Evolution of the streamwise vortices in a coaxial jet controlled with micro flap actuators

Abstract

A coaxial jet was actively controlled by a MEMS-fabricated micro flap actuator nozzle. The effect of different control modes on secondary azimuthal instabilities and the evolution of streamwise vortices were investigated by applying stereoscopic PIV to the cross-stream plane of the jet. Forcing with non-symmetric modes, in particular the least-stable helical mode, accelerates the evolution of the streamwise vortices through the enhancement of azimuthal instabilities. Although forcing is applied to the outer shear layer of the outer jet, the control effect is most pronounced in the inner shear layer of the inner jet. Unlike in the natural jet, streamwise vortices appear in the inner shear layer of the controlled jet. For forcing with the fundamental axisymmetric mode, a Strouhal number of the order of unity maximise the azimuthal instabilities and hence the counts of the streamwise vortices. The present result is in accordance with our previous experimental findings in the longitudinal plane, where the evolution of the primary vortices and mixing between the inner and the outer jets were examined through 2D-PIV and PLIF (Kurimoto et al., 2004, Active control of coaxial jet mixing with arrayed micro actuators. Transactions of the Japanese Society of Mechanical Engineers, pp. 31–38.) This emphasises the connection between primary and streamwise vortices and their significance in the mixing enhancement process. It is also found that the azimuthal wavelength under the present control scheme is almost the same as that of the natural jet and independent of the streamwise position.