Influence of mass flux ratio on the evolution of coaxial synthetic jet

This paper highlights a direct numerical simulation study on the flow field of a coaxial synthetic jet (CSJ) generated from two independently controlled synthetic jet actuators, which are combined coaxially with $0°$ orientation angle. The jet is issued into a quiescent environment from inner and annular openings (orifices) with equal hydraulic diameters, employing an oscillating boundary. Seven different mass flux ratios (⁠$Mr$⁠) such as $0.25$⁠, $0.5$⁠, $0.75$⁠, $1.0$⁠, $1.5$⁠, $2.0$⁠, and $3.0$ are considered for the study. The average velocity (⁠$Uavg$⁠) of inner jet, measured at orifice exit, is kept at $0.7$ m/s (Reynolds number, $Re=135$⁠), and the same is varied for the annular jet to achieve the desired $Mr$ s. The influence of $Mr$ s on the vortex rings, evolved from inner and annular orifices, along with their dynamics, is predicted by furnishing the instantaneous flow field. Also, we examine the effect of $Mr$ s on the mean flow parameters of the CSJ. Moreover, the CSJ flow field is compared with the inner cavity synthetic jet (SJ), and annular cavity SJ under identical conditions, to demonstrate the superior performance of the CSJ over the single cavity SJs. For CSJ, the azimuthal instability of the evolved vortex rings can be triggered by decreasing the $Mr$⁠, which results in a wide jet. For $Mr≥1.0$⁠, the CSJ retains its axisymmetric nature, and the interaction of vortex rings emanating from the inner and annular cavities influences the strength and spreading of the CSJ. The modal decomposition of the instantaneous flow field is also performed using proper orthogonal decomposition method to gain insight of the coherent vortical structures present in the modes. The study will be useful for deploying such novel coaxial synthetic jets in various applications.