Structure and Mixing Properties of Unsteady Round Nonbuoyant Turbulent Jets and Puffs in Still Gases

A theoretical and experimental study of the temporal development of unsteady round nonbuoyant turbulent jets (starting jets) and puffs (interrupted jets) is described, limited to sources in still and unstratified environments. The experiments involved dye-containing fresh water sources injected vertically downward into fresh water within a large windowed tank with injector passage length/diameter ratios of 50. Time-resolved video images of the flows were obtained using a CCD camera. Test conditions were as follows: jet exit diameters of 3.2–12.7 mm, jet exit Reynolds numbers of 1450–11700, volume of injected fluid for puffs up to 80 passage diameters long, and penetration lengths up to 100 source diameters. Near-source behavior varied significantly with source properties but the flows generally became turbulent near the jet exit with self-preserving behavior observed at distances greater than 20–30 source diameters from the source. Within the self-preserving region, both the normalized streamwise penetration distance and the normalized maximum flow radius varied as functions of time to the following powers, in agreement with estimates for self-preserving turbulent flows: 1/2 for starting nonbuoyant jets and 1/4 for nonbuoyant puffs.