On the near field of single and twin circular synthetic air jets

In this work the flow features in the near field of single and twin synthetic jets are experimentally investigated. An acoustic resonator is designed as a cavity split in two equal sub-cavities by the membrane of a loudspeaker. Each sub-cavity is connected to the quiescent ambient by a cylindrical nozzle (a pipe) thus two circular synthetic jets in phase opposition are obtained. The influence of jets interaction has been evaluated by varying the jet-axes-distance (1.1, 3 and 5 nozzle diameters) at a fixed operating condition (Reynolds number equal to 6700 and Strouhal number equal to 0.046); the jets pulsating frequency is kept equal to the sub-cavities resonance frequency and the results are compared with those of the single circular synthetic jet in the same conditions. ow is characterized with PIV phase averaged measurements. Results for the single synthetic jet are in good agreement with the ones present in the literature and a potential core region is defined for the phase averaged flow field of synthetic jets. Twin circular synthetic jets at jet-axes-distance equal to 3 and 5 diameters show the same behavior with respect to the single circular synthetic jet. Differently, strong interaction effects are seen at jet-axes-distance equal to 1.1 diameters: in particular a double vortex ring structure is visualized. This causes a different velocity distribution in the jet core, higher axial velocities and a lower jet width.