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Eidgenössische Technische Hochschule Zürich, ETHZ (2014)

Simulation of the saltation process for aeolian sand transport

Carneiro Martins, Marcus Vinícius

Titre : Simulation of the saltation process for aeolian sand transport

Auteur : Carneiro Martins, Marcus Vinícius

Etablissement de soutenance : Eidgenössische Technische Hochschule Zürich, ETHZ

Grade : Doctor of Sciences ETH Zurich 2014

Résumé
The transport of sand by wind commonly seen in dune motion reveals a complex interaction between granular media and a fluid. In saltation, the air lifts and accelerates sand grains that, in a chain reaction, eject others as they impact back on the ground. We simulate aeolian sand transport in two and three dimensions using the Discrete Elements Method. Our model simulates the behavior of a disordered particle bed under the influence of a logarithmic wind profile. The wind dynamically changes according to the momentum extraction of the saltating grains. We study a transition in the saturated flux for aeolian saltation obtained by our numerical model. The discontinuity is followed by a coexistence interval with two metastable solutions. We analyze the metastable behavior in the presence of perturbations and lift forces. The saturated flux in 3D simulations was reasonably fitted with the model proposed by Owen, q = A0(u 3 ∗ρw/g)(1 − uτ/u∗), with A0 = 1.6 and uτ = 0.18 m/s. Experimentally, we verify in the wind tunnel the fluid and dynamic thresholds at uτ = 0.17 m/s and uD = 0.145 m/s, displaying hysteresis for low wind shear velocities. Close to the onset of Aeolian particle transport through saltation we find in wind tunnel experiments a regime of intermittent flux characterized by bursts of activity. A saltation model including the windentrainment from the turbulent fluctuations can reproduce these observations and gives insight about their origin. The duration of the bursts follows an exponential probability distribution. The time intervals between each burst decrease on average as the wind shear velocity increases until the sand flux becomes continuous. Finally, we discuss the role of mid-air collisions in Aeolian transport. We find that, surprisingly, these collisions do enhance the overall flux substantially. The effect depends strongly on restitution coefficient and wind speed. We can explain this observation as a consequence of a “soft bed” of grains which floats above the ground and reflects the highest flying particles. We make the unexpected observation that the flux is maximized at an intermediate restitution coefficient of about 0.7, which is comparable to values experimentally measured for collisions between sand grains. We also distinguish saltons, creepers and leapers in saltation.

Mots clés : AEOLIAN SEDIMENT TRANSPORT ; AEOLIAN EFFECTS ; DISCRETE DYNAMICAL SYSTEMS

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