Self-preserving properties of unsteady round bouyant turbulent plumes and thermals in still fluids

The self-preserving properties of round buoyant turbulent starting plumes and starting jets in unstratified environments. The experiments involved dye-containing salt water sources injected vertically downward into still fresh water within a windowed tank. Time-resolved images of the flows were obtained using a CCD camera. Experimental conditions were as follows: source diameters of 3.2 and 6.4 mm, source/ambient density ratios of 1.070 and 1.150, source Reynolds numbers of 4,000-11,000, source Froude numbers of 10-82, volume of source fluid for thermals comprising cylinders having the same cross-sectional areas as the source exit and lengths of 50-382 source diameters, and streamwise flow penetration lengths up to 110 source diameters and 5.05 Morton length scales from the source. Near-source flow properties varied significantly with source properties but the flows generally became turbulent and then became self-preserving within 5 and 20-30 source diameters from the source, respectively. Within the self-preserving region, both normalized streamwise penetration distances and normalized maximum radial penetration distances as functions of time were in agreement with the scaling relationships for the behavior of self-preserving round buoyant turbulent flows to the following powers: time to the 3/4 power for starting plumes and to the 1/2 power for thermals. Finally, the virtual origins of thermals were independent of source fluid volume for the present test conditions.