Self-Preserving Properties of Unsteady Round Nonbuoyant Turbulent Starting Jets and Puffs in Still Fluids

Abstract

The self-preserving properties of round nonbuoyant turbulent starting jets, puffs, and interrupted jets were investigated both experimentally and theoretically for flows in still and unstratified environments. The experiments involved dye-containing fresh water sources injected into still fresh water within a large windowed tank. Time-resolved video images of the flows were obtained using a CCD camera. Experimental conditions were as follows: jet exit diameters of 3.2 and 6.4 mm, jet exit Reynolds numbers of 3000–12,000, jet passage lengths in excess of 50 injector passage diameters, volume of injected fluid for puffs and interrupted jets up to 191 source diameters, and streamwise penetration lengths up to 140 source diameters. Near-source behavior varied significantly with source properties but the flows generally became turbulent within 5 source diameters from the source and self-preserving behavior was generally 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 in agreement with estimates for self-preserving turbulent flows to the following powers: 1/2 for starting nonbuoyant jets and 1/4 for nonbuoyant puffs and interrupted jets. Effects of injected fluid quantity for self-preserving puffs and interrupted jets could be handled by correlating the location of the virtual origin as a function of the volume of the injected fluid represented by the number of passage lengths of injected fluid. In particular, the virtual origin for puffs was independent of injected fluid volume for injected passage lengths less than 120 but became proportional to the injected fluid volume thereafter, defining a boundary between puff and interrupted-jet behavior.