Active Control of Coaxial Jet Mixing with Arrayed Micro Actuators

Active mixing control of gaseous coaxial jets is achieved using magnetic flap actuators arranged on the inner periphery of the outer annular nozzle. The spatio-temporal velocity fields and scalar mixing are studied through a particle image velocimeter and a laser-induced fluorescence method. Flap motion forces the outer shear layer to roll up into large-scale ring vortices. When the momenum flux of the annular jet is much larger than that of the central jet, the inner shear layer also rolls up into large-scale vortices, which pinch off the central jet significantly. Near-field mixing process is dominated by the ring vortices, while streamwise vortices generated through the breakdown of the ring vortices are responsible for the mixing at the downstream. The near-field mixing is characterized by the length scale of ring vortices and the Strouhal number, which is defined as the ratio between the dimension and shedding interval of the vortices. When the Strouhal number is unity and the vortex size is comparable to the nozzle radius, the mixing is enhanced most significantly since the near-field is most densely populated with the large-scale vortices.