Transport by wind of granular materials of different grain shapes and densities

Aeolian transport rates were measured for three sands: a quartz sand (relative density 2.68, sphericity 0.73), a shelly sand (relative density 2.64, sphericity 0.59, carbonate content 67%), and granular aluminium oxide (relative density 3.95, sphericity 0.67). Low sphericity depresses the transport rate, particularly at high wind speeds; high density also does so but the effect is more marked at low transporting wind speeds. The sand of low sphericity undergoes longer saltations than the other materials, but is dislodged less frequently than they are and is transported less freely in a given wind. Of the more spherical materials, the denser was the least effective at extracting energy from the wind for grain transport in the range of our experiments. Both of the more spherical materials showed evidence of a fairly sudden transition of transport behaviour at modest wind speed. It is inferred that this marks the transfer of the function of grain dislodgement from direct wind action to inter-granular collision. The transition did not occur in experiments on the material of lower sphericity. For a given transport rate, wind speed near the bed is highest for the grains of low sphericity (by a considerable margin) and lowest for the more compact quartz grains. Propensity for transport by wind is greatest for the quartz sand, less so for the heavy material and least for the material of low sphericity.