Models for airflow velocity profiles in natural settings: Accounting for atmospheric conditions and secondary flow over eolian dunes.

摘要

The link between airflow and eolian sand transport has been the subject of continuous research since Bagnold published The physics of blown sand and desert dunes (1941). While he made significant strides in quantifying eolian sand transport under the controlled conditions of a wind tunnel, applying his work to natural settings has proved problematic for most subsequent research for three reasons; (1) atmospheric conditions, (2) sloping surfaces, and (3) secondary airflow on the lee of dunes.; In order to address the effects of atmospheric convection, airflow and temperature data were collected over a flat substrate at White Sands National Monument in New Mexico. The data reveals that during the day, when solar insolation is heating the surface, near-surface atmospheric conditions become unstable thereby enhancing convection and vertical mixing resulting in decreasing shear velocity with height. At night, the near-surface atmospheric conditions are stable thereby reducing convection and vertical mixing, resulting in stratified airflow and increased shear velocity with height. Unless this atmospheric effect is accounted for, estimates of sediment transport rates may be off by as much as a factor of 15 times when wind speeds are near threshold velocity.; On the sloping stoss side of a sand dune airflow is accelerated causing compression of flow streamlines. This compression necessitates an increase in shear velocity up the stoss slope. However, measurements over 14 dunes shows that compression occurs very close to the surface and, as a consequence, in the overlying flow where measurements are typically made, an overall decrease in shear stress occurs up the slope. Airflow measurements taken more than a few centimeters above the surface will underestimate sand transport rates.; Airflow patterns downwind of a dune with transverse separated flow, consist of a back-flow eddy that extends about four dune heights downwind from the brink of the dune. Beyond the back-flow eddy the vertical velocity profiles can be divided into four vertical regions based upon segments separated by 'kinks' in the velocity profiles, from top down: (1) the interior, a low shear region above the dune; (2) the transition zone with high shear stress; (3) the lower wake with low shear and low wind speeds; and (4) the internal boundary layer of high shear flow, never exceeding a few tens of centimeters in height. This region is perhaps the most important because it controls the near-surface shear stress that drives sand transport.