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.

Automated summary

The study reveals that both the Reynolds number and porosity play significant roles in the impingement of synthetic jet vortex rings onto a porous wall. As the Reynolds number increases, the vorticity cancellation becomes the dominant factor affecting the generation of transmitted vortex rings. Viscous effects are found to be the main cause of incoherence in vortex rings at low Reynolds number, while transition leads to enhanced turbulent kinetic energy at high Reynolds number.