Grain transport rates in steady and unsteady turbulent airflows

  • G. R. Butterfield
Part of the Acta Mechanica Supplementum book series (ACTA MECH.SUPP., volume 1)


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, time-dependent 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.


Wind Tunnel Transport Rate Sand Transport Internal Boundary Layer Saltation Layer 
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Copyright information

© Springer-Verlag Wien 1991

Authors and Affiliations

  • G. R. Butterfield
    • 1
  1. 1.Department of Geography, Queen Mary and Westfield CollegeUniversity of LondonLondonUK

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