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 242 μ m or 320 μ m. 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 modeled using a numerical saltation model, and predicted grain speeds agree fairly well with experimental results obtained from laser‐Doppler anemometry. The use of laser‐Doppler anemometry to study aeolian saltation is thoroughly discussed and some pitfalls are identified. At 80 mm height the ratio between air speed and grain speed is about 1.1 and from there it increases toward the bed so that at 5 mm it is about 2.0. All grain speed profiles converge toward a common value of about 1 m/s at 2−3 mm height. Moreover, the estimated launch velocity distributions depend only very weakly on the friction speed in contrast to what has often been assumed in the literature. Flux density profiles measured with a laser‐Doppler appear to be similar to most other density profiles measured with vertical array compartment traps; that is, two exponential segments will fit data between heights from a few millimeters to 100−200 mm. The experimental flux density profiles are found to agree well with model predictions. Generally, validation rates are low from 30 to 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.