study.