The vertical variation of particle speed and flux density in aeolian saltation: measurement and modeling

Particle dynamics in aeolian saltation has been studied in a boundary layer wind tunnel above beds composed of quartz grains having diameters of either 254 μm or 320 μm. The cross section of the tunnel is 600 mm × 900 mm and its thick boundary layer allows precise estimation of the fluid friction speed. Saltation is modelled using a numerical saltation model and predicted grain speeds agree fairly well with experimental results obtained from laser-Doppler anemometry. At 80 mm height the ratio between air speed and grain speed is about 1.1 and from there it increases towards the bed so that at 5 mm it is about 2.0. All grain speed profiles converge towards a common value of about 1 m/s at 2-3 mm height. Flux density profiles, measured with a laser-Doppler appear to be similar to most other density profiles measured with vertical array compartment traps, i.e. two exponential segments will fit data between heights from a few mm to 100-200 mm. The experimental flux density profiles are found to agree well with model predictions. Generally validation rates are low from 30-50 % except at the highest level of 80 mm where they approach 80 %. When flux density profiles based on the validated data are used to estimate the total mass transport rate results are in fair agreement with measured transport rates except for conditions near threshold where as much as 50 % difference is observed.