Grain scale simulations of loose sedimentary beds: the example of grain-bed impacts in aeolian saltation |
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| Authors: | PETER K HAFF ROBERT S ANDERSON† |
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| Institution: | Department of Civil and Environmental Engineering, Department of Geology, Duke University, Durham, North Carolina 27708–0287, USA;Department of Earth Sciences, University of California, Santa Cruz, California 95064, USA |
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| Abstract: | Sediment transport by wind is one of many processes of interest to the geomorphologist in which grain to grain contacts play an important role. In order to illustrate the modelling of collections of frictional, inelastic sedimentary grains with the particle dynamics method (PDM), we use the grain impact process in aeolian saltation as a specific example. In PDM, all the forces on each particle are evaluated at a sequence of small time-steps, and the Newtonian equations of motion are integrated forward in time. Interparticle forces at grain contacts are treated as springs with prescribed stiffness (normal force) and by a Coulomb friction law (tangential force); particle inelasticity is represented by spring damping. The granular splash resulting from saltation impacts is assessed for sensitivity to the choice of grain properties, and the integration time-step. We find that for the range of impact speeds and impactor masses relevant to aeolian settings, grain splashes are relatively insensitive to grain stiffness, grain inelasticity and grain friction, and that the pattern of ejection from the bed is largely controlled by bed microtopography. A large set of impact realizations involving a variety of impact points on a small set of target beds is used to collect the appropriate statistics for describing the stochastic splash process. The splash function representing these statistics is then available for use in calculations over longer time-scales, such as the evolution of the saltation curtain. The details given here will enable the interested reader to adapt PDM modelling to other types of clastic sedimentary systems. |
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