Grain transport rates in steady and unsteady turbulent airflows

Abstract

Summary. Wind tunnel and field experiments are reported in which continuous, synchronous measurements of grain transport rates and near-bed velocity profiles were made at one second intervals to assess mass-flux response to velocity variations. Resulting grain flux and velocity series demonstrate the variability concealed by conventional time-averaged data. In steady tunnel winds, timedependent mass transport rates are found to correlate better with fluctuations in mean velocity near the top of the saltation layer than with estimates of instantaneous shear stress. Quasi-periodic oscillation (20-30 seconds) of near-bed mass-flux and flow velocity in the lower regions of the inner boundary layer is evident in such airflows as the saltation system moves towards equilibrium with a developing bed form and confined boundary layer. This phenomenon may not occur in nature at these time-scales, however.In systematically unsteady airflows, the time constant between flux rate and velocity near the top of the saltation layer is shown to be of order one second, tentatively confirming Anderson and Haff's [2] calculations of saltation response time. Mass-flux also correlates well with large time-dependent variations in velocity in this region. Without grain replenishment, progressive surface re-sorting induces non-stationarity in grain flux under all observed flow regimes. Mass-flux and velocity histories measured on dunes show no correspondence. This difference is attributed to the stochastic nature of three-dimensional turbulence, the larger integral scales of atmospheric flows, measurement noise, and the effects of flow non-uniformity on satisfactory definition of shear velocity. Unsteady velocity profiles over a transverse dune are shown to be non-logarithmic above 20 cm but log-linear velocity segments of variable extent are found within the upper saltation layer.

Rates of aeolian transport on a beach in a temperate humid climate

“…According to Butterfield (1991) rates of transport respond almost instantaneously (1-2 s) to velocity accelerations. Butterfield states that the grain flux under steady wind speeds are more variable than might have been expected from time-average values.…”

Vegetation in Drylands: Effects on Wind Flow and Aeolian Sediment Transport

“…This is particularly problematic in the field, where the dynamic complexity of the flow structures cannot be controlled in the same way as in modelled or wind tunnel environments [104]. This issue is pertinent given the evidence that turbulence is an important driving force behind sediment entrainment and transport in aeolian environments [105][106][107][108][109][110][111][112][113][114].…”

Turbulent characteristics of saltation and uncertainty of saltation model parameters

“…The most well known is the Owen (Owen, 1964) saltation model, which predicts that the vertically integrated saltation flux is proportional to u * cubed, where u * is friction velocity, defined as u * = √ τ/ρ with τ being surface shear stress (N m −2 ) and ρ being air density (kg m −3 ). A dedicated investigation on turbulent saltation was conducted by Butterfield (1991), which revealed the significant variability of saltation fluxes concealed in conventional time-averaged data. Stout and Zobeck (1997) introduced the idea of saltation intermittency and pointed out that even when the averaged u * is below the threshold friction velocity, u * t , saltation can still intermittently occur.…”