Duty cycle effect on jet control based on a single unsteady minijet

An experimental investigation has been carried out to assess the mixing promoting capability on a turbulent round jet of Reynolds number 8000 controlled by a single unsteady radial minijet. The influence of the duty cycle d_c of the unsteady minijet on jet mixing characteristics has been investigated. The decay rate K of the jet centerline mean velocity is strongly influenced by the duty cycle. Moreover, the optimum duty cycle increases with increasing C_m. The increase in duty cycle is closely linked to the depth of the minijet penetration and the generation of mushroom-like structures, resulting in greatly enhanced jet mixing. Flow visualization results reveal that at small C_m the unsteady injection can only affect the shear layer and suppress the vortex-pairing on the injection side, thus resulting in the jet column to flap. However, at larger C_m the unsteady injection may have penetrated the main jet, causing a highly turbulent jet, irrespective of d_c.