One‐dimensional transitional behaviour in saltation

Abstract

One-dimensional simulations of the unsteady saltation process show that the transport rate's response depends on the amplitude and frequency of the wind fluctuations. At frequencies higher than f % 0Á5 Hz the transport rate was found not to respond to the wind changes. The initial overshoot reported by previous investigators was found not to appear for simulation heights smaller than 50 to 60 cm. This is due to the fast propagation of the grains' influence upward in the flow and the immediate deceleration of the wind. Confirmation of these findings comes from reports of experiments conducted in wind tunnels of different sizes.Further test calculations show that the discretization time step size Át has an influence on the model's temporal behaviour. The reason for this is the better coupling of the wind±sand system when a smaller Át is used. The implications of bed area choice on the statistical accuracy of predicted transport rate is also demonstrated. In the one-dimensional case the grain cloud's total forward momentum equals transport rate, which is independent of model geometry.

Wind tunnel experimental investigation of sand velocity in aeolian sand transport

“…In order to explore the fundamental internal mechanism in aeolian sand transport, many studies have been done using theoretical models and experiments in both wind tunnel and field settings (e.g., Bagnold, 1941;Owen, 1964;Willetts and Rice, 1986;Ungar and Haff, 1987;Anderson andHaff, 1988, 1991;Willetts, 1991, 1993;Greeley et al, 1996;Spies et al, 2000;Zou et al, 2001;Ni et al, 2002;Namikas, 2003;Dong et al, 2004a;McKenna Neuman, 2004;Cornelis et al, 2004;Bauer et al, 2004;Wiggs et al, 2004). The movement of grains is adapted to the near-surface wind velocity profile by a complex momentum exchange, and the trajectories of saltating grains are also dependent on many complex impact factors, such as sand size and shape, wind strength and frequency (Spies et al, 2000), sand bed topography, and environmental temperature and humidity (McKenna Neuman, 2004;Wiggs et al, 2004;Davidson-Arnott et al, 2005).…”

“…The movement of grains is adapted to the near-surface wind velocity profile by a complex momentum exchange, and the trajectories of saltating grains are also dependent on many complex impact factors, such as sand size and shape, wind strength and frequency (Spies et al, 2000), sand bed topography, and environmental temperature and humidity (McKenna Neuman, 2004;Wiggs et al, 2004;Davidson-Arnott et al, 2005). The complexity of the saltation problem is apparent.…”

Timescale dependence of aeolian sand flux observations under atmospheric turbulence

“…McEwan and Willetts [] noted that adjustments in the velocity profile persist for up to 40 s and that the velocity profile deviates from a logarithmic form during transient adjustments. At shorter times, there are also lags for saltation flux to respond to near‐surface winds [e.g., Anderson and Haff , ; Butterfield , ; Spies et al , ]. In addition, presence of separate aerodynamic and collision thresholds introduces path dependence in prediction of overall sediment flux [ Rasmussen and Sorensen , ; Kok , ], even without the complicating effect of local changes in τ c related to surface sediment moisture and cohesiveness variations [e.g., Gillette et al , ; Davidson‐Arnott and Bauer , ].…”

Formation and behavior of aeolian streamers

“…[3] Temporal variability in aeolian transport has received considerable attention in recent years, both with respect to saltation response to rapid fluctuations in airflow forcing -primarily investigated in wind tunnels [Shao and Raupach, 1992;Butterfield, 1998;Spies et al, 2000], by numerical simulations [Anderson and Haff, 1988;McEwan and Willetts, 1991] and in terms of transport intermittency under natural winds [Lee, 1987;Stout and Zobeck, 1997;Bauer et al, 1998]. Such studies suggest that the temporal limit of sand transport response to wind speed fluctuations is on the order of 1 Hz.…”