Laboratory observations of interactions of forced plumes with stratified shear layers

Plumes of fluid are often observed in nature to interact with stratified sh ear layers. Examples of this include chimney plumes hitting inversion-layer ceilings; sewage plumes impinging on unmixed fresh/saltwater interfaces; d escending plumes of cold water formed at ice-leads interacting with the oce anic thermocline; and volcano plumes interacting with atmospheric interface s. Controlled laboratory studies of these phenomena have not previously bee n described in the literature, and as a result there is a lack of understan ding regarding their morphology and dynamics. Thus, a novel set of experime nts is described here in which the behaviour of a turbulent plume is observ ed in the presence of a two-layer ambient. The lower layer, into which the plume initially emerges, is quiescent and at a relatively high density. The upper layer is forced to how uniformly across the top of the lower layer, and has a lower density. The flow of the resulting plume is characterised b y (a) its vertical and lateral spreading in the lower layer; (b) the nature of its extension upstream and downstream at the interface; and (c) the ext ent to which it penetrates into the upper layer. The behaviour is found to be governed by three non-dimensional parameters: the initial gradient Richa rdson number of the interface Ri(G), the ratio of the upper layer crossflow speed to the speed of the plume when it first impinges on the interface U- F/U-PI, and the ratio of the plume Monin-Obukhov lengthscale to the lower l ayer depth L-MO/H-L. Regime diagrams are presented showing the effects of c hanging these parameters on the plume flow, quantitative relationships are determined, and practical applications of the results are considered. (C) 2 000 The Japan Society of Fluid Mechanics and Elsevier Science B.V. All righ ts reserved.