On the behaviour of advected plumes and thermals

The results of an experimental investigation into the behaviour of advected plumes are presented. Measurements of tracer concentration are made using laser-induced fluorescence and image processing techniques. These measurements provide the basis for studying the bulk properties and fluctuation statistics of the flows. The experimental results show that the appropriate characteristic velocity for predicting the centreline location of an advected plume is the cross-sectional average velocity and not the centerline velocity (which has been utilised for this purpose in the past). The reason for this is discussed in the context of the large scale turbulent structures evident in the flow. In addition, it is shown that reasonably accurate predictions of the bulk properties of advected plumes can be made by assuming the behaviour of an advected plume is similar to that of a plume discharged in a still ambient fluid (a still plume), which is simply advected by the ambient current. Notable differences can be found in the fluctuation statistics of still and advected plumes. Detachment of tracer from the plume is observed, but it does not have any significant impact on the bulk properties of the flow. In the context of the advected plume experimental results the behaviour of an advected thermal is discussed, with particular reference to the appropriate characteristic velocity for defining its path. The data set also enables us to locate the transition from an advected plume to an advected thermal with some confidence