  An experimental study is reported which investigates the head-on collision of
a laminar vortex ring of diameter D (Re{\Gamma}= 3000) on a fakir-like surface
composed of circular posts of height h*=0.068 located on a planar bed. Lattices
of the posts in hexagonal and random distribution (average porosity of e = 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 rough wall, a secondary ring is observed which is pushed
outwards and is not wrapped around the primary ring as in flat wall impact.
Between both rings of opposite sign vorticity, a strong fluid rebound is
induced. The hexagonal lattice causes the rapid growth of further secondary
vortex structures in a regular mode number N=6 arrangement at the outer edge of
the primary ring in form of six lobes which are aligned with the orientations
of preferential pathways in the layer. At the outer tip of the lobes radial
wall-jets are generated. Rotating the fakir geometry around the centre of
impact also rotates the jets location and direction accordingly. A surface with
random lattice of the posts at the same average number density is not able to
repeat this observation and no regular secondary flow pattern is visible until
full breakdown of the ring. The results show that a tailored arrangement of
such posts can be used for near-wall flow control when patterns of preferred
pathways in the posts layer lock-on with existing instability modes such as in
impacting jet flows or in turbulent boundary layer flows.
