Mixing Enhancement of Compressible Jets by Using Unsteady Microjets as Actuators

Lateralsteady/unsteadyinjectionofanarrayofmicrojets,whichareplacedalongthecircumferenceofthenozzle 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 modesof the microjets wereinvestigated: axisymmetric and antisymmetric injections. Fully expanded and underexpanded primary jets issued from a convergent nozzle are considered in thepresent study. Theunsteady microjets wereinjectedata Strouhal number Srof0.16, 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 e ow rate. Results of the mean e owe eld showed that the antisymmetric injection has a higher spreading ratethan the steady and unsteady axisymmetric injections in terms of centerlinevelocity decay. Thoseresults were cone rmed 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 e ow control.