Evolution of flow structure from a coaxial synthetic jet

Abstract

• Influence of phase differences (∅) on novel coaxial synthetic jet (CSJ) is studied. • Azimuthal instability widens the jet for ∅ = 45°, and 90°. • Vortex pairing results in a strong jet for ∅ = 180°. • Entrainment ability of CSJ is high compared to single cavity synthetic jets. • Modulation of CSJ can be achieved with ∅ without investing any extra power. In this work, we propose and demonstrate a novel coaxial synthetic jet (CSJ) in which two cavities are arranged coaxially with 0° 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 (∅ = 0°, 45°, 90°, 135°, and 180°) 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 ∅ = 0° 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 ∅ = 180° as compared to ∅ = 0°. Merging and pulling mechanisms in case of ∅ = 45° and 90° enhances the azimuthal instability resulting in a wide and strong jet as compared to ∅ = 0°. 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.