| 研究概要 |
In numerical simulations such as quasi-DNS ("qDNS") that track individual particles in liquid, it is necessary to model the interparticle contact forces, notably the viscous resistance. However there remain fundamental gaps in physical understanding of this phenomenon that retard further progress of qDNS. We have further improved our earlier models in which the deformation of the particle near the contact point is characterized by just two numbers, deformation on the axis, and equivalent contact width. On the experimental side of this issue, we used a high-speed video camera to record impacts of spherical particles onto target plates with carefully controlled roughness in the range of 0.2 to 25 microns, which had not been fully investigated previously. Roughened elastic targets, made of glass, yielded stronger rebound velocity that increased with roughness, in accord with the work of Barnocky and Davis. This contrasts with stainless steel targets, for which roughness did not yield stronger rebound, pointing out the importance of energy dissipation by plastic asperity deformation.
An important phase of sediment transport occurs during overland sheet flow, when the gap between the free surface and the particle may be on the order of particle diameter. In this case standard tools for estimating "drag", "virtual inertia", etc cannot be applied. In response, Wells has adapted earlier theoretical relationships to include a rigid particle near a free surface in viscous fluid; this will permit rigorous analysis of the complex forces at work in such a situation.
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