Spread of a surface plume in unsteady and steady flows

Abstract

A series of laboratory measurements was undertaken in unsteady and steady flows to determine the spread and mixing of surface plumes, emitting from a heat source without momentum. Instantaneous temperatures were measured in the vertical and lateral directions at various downstream sections; similar measurements were made when the ambient currents were steady. The mean temperature profiles in the lateral and vertical directions were of the Gaussian form in steady and unsteady flows. The outstanding results were those obtained when the flow was unsteady; the plume dilution, the plume spread and the turbulence intensity of temperature fluctuations were then larger than when the current was steady - their values were the maximum in decelerating flow. Instantaneous velocities were measured at a section upstream from the heat source in unsteady and steady flows; the results showed that the mean velocity profiles were logarithmic irrespective of the flow state. Both the turbulence intensity in the mean flow direction and the Reynolds shear stress in the decelerating flows were substantially larger than those measured when the flows were accelerating. These results agreed reasonably well with those obtained in estuarine flows. The temperature rise at the heat source was 10°C, hence the present results are also applicable to the case in which buoyancy is produced by a difference in concentration of dissolved materials.