Aeolian Sand Transport in Out-of-Equilibrium Regimes

We investigate the relaxation process toward the equilibrium regime of saltation transport in the context of nonuniform conditions. Relaxation phenomena can be described in terms of a characteristic length scale that measures the distance for the particle flux to adapt to a spatial change in flow or boundary conditions. We conducted wind tunnel experiments to document the influence of the upwind mass flux on the relaxation process. For zero upwind mass flux conditions, the relaxation process is monotone and the relaxation length is independent of the wind strength. In contrast, for nonzero upwind flux conditions (obtained by releasing particles in the flow from a finite height), the relaxation process is nonmonotone and is well captured by damped harmonic oscillations. Importantly, the relaxation length increases with increasing air flow velocity but is almost insensitive to the magnitude of the upwind flux. Our experimental outcomes clearly indicate that the relaxation of far from equilibrium transport regimes strongly deviates from a simple exponential behavior.