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

Abstract

An experimental study is reported which investigates the head-on collision of a laminar vortex ring of diameter D (Γ = 3000) on a fakir-like surface composed of circular posts of height h/D=0.068 located on a planar bed. 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 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.

Sediment erosion in flexible canopies by vortex impact

“…Stronger transport is observed along the preferential pathways where the filament bends less than in the directions of higher canopy resistance. Note that the impacting vortex ring generates a pattern of six radial jets which are aligned with the preferential pathways in the hexagonal pattern, as shown in our previous study (Li and Bruecker 2018).…”

“…As a striking feature of our experiment, the crater is not perfect circular but shows a hexagonal pattern of the crest, orientated in the same way as the hexagonal pattern of the canopy (see (Li and Bruecker 2018)). The local maxima along the crest of the crater are aligned with the orientation of the preferential pathways in the canopy.…”

“…The contours of the displacement field are not perfect axisymmetric but show a footprint of the hexagonal structure of the canopy. Previous studies in our lab haven proven that the vortex ring deforms into a hexagonal pattern after impact due to a lock-in effect of the azimuthal instability with the hexagonal grid (Li and Bruecker 2018). Interestingly, the peak values of Qr occur not along the preferential pathways along = 0°+ * 60° ( = 1,2,3 … ) but in between at angles of = 30°+ * 60° ( = 1,2,3 … ).…”

“…The experimental setup is shown in Fig. 1 following our previous study on the vortex interaction with a rough wall (Li and Bruecker 2018). A piston-cylinder nozzle (nozzle outlet diameter 30mm) is installed at the top of the liquid tank to generate the vortex ring with a radius of = 2 ⁄ = 22 mm at an initial travelling speed of 0 = 400 mm/s.…”

A large-eddy simulation study on vortex-ring collisions upon round cylinders

“…Their results show that the aperture-to-ring radius ratio plays a key role in determining the behaviour of the collision process, especially with respect to how much of the primary vortex-ring will be blocked or if a new vortex-ring will be formed downstream of the aperture with partial blockage. In addition, Li and Bruecker (2018) investigated vortex-rings colliding with surfaces with protruding slender circular posts and observed flow transitional differences between random and hexagonal post arrangements. Posts arranged randomly did not have appreciable coherent impact upon the vortex-ring after the collision, while the hexagonally arranged posts led to faster growths in the secondary vortex structures and eventual formation of six distinct lobes.…”