Coherent structure evolution and entrainment effect of dual synthetic jets: effect of operating frequency

Dual synthetic jets are formed by the downstream merging of two synthetic jets with a phase difference of 180°, featuring stronger momentum flux, greater penetration depth, and lower operating noise. This technology has broad application prospects in the field of flow control. This study measured the instantaneous flow fields of dual synthetic jets with an aspect ratio R_AR = 10 at operating frequencies f_A = 650, 850, 1050, and 1250 Hz by high-frequency particle image velocimetry. Coherent structures in the phase-averaged and time-averaged flow fields were analysed based on the velocity triple decomposition method and four-quadrant rule, and the entrainment capacity of the dual synthetic jets was quantitatively evaluated through mass flow rate and momentum flux. The results show that the self-support phenomenon accelerates the curvature change of the vortex ring, inhibiting the formation of inner vortices while pulling the entire vortex ring. These causes the primary vortex to deform and split off to form a secondary vortex, and promotes the transformation of minor-axis plane vorticity into the major-axis plane, leading to faster loss of coherence and collapse of the primary vortex. The time-averaged flow fields at f_A = 650 and 850 Hz exhibit a high degree of similarity, with the turbulent kinetic energy being relatively low and distributed over a large range. The periodic kinetic energy plays a dominant role in the flow field, and is mainly concentrated near the orifices. As the operating frequency increases, primary vortices are more susceptible to collapse, leading to a rapid loss of coherence. The turbulent kinetic energy caused by vortices breaking and merging becomes dominant in the flow field (f_A = 1050 and 1250 Hz), and the periodic kinetic energy is mainly concentrated in the region where primary vortices are formed. The entrainment capacity of dual synthetic jets is not solely determined by the jet Reynolds number (Re_j) or the Strouhal number (St_j), but is also closely related to the pattern of vortex evolution. The generation and development of secondary vortices can enhance the entrainment capacity, allowing the jet to maintain a high mass flow rate and momentum flux even at lower Re_j. This process is regulated by the operating frequency, which alters the evolution pattern of the primary vortex and thereby changes the streamwise evolution of the mass flow rate and the momentum flux. This study can provide a reference for the parameter optimization and engineering application of the dual synthetic jet actuator.