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 (Re-sj = 300, 600, and 900) and porosity (phi = 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 Re-sj is low (Re-sj = 300), phi 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 (phi = 20%) have been observed in the synthetic jet/porous wall interaction. As Re-sj increases to 900, the triggered Kelvin-Helmholtz instability promotes the vorticity cancellation at a low porosity (phi = 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 Re-sj, the incoherence of the vortex ring is mainly due to the viscous effects. However, for a high Re-sj, 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 phi for all Re-sj, while the loss of the kinetic energy transport is nonlinearly dependent on phi. Incorporating phi, this study presents a more comprehensive similarity parameter, phi In(Re-sj(2) d(h)*(3)), to characterize the synthetic jet/porous wall interaction. Published under license by AIP Publishing.