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Abstract
The approximate variation of the mean properties in an axisymmetric jet and a two dimensional jet with an ambient coflow in an infinite duct have been determined using the excess momentum equation and an entrainment function. The turbulent velocity flux, consisting of a portion due to the excess jet velocity and a portion due to the turbulent coflow velocity, is carried by a velocity approximately equal to the top hat velocity. The entrainment into this flow is driven by the excess jet velocity and the entrainment function varies to allow for the change in entrainment from the strong jet to the weak jet. The entrainment constant is determined from the data of Nickels and Perry [14] for the axisymmetric case and the data of Bradbury and Riley [6] for the two dimensional case. All coflow experiments are in ducts and the effect of the duct is explored for the axisymmetric case. This paper is a prelude to the study of a buoyant jet in a coflow and a buoyant jet in a crossflow.