Recirculating Flow Generated by Line-Source Bubble Plumes

Abstract

The flow structure in the far field of line-source bubble plumes in shallow water is investigated. Recirculating cells form at both sides of the plume. Cell lengths, as given by other investigators, vary between 2.5 and 7 times the water depth. The variations are caused in part by different definitions of cell length and by the different geometries of the experimental setups. In the present study the influence of the cross-sectional geometry of the channel on the cell structure is investigated experimentally. Experiments have been made in a tank with dimensions 1 × 1 × 40 m. A dividing wall has been used to vary the width of the channel. Flow velocities in the plane of symmetry have been measured by use of a propeller anemometer. These results have been supplemented by measurements with an acoustic Doppler velocimeter. Different criteria for the definition of the cell length have been examined, such as the surface flow rate, the surface velocity, and surface flow patterns. The velocity distributions show a strong influence of the cross-sectional geometry. For values of the depth-to-width ratio higher than unity the cell length is rather short. In a very wide channel three-dimensional effects occur. The aspect ratio is found to be a characteristic parameter for the flow. For an analysis of the flow in the far field, a model based on an analogy of the surface flow with a free (half) jet with counterflow is proposed.