A comprehensive investigation has been made of turbulent mixing in a system consisting of a cylindrical duct into which there is pumped an uniform stream of air and down whose axis there is injected from a small nozzle a more concentrated stream of air which generates a turbulent jet. The work covers the regime in which the inducted stream of air is too meagre to satisfy the entrainment needs of the jet, causing the jet to feed itself through a recirculatory flow of downstream gases. The flow pattern is governed by a similarity parameter which has been named the throttling number. The problem was, therefore, to study the mixing field of the "ducted jet" as a function of the throttling number. The fields of the mean velocity and of the mean concentration of a species marking the nozzle fluid have been analyzed theoretically, and have also been studied experimentally. The sol-scattered light technique developed by R. E. Rosensweig has been used to determine the characteristics of the turbulent fluctuations in concentration of an oil-fog marking the nozzle fluid. The field of the intensity of the fluctuations has been mapped, the spectrum of the fluctuations has been studied, and two-point lateral correlation coefficients have been measured. The field of the mean static-pressure was mapped, and from the data information has been elicited concerning the velocity fluctuations in a turbulent jet.

It was found that the profiles, of the mean velocity and the mean concentration are self-preserving in shape. The profiles were characterized, as to scale, by the velocity or concentration excess on the jet axis and by the radius at which the velocity or concentration excess was one-half that on the axis. The ducted jet was found to behave like a free jet at the operating condition at which recirculation just disappeared. Recirculation caused the jet to spread more rapidly. The ratio between the semivelocity and semiconcentration radii was a constant, and indicated an eddy Schmidt number of 0.67. The shape of the profile of the intensity of concentration fluctuations was found to be self-preserving at stations further than 30 nozzle radii downstream of the nozzle. The ratio of the concentration fluctuation intensity to the mean concentration appears to near constancy only at stations more than 200 nozzle radii downstream of the nozzle, and the same is true of the intensity of velocity fluctuations relative to the mean velocity. The shape of the profile of the static-pressure defect in the jet was self-preserving, The spectra of concentration fluctuations in the jet and the curves of lateral correlation coefficient vs. separation distance were self-preserving in shape. The integral scales calculated from the spectra and correlations approach a constant ratio with the radius of the mean flow in the jet only at very large distances downstream of the nozzle.