How do neighbors affect incipient particle motion in laminar shear flow?

Abstract

We experimentally study how neighboring particles affect the incipient motion of particles on regular substrates and exposed to a laminar shear flow. To this end, we determine the critical Shields number and determine whether the particle rolls or slides. The substrates consist of a monolayer of fixed spheres of uniform size that are regularly arranged in triangular and quadratic configurations. Neighboring particles influence the incipient motion by shielding to the shear flow and may inhibit continuous motion once they are in direct contact with the particle. At the low particle Reynolds numbers studied, neighboring spheres on the monolayer only affect the incipient particle motion if they are closer than about 3 particle diameters. Direct contact inhibits continuous motion and results in a strong increase of the critical Shields number. For identical beads, we found two different regimes for the onset of continuous motion. Depending on the substrate geometry, the upstream particle may start to roll like a single particle passing the downstream neighbor or it may push its downstream neighbor forward. In the latter case, the downstream sphere rolls while the upstream bead slides in contact with the downstream neighbor. Both regimes yield about the same critical Shields number although the critical Shields number for single particle motion differs by about 50%. If particle contact is avoided by a sudden jump in the Shields number, the critical Shields number for onset of continuous particle motion can be reduced considerably. Finally, the lowest critical Shields numbers for dislodging buried beads in the configurations studied coincides with the critical Shields number for incipient motion of irregular granular beds.

The Cessation Threshold of Nonsuspended Sediment Transport Across Aeolian and Fluvial Environments

“…The former criteria are typically used for turbulent fluvial flows (Buffington & Montgomery, 1997;Miller et al, 1977;Paphitis, 2001, and references therein), whereas the latter criteria are preferred for laminar fluvial (Govers, 1987;Yalin & Karahan, 1979) and turbulent aeolian flows (Bagnold, 1936(Bagnold, , 1937(Bagnold, , 1941Burr et al, 2015;Carneiro et al, 2015;Chepil, 1945;Dong et al, 2003;Greeley et al, 1976;Iversen et al, 1976;Iversen & Rasmussen, 1994;Iversen & White, 1982;Lyles & Krauss, 1971;Webb et al, 2016) because bulk transport can be easily visually identified in these environments: by the formation of grain carpets in laminar fluvial flows and by very large particle hops in turbulent aeolian flows. A further situation sometimes studied in the laboratory is the beginning motion of a single particle on top of a prearranged substrate (Agudo et al, 2014(Agudo et al, , 2017(Agudo et al, , 2018Agudo & Wierschem, 2012;Celik et al, 2010;Charru et al, 2007;Deskos & Diplas, 2018;Diplas et al, 2008;Fenton & Abbott, 1977;Kudrolli et al, 2016;Valyrakis et al, 2010Valyrakis et al, , 2011Valyrakis et al, , 2013. Moreover, it is worth highlighting that Salevan et al (2017) proposed a fundamentally different criterion for the onset of significant motion based on analyzing the particle velocity distribution of all particles, including those nearly static ones that belong to the bed surface.…”

The Horizontal and Vertical Characteristics of Aeolian Dust from Riverbed

“…On the other hand, smaller particle might be hidden by the larger particle at the same flat that obstructed the fine particle resuspension. It was called the shielding effect of particle motion (Agudo et al, 2014;Wang and Arson, 2016). Therefore, the fraction of fine particle suspension was lower than coarse particle during the slower wind speed condition.…”

Threshold of Motion Conditions Under Stokes Flow Regime and Comparison With Turbulent Flow Data

“…Table includes a list of representative experimental studies that focused on sediment transport phenomena occurring at the lower end of laminar flow conditions (Agudo et al, ; Agudo & Wierschem, ; Charru et al, ; Charru et al, ; Hong et al, ; Lobkovsky et al, ; Loiseleux et al, ; Ouriemi et al, ). In this type of experiments, highly viscous fluids together with very fine, yet noncohesive, sediment particles are typically used to achieve very small Re P values.…”