Experimental investigation on the impingement of synthetic jet vortex rings onto a porous wall

This paper presents an experimental study on the effects of the Reynolds number (Resj = 300, 600, and 900) and porosity (φ = 20%-85%) on synthetic jet vortex rings impinging onto a porous wall. Laser-induced fluorescence and particle image velocimetry are used to acquire flow information qualitatively and quantitatively. When Resj is low (Resj = 300), φ plays a key role in determining the formation of transmitted vortex rings downstream. For the first time, a row of individual small-scale vortex rings that form at the lowest porosity (φ = 20%) have been observed in the synthetic jet/porous wall interaction. As Resj increases to 900, the triggered Kelvin-Helmholtz instability promotes the vorticity cancellation at a low porosity (φ = 30%), and thus contributes to the formation of a transmitted vortex ring. It is concluded that the vorticity cancellation is the dominant factor affecting the generation of a transmitted vortex ring. Time-averaged characteristics indicate that for a low Resj, the incoherence of the vortex ring is mainly due to the viscous effects. However, for a high Resj, it is the transition that leads to a significant enhancement in the turbulent kinetic energy. Measurements of flow macroscopic parameters show that the loss of the momentum flux exhibits a linear relationship with φ for all Resj, while the loss of the kinetic energy transport is nonlinearly dependent on φ. Incorporating φ, this study presents a more comprehensive similarity parameter, φln(Resj2 d h∗ 3), to characterize the synthetic jet/porous wall interaction.