On the interaction between turbulent vortex rings of a synthetic jet and porous walls

Abstract

Time-resolved particle image velocimetry was applied to investigate turbulent vortex rings generated by a synthetic jet impacting porous walls. By varying the nondimensional diameter of the hole (dh∗ = 0.067-0.20), we chose four porous walls with a constant porosity (φ = 75%) to examine the effect of their geometry on this vortex rings/porous wall interaction. In upstream flow, the strength of the wall shear layer induced by the vortex rings decreased as dh∗ increased, and so did the radial spreading of the primary vortex ring. In downstream flow, with an increase in dh∗, the coherence of the transmitted vortex rings gradually weakened owing to insufficient vorticity cancellation and enhanced Kelvin-Helmholtz (K-H) instability. For dh∗ = 0.20 (the largest hole diameter), the transmitted vortex rings downstream were the most disrupted and lost coherence quickly to survive the shortest axial distance. The results of velocity triple decomposition show that in the case of the impact of a turbulent synthetic jet on a porous wall, the wall can effectively reduce the ratio of fluctuation energy to total flow kinetic energy. Because of the low contribution of fluctuation energy to the total kinetic energy, the porous wall with the largest hole (dh∗ = 0.20) exhibited the best quality of the downstream flow field. Therefore, for flow control, we propose using a porous wall with a hole of a large diameter to effectively disrupt vortical structures and achieve a better quality of flow under the given conditions.