Vortex rings impinging on permeable boundaries

Abstract

Experiments with vortex rings impinging permeable and solid boundaries are presented in order to investigate the influence of permeability. Utilizing Particle Image Velocimetry, we compared the behaviour of a vortex ring impinging four different reticulated foams (with permeability k ~ 26 - 85 × 10-8 m2) and a solid boundary. Results show how permeability affects the stretching phenomena of the vortex ring and the formation and evolution of the secondary vortex ring with opposite sign. Moreover, permeability also affects the macroscopic no-slip boundary condition found on the solid boundary, turning it into an apparent slip boundary condition for the most permeable boundary. The apparent slip-boundary condition and the flux exchange between the ambient fluid and the foam are jointly responsible for both the modified formation of the secondary vortex and changes on the vortex ring diameter increase.Peer ReviewedPostprint (published version

Quantitative visualization of vortex ring structure during wall impingement subject to background rotation

“…Yamada et al (1982) and Walker et al (1987) experimentally observed fingering of vortex ring after the impingement. Interaction with other type of solid objects was reported by Naaktgeboren et al (2012), Cheng et al (2014), andMujal-Colilles et al (2015) for permeable plates. Suzuki and Kumagai et al (2007), Masuda et al (2012), Bethke et al (2012), and Misaka (2012) studied particulate layers transported by a vortex ring.…”

Direct effects of boundary permeability on turbulent flows: observations from an experimental study using zero-mean-shear turbulence

“…Consequently, reticulated polyether foams have also been used in previous studies investigating permeable boundaries (see, for example, Manes et al. 2011; Mujal-Colilles, Dalziel & Bateman 2015). The absolute permeability, , of each foam used is shown in table 1, which was determined using a constant head permeameter test (British Standards Institution 2010) in which the permeability was determined from measurements of the pressure drop across a sample at a given (constant) volume flow rate.…”

“…The geometry of these permeable media are ideal for studying the effects of permeability on wall turbulence as the high porosity and small filament thickness of the foams minimises the influence of the roughness of the interface between the porous layer and the fluid above. Consequently, reticulated polyether foams have also been used in previous studies investigating permeable boundaries (see, for example, Manes et al 2011;Mujal-Colilles, Dalziel & Bateman 2015). The absolute permeability, K, of each foam used is shown in table 1, which was determined using a constant head permeameter test (British Standards Institution 2010) in which the permeability was determined from measurements of the pressure drop across a sample at a given (constant) volume flow rate.…”

“…That is, studies investigating the interaction of a vortex ring with solid impermeable boundaries report a rebound of the vortex ring away from the boundary (see for example Walker et al 1987;Orlandi & Verzicco 1993;Munro, Bethke & Dalziel 2009;Munro 2012) which appears similar, in concept, to the antisplat self-generation mechanism. However, studies investigating the interaction of vortex rings with permeable boundaries indicate that vortex ring rebound is inhibited as boundary permeability increases (Mujal-Colilles et al 2015); the relaxation of the kinematic blocking condition facilitates penetration of the flow into the boundary and a reduction in instantaneous shear such that the vorticity that acts to eject fluid away from the boundary is weaker. The results presented here are consistent with these observations; we have observed a relaxation of the kinematic blocking effect of the boundary ( § 3) that reduces the strain rate of fluid elements as they impinge onto the boundary ( § 4.1).…”

“…However, studies investigating the interaction of vortex rings with permeable boundaries indicate that vortex ring rebound is inhibited as boundary permeability increases (Mujal-Colilles et al. 2015); the relaxation of the kinematic blocking condition facilitates penetration of the flow into the boundary and a reduction in instantaneous shear such that the vorticity that acts to eject fluid away from the boundary is weaker. The results presented here are consistent with these observations; we have observed a relaxation of the kinematic blocking effect of the boundary (§ 3) that reduces the strain rate of fluid elements as they impinge onto the boundary (§ 4.1).…”