Turbulence reduction in the mixing layer of a plane jet using small cylinders

The influence of small diameter cylinders placed at the nozzle exit plane on the development of a plane jet mixing layer is investigated using a single hot wire at a jet Reynolds number of about 23000. There is a reduction in both the growth rate and the momentum thickness of the jet as well as a 20% reduction in the maximum value of u', the rms longitudinal velocity fluctuation u. The jet virtual origin is shifted downstream and distributions of the normalized mean velocity, u', skewness and flatness factors of u exhibit much better similarity than in the absence of the cylinder. Spectral measurements indicate that downstream of the cylinder, the peak amplitude at the roll-up or instability frequency is greatly reduced or even suppressed. Two different diameter cylinders, placed independently at several locations in the shear layer yielded similar results. A model based on an interaction between the organized motion in the mixing layer and the vortical structures shed by the cylinder is consistent with our observations.