Rotating-disk-type flow over loose boundaries

Abstract

Rotating-disk-type flow of a liquid over a loose boundary, such as a layer of sand, is investigated. For this flow the formation of a new large-scale spiral pattern has been discovered. The new pattern is reminiscent of the Type-I spiral-vortex structures which characterize the laminar-turbulent transition region of boundary layers over rigid rotating disks. Flow visualizations reveal that the new pattern and the Type-I spiral vortices co-exist in the loose-boundary flow. The research investigating the origin of the new largescale pattern is reviewed. Then photographs from flow visualizations are analysed to obtain estimates for the critical Reynolds number for which Type-I spiral vortices first appear for the loose-boundary flow and for the critical Reynolds numbers for the laminar-turbulent transition of the boundary layer. The results suggest that Type-I vortices appear at much lower Reynolds numbers over loose boundaries in comparison with flow over rigid rotating disks and that transition also appears to be advanced to much lower Reynolds numbers. The discussion of the results suggests that advanced transition arises from disturbances introduced into the flow after the loose boundary has been mobilized and not from disturbances associated with the roughness that the surfaces of the granular layer represents to the flow while grains are at rest.

Morphodynamics of a sediment bed in a fluid-filled cylinder during spin-down: An experimental study

“…In contrast, experiments with Ro −1 = 0 show significant particle suspension and motion is observed to be more turbulent, leading to the formation of patterns composed of a number of spirals with small features (such as ripples). These patterns are reminiscent of previous experiments performed in granular beds [28][29][30], where it is discussed that type I and type II instabilities (as described by Faller [44] and by Savas [40]) play a role in the formation of patterns. This is also consistent with the lack of smaller features as the Ro −1 value increases since instabilities are suppressed with the presence of background rotation.…”

“…These boundary layers and intermediate cases are three-dimensional prototypical flows [22] used as models for turbomachinery [23] and geophysical swirling flows [24][25][26][27]. * m.duran.matute@tue.nl Previous studies on the interaction between a spin-up or spin-down flow and a granular bottom inside of a cylinder [28][29][30] have observed the emergence of spiral patterns on the sediment bed. In particular, these studies focus on the number of ripples or spiral arms that form on the azimuthal angle of the spiral arms and on the generating mechanism.…”

Micrometric Granular Ripple Patterns in a Capillary Tube

“…In these contexts observations of pattern-formation phenomena in microfluidic two-phase flows have been described [3][4][5][6]. We report a new such pattern that appears to be closely related to the well-known type of macroscopic sand-ripples [7][8][9][10][11] one finds on beaches or on the bottom of the ocean and rivers.…”

“…The pattern wavelength is nondimensionalized as (=d G ) and a mobility parametersimilar to a Shields parameter -is defined as (compare Ref. [12]; see also [10,11,13,14])…”

Instabilities of flows due to rotating disks: preface

“…Thomas and Zoueshtiagh [20] review their earlier results where they showed that the presence of a sandy bed produces a new type of spiral pattern. New visualizations are also presented showing that these new patterns are in addition to, and co-exist with, the one discovered by Gregory, Stuart and Walker.…”