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Shear stress partitioning in aeolian saltation -- (undated)
Keld R. Rasmussen, (Aarhus) geolkrr@geo.au.dk
In the saltation layer moving grains carry part of the momentum while in the flow above the horizontal momentum is carried entirely by the fluid. Thus the saltation process leads to partitioning of the shear stress between the wind flow and the saltating particles which results in an increase in the horizontal wind speed u (expressed as a deviation from the logarithmic wind law). The influence is serious because this speed-up will create a convexity in the near bed velocity profile and bias estimation of the bed shear stress from velocity profile data. The present study investigates the influence from particle size and mass transport on shear stress partitioning. It also investigates the dynamic properties of the saltating particles in terms of their number and horizontal speed. Experiments have been made in a 20 m long aeolian wind tunnel where quasi-steady state saltation takes place in the working section approximately 15 m downstream from the entry. For small particles vertical profiles of the horizontal air flow velocity can be measured using a robust split-fiber sensor which records the (2-D) velocity vector so that its horizontal (u) and vertical (w) components can be deduced. Simultaneously the average particle speed can be measured using laser Doppler anemometry. For 125 ým particles data show that the average forward particle speed is less than the corresponding flow velocity up to about 35 mm height while for the 320 ým the average forward particle speed is less than the flow velocity up to about 35 mm height. Deviations in u from the log-normal profile are noted below 20 mm height and at 10 mm the increase in flow velocity is approximately 15% as compared to the logarithmic profile. Below that measurements with the split-fibre sensor are impossible because of rapid erosion of its coating which changes the calibration function. For larger particles, data for particle flux and velocity show that there are systematic changes in the velocity distribution with height. Thus the average particle speed gradually approaches that of the air flow and the variation in speed decreases.