Sediment resuspension and erosion by vortex rings

Abstract

Particle resuspension and erosion induced by a vortex ring interacting with a sediment layer was investigated experimentally using flow visualization (particle image velocimetry), high-speed video, and a recently developed light attenuation method for measuring displacements in bed level. Near-spherical sediment particles were used throughout with relative densities of 1.2–7 and diameters (d) ranging between 90 and 1600 μm. Attention was focused on initially smooth, horizontal bedforms with the vortex ring aligned to approach the bed vertically. Interaction characteristics were investigated in terms of the dimensionless Shields parameter, defined using the vortex-ring propagation speed. The critical conditions for resuspension (whereby particles are only just resuspended) were determined as a function of particle Reynolds number (based on the particle settling velocity and d). The effects of viscous damping were found to be significant for d/δ<15, where δ denotes the viscous sublayer thickness. Measurements of bed deformation were obtained during the interaction period, for a range of impact conditions. The (azimuthal) mean crater profile is shown to be generally self-similar during the interaction period, except for the most energetic impacts and larger sediment types. Loss of similarity occurs when the local bed slope approaches the repose limit, leading to collapse. Erosion, deposition, and resuspension volumes are analyzed as a function interaction time, impact condition, and sediment size.

The interaction of a vortex ring with a sloped sediment layer: Critical criteria for incipient grain motion

“…With this factor eliminated, experimental data exhibit clear agreement with the increased-mobility trend predicted by the theory. 19,20 This paper extends the work of Munro et al 10 and describes experiments to investigate how the flow-field of an isolated vortex ring, made to interact with a sediment layer, gives rise to incipient sediment motion. The vortex ring is a simple coherent vortex structure, which can be easily and consistently produced in the laboratory and has been used in a range of previous studies as an idealised model for turbulent entrainment.…”

“…10 Over this short distance, very little ambient, irrotational fluid is entrained within the ring structure (by diffusion). 21,24 Hence, prior to being affected by the presence of the sediment layer, the ring diameter (D) and axial propagation speed (U) were essentially constant.…”

“…With this system, the ring diameter D and, most importantly, the propagation speed U were controlled by adjusting the pump stroke time. 10,25 [For the experiments reported here, the measured stroke times were within the range 0.1-0.9 s.] For a given sediment type and bed slope (β), the critical conditions for incipient motion were determined by repeating an experiment, each time systematically increasing the pump stroke time (thereby decreasing U) until grain motion was only just observed to occur, with any further increase in stroke time resulting in no motion. During this procedure, the sediment layer was re-scraped and made flat prior to each repeat.…”

“…1). 10 For the experiments described here, a fixed stroke length of 20 cm was used throughout, which produced a fluid slug of length L 0 = 9.5 cm (i.e., L 0 /D 0 = 2.4). The pump actuation rate was controlled by attaching the pump handle to a motor-driven spool by a length of nylon cord (see Fig.…”

“…9 Recent studies have attempted to address some of these issues by analysing the role played by (unsteady) coherent vortex structures in the entrainment and transport of sediments. For example, Munro et al 10 studied sediment erosion induced by an isolated vortex ring interacting with a layer of artificial (transparent) sediment. A light attenuation 11 technique was used to obtain detailed time-resolved measurements of sediment erosion levels during the interaction period, and over the entire impact region.…”