Near-Field Entrainment in an Impulsively Started Turbulent Gas Jet

ABSTRACT The entrainment characteristics of an impulsively started gas jet injected into quiescent atmosphere was studied by means of 2-D visualisations and Laser Doppler velocimetry. The focus of the study was on the near field behaviour where the velocity profiles are not self-similar unlike that far downstream and the unsteady jet head structure plays a relevant role in the early jet development. Direct measurements of the size of the unsteady leading jet structure shows that its length is larger than 12 nozzle diameters, in the near field region investigated, 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 of entrainment in the quasi-steady part of the jet. It 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 jet 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 larger times to reach equilibrium.