Near-field entrainment in an impulsively started turbulent gas jet

The entrainment characteristics of an impulsively started gas jet injected into quiescent atmosphere was studied by means of two-dimensional visualizations and laser Doppler velocimetry. The focus was on near-field behavior, where the velocity profiles are not self-similar, unlike those far downstream, and the unsteady jet ahead structure plays a relevant role in the early jet development. In the investigated near-field region, direct measurements of the size of the unsteady leading jet structure shows that its length is larger than 12 nozzle diameters and grows linearly with time. The present analysis indicates that early in the near field the jet volume becomes larger than the injected one and suggests a possible mechanism for surrounding gas entrainment into the jet head, which differs significantly from the mechanism responsible for entrainment in the quasi-steady part of the jet. It is follows that the frequently used model that assumes the head vortex growing due to the mass entrainment from the steady-state jet cone only should be modified when dealing with the initial part of the injection. A further result is that the attainment of the steady-state conditions is faster on the jet axis than at the edges, where the mixing mechanism requires longer times to reach equilibrium.