Abstract
Large-scale vortical structures and associated flow development in an axisymmetric jet are actively controlled by manipulating the initial shear layer with arrayed miniature jet actuators. The periodic radial miniature jet injections are realized by using a rapid-response pneumatic servovalve, and sinusoidal/pulsed flowing are employed in the present study. The spatio-temporal primary jet structures are investigated through phase-locked particle image verocimetry (PIV). In the pulsed configuration, it is found that intense vortex rings are produced in phase with the periodic control input regardless of the valve-driven frequency f_v examined. When the Strouhal number Stv, which is defined with f_v, is larger than unity, the vortex rings are shed continuously and thus the jet flow development is prompted with low periodic velocity fluctuation. The diameter of the vortices becomes small as Stv is increased, so that the transport range of the jet and ambient fluids can be controlled by changing Stv. In addition, the evolution of counter-rotating streamwise vortices is also confirmed. These streamwise vortices are formed as a result of tilting of the primary jet azimuthal vorticity by the miniature jet impinging on the primary jet shear layer. It is conjectured that they contribute the mixing enhancement in the azimuthal direction.
