The saltations of different sized particles in aeolian sand transport

Abstract

Wind tunnel experiments were performed in order to investigate the effect of mixing on the aeolian transport of sands with different grain sizes. Two types of sand with different grain size distributions and an equal-mass binary mixture of these sands were used. Comparing the gradients of their measured mass flux profiles and some published profiles for mixed sand transport with those for nearly uniform sand transport, it was found that for both of these types of sand bed, the negative gradient of the mass flux profile on a log-linear plot varies with the mass averaged grain size of the sand bed according to the same power law. Hence it can be deduced that, during the aeolian transport of mixed sand beds, the mean vertical ejection speeds of different sized grains are nearly identical to that for the transport of monospecific-sized sand with the same mass averaged grain size. Theoretical analysis was undertaken to explore the characteristics of ejections of different sized grains during the aeolian transport of sand with mixed-size grains. It is proposed that the mean ejection angles and the mean ejection speeds for sand grains of different sizes are nearly identical and are equivalent to that for monospecific-sized sand with the same mass averaged grain size. It was also evident that the ratio between the transport rate of each grain size group expressed as the fraction in the whole transport rate and its mass fraction in the mixed-size sand is a combined consequence of both the wind effect on the mean saltation distance of different sized grains and the effect of the mass fraction of each grain size group in the original mixture on its ejection potential responding to an impact of a saltating sand grain.