Evolution of Coaxial Synthetic Jet with Different Velocity Ratios *

In this study, we propose and demonstrate two synthetic jets which are arranged coaxially with 0^o orientation angle to form a coaxial synthetic jet (CSJ). Two diaphragms are mounted independently with two cavities such that their oscillating amplitude and frequency can be varied separately to achieve various velocity ratios (V_r), which is the ratio of spatial and time-averaged velocity at the inner orifice exit to that at the annular orifice exit. Both the diaphragms are oscillated at 50 Hz frequency, and the amplitude is varied such that various V_r of 0.5, 1, 2, and 4 can be achieved. The CSJ is associated with two vortex rings: (a) single toroidal inner vortex ring (VR_i) generated from the inner orifice, and (b) double toroidal annular vortex ring (VR_a) generated from the annular orifice. The effect of different V_r along with the effect of absolute velocity for a constant V_r on the evolution and interaction of these vortex rings, and the effect of this interaction on the flow field of the CSJ is studied. For V_r = 0.5, as the velocity output of the annular jet is twice the inner jet, the diameter of the outer ring of the VR_a is large as compared to V_r = 1, and 2 resulting in more azimuthal instability making the jet wider. Also, the breaking of the VR_i into streamwise vortices near the orifice exit contributes in widening the jet for V_r = 0.5. For V_r = 1, the VR_i advances faster in the downstream direction as compared to the VR_a due to the presence of low-pressure region at the core of the outer ring of the VR_a generated in the previous cycle. This makes the CSJ strong and, also the additional annular jet around the inner jet makes the CSJ wide as compared to a single SJ. Overall, the study briefs the nature of CSJ on the basis of different velocity ratios and absolute velocities, which can be helpful for effective utilization of the CSJ in flow control and heat transfer applications.