Relaxation processes in Aeolian transport

Houssem Selmani,Pascal Dupont,Ahmed Ould El Moctar,Rabah Zegadi,Alexandre Valance,F Radjai,S Luding,J.Y Delenne,S Nezamabadi

doi:10.1051/epjconf/201714003055

Abstract

We investigate experimentally the relaxation process toward the equilibrium regime of saltation transport in the context of spatial inhomogeneous conditions. The relaxation length associated to this process is an important length in aeolian transport. This length stands for the distance needed for the particle flux to adapt to a change in flow conditions or in the boundary conditions at the bed. Predicting the value of this length under given conditions of transport remains an open and important issue. We conducted wind tunnel experiments to document the influence of the upstream particle flux and wind speed on the relaxation process toward the saturated transport state. In the absence of upstream particle flux, data show that the relaxation length is independent of the wind strength (except close to the threshold of transport). In contrast, in the case of a finite upstream flux, the relaxation length exhibits a clear increase with increasing air flow velocity. Moreover, in the latter the relaxation is clearly non-monotonic and presents an overshoot.

Highlights

Saltation is known as the primary mode of sediment transport in air and characterizes the movement of the particles jumping along the sand surface in ballistic trajectories.[1,2,3,4]
We investigate experimentally the relaxation process toward the equilibrium regime of saltation transport in the context of spatial inhomogeneous conditions
We carried out wind tunnel experiments to document the relaxation process towards the equilibrium regime of aeolian sand transport in the context of spatial inhomogeneous configuration by varying the upstream particle flux and wind speed

Summary

Introduction

Saltation is known as the primary mode of sediment transport in air and characterizes the movement of the particles jumping along the sand surface in ballistic trajectories.[1,2,3,4]. When a steady wind blows over a flat erodible bed, an equilibrium between the flow and the transported particles is achieved and the mass flux, Q, reaches a saturated value Q = Qsat. If we consider an unsteady or inhomogeneous situation, in which the wind velocity is subjected to a temporal or spatial change, the relaxation process towards the new saturated state takes a certain characteristic time or length [5, 6]. By linearizing the problem around the saturated state, one can describe the relaxation process by a first order differential which takes the following form for spatial inhomogeneous situations: ∂Q ∂x = −Q − Qsat Lsat (1). Where Lsat is the characteristic length scale of the relaxation process and Qsat is the saturated state corresponding to the new equilibrium transport regime. A similar first order differential equation can be written for unsteady situations where the space variable is replaced by the time and the saturation length by the saturation time τsat

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Conclusion