Saltation on Earth and extraterrestrial atmospheres
Abstract
We present an approximate analytical analysis of particle periodic motion over granular beds. The analysis is based on the calculation of approximate solutions for average periodic trajectories of particles that are accelerated by the turbulent shearing of a fluid, between collisions with the bed. We focus on the case in which the mean fluid motion is strong enough to sustain the saltation of the particles, as continuing rather than intermittent. From these solutions, we determine the relations between the horizontal particle flux, the strength of the shearing flow and the particle take-off velocity over a range of the grain-to-fluid mass density ratios that vary between those for sand in air and sand in water, in saltation over rigid bumpy and erodible beds. For saltation over rigid bumpy beds, we predict that there is range of particle flux that the fluid can sustain at a given Shields parameter, irrespective of the density ratio. That range presents a maximum, which corresponds to the maximum transport capacity of the flow, before particles begin to be deposited. To our knowledge, these findings have been previously demonstrated in the case of aeolian transport only. For saltation over erodible beds, there is only one horizontal particle flux associated with a given Shields parameter. The analytical solution indicates that this flux scales linearly with the Shields parameter in aeolian transport, while it is roughly proportional to the Shields number to the power of 3/2 near aquatic transport. These predictions are in agreement with the scaling laws drawn from sand transport experiments in air and water. We also highlight that saltation regimes in air and water are different in nature: aeolian saltation is limited by the splash, while the aquatic saltation is not. Interestingly, for intermediate values of the density ratio (typically between 30 and a few hundred), we find a crossover regime: the particle flux is proportional to S3/2 close to the threshold and linear in S at larger values of the Shields parameter. This crossover regime is expected to be relevant for sediment transport in extraterrestrial atmospheres such as on Venus and Titan, where the density ratios are approximately 40 and 200, respectively.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFMEP23D..02V
- Keywords:
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- 1847 Modeling;
- HYDROLOGYDE: 1862 Sediment transport;
- HYDROLOGYDE: 4430 Complex systems;
- NONLINEAR GEOPHYSICS