Vortex interaction with a rough wall formed by a hexagonal lattice of posts

An experimental study is reported which investigates the head-on collision of a laminar vortex ring of diameter D (ReΓ= 3000) on a fakir-like surface composed of slender circular posts protruding out of a planar layer. Lattices of the posts in hexagonal and random distribution (average porosity of ϵ = 0.94 in the layer) are compared to each other with respect to the plain wall. Prior to impact, the vortex ring develops the early state of natural azimuthal instabilities of different mode numbers N = 5-7 competing with each other. While impacting with the wall, the hexagonal lattice causes the rapid growth of secondary vortex structures in a regular mode number N = 6 arrangement at the outer edge of the primary ring in the form of six lobes which are aligned with the orientations of preferential “pathways” in the lattice. At the outer tip of the lobes, radial wall-jets are generated. Rotating the layer with the hexagonal lattice results in the same rotation of the secondary flow pattern with the jets’ orientation lock-in with the orientation of the lattice. The layer with random distribution of the posts at the same number density is not able to repeat this observation and no regular secondary flow pattern is seen. The results show that a tailored arrangement of such posts can be used for near-wall flow control such as in impacting jet flows or in boundary layer flows when instability modes in the flow lock-in with the wall pattern consisting of preferred pathways in the posts’ layer.