Evolution of flow structure from a coaxial synthetic jet

In this work, we propose and demonstrate a novel coaxial synthetic jet (CSJ) in which two cavities are arranged coaxially with 0 degrees orientation angle with inner and annular orifices of equal hydraulic diameter. The flow behavior of the CSJ is studied at a low Reynolds number; Re = 135. The Navier-Stokes equations are solved (Direct Numerical Simulation) with the finite volume solver of openFoam, an open-source platform for numerical simulations. During the evolution of CSJ, two vortex rings IVR (inner vortex ring), and AVR (annular vortex ring), which are single toroid and double toroid in nature, are emanated from inner and annular orifices, respectively. Both the inner and annular jets are operated at equal mass fluxes. The evolution and interaction of the IVR, and AVR are studied by providing five-phase differences ( empty set = 0 degrees, 45 degrees, 90 degrees, 135 degrees, and 180 degrees) between the inner and outer diaphragms, and also its influence on the nature of the CSJ is highlighted. The phase-averaged and time-averaged flow fields are analyzed, and the CSJ is compared with the single cavity synthetic jet (SJ) at Re = 135, which is obtained by operating the inner cavity and annular cavity alone. The CSJ with empty set = 0 degrees reveals a wide and strong jet as compared to the single cavity SJ. The pairing of IVR and AVR results in a narrow and strong jet for empty set = 180 degrees as compared to empty set = 0 degrees. Merging and pulling mechanisms in case of empty set = 45 degrees and 90 degrees enhances the azimuthal instability resulting in a wide and strong jet as compared to empty set = 0 degrees. A significant increase in the integrated jet area is observed for CSJ, which evidences the increase in entrainment ability of the CSJ. This novel device may find wide applicability for various engineering applications.