Mixing enhancement of compressible jets by using unsteady microjets as actuators

Lateral steady/unsteady injection of an array of microjets, which are placed along the circumference of the nozzle exit of a primary jet at equal intervals of azimuthal angle, was studied experimentally to clarify the characteristics of mixing and noise of compressible primary jets. Two modes, of the microjets were investigated: axisymmetric and antisymmetric injections. Fully expanded and underexpanded primary jets issued from a convergent nozzle are considered in the present study. The unsteady microjets were injected at a Strouhal number Sr of 0.16, based on the nozzle exit diameter and the velocity at the nozzle exit, for two cases of total mass injection: 4 and 6% of primary jet mass flow rate. Results of the mean flowfield showed that the antisymmetric injection has a higher spreading rate than the steady and unsteady axisymmetric injections in terms of centerline velocity decay. Those results were confirmed from linear stability analysis, which showed that antisymmetric modes for natural disturbances are more unstable in the downstream region than the corresponding axisymmetric modes. Moreover,: reduction in the radiated noise was observed in steady axisymmetric injection. Thus, these results suggest that unsteady microjets have the potential for future use as a device for shear flow control.