Walker, I.J., and Nickling, W.G., 2003. Simulation and measurement of surface shear stress over isolated and closely spaced transverse dunes in a wind tunnel. Earth Surface Processes and Landforms, 28: 1111-1124.
ABSTRACT
Topographic interactions generate multidirectional and unsteady airflow that limits the application of velocity profile approaches for estimating sediment transport over duties. Results are presented from a series of wind tunnel simulations using Irwin-type surface-mounted pressure sensors to measure shear stress variability directly at the surface over both isolated and closely spaced sharp-crested model duties. Findings complement existing theories on secondary airflow effects on stoss transport dynamics and provide new information on the influence of lee-side airflow patterns on dune morphodynamics. For all speeds investigated, turbulent unsteadiness at the dune toe indicates a greater, more variable surface shear, despite a significant drop in time-averaged measurements of streamwise shear stress at this location. This effect is believed sufficient to inhibit sediment deposition at the toe and may be responsible for documented intermittency in sand transport in the toe region. On the stoss slope, streamline compression and flow acceleration cause an increase in flow steadiness and shear stress to a maximum at the crest that is double that at the toe of the isolated dune and 60-70 per cent greater than at flow reattachment on the lower stoss of closely spaced duties. Streamwise flow accelerations, rather than turbulence, have greater influence on stress generation on the stoss and this effect increases with stoss slope distance and with incident wind speed. Reversed flow within the separation cell generates significant surface shear (30-40 per cent of maximum values) for both spacings. This supports field studies that suggest reversed flow is competent enough to return sediment to the dune directly or in a deflected direction. High variability in shear at reattachment indicates impact of a turbulent shear layer that, despite low values of time-averaged streamwise stress in this region, would inhibit sediment accumulation. Downwind of reattachment, shear stress and flow steadiness increase within 6 h (h = dune height) of reattachment and approach upwind values by 25 h. A distance of at least 30 h is suggested for full boundary layer recovery, which is comparable to fluvial estimates. The Irwin sensor used in this study provides a reliable means to measure skin friction force responsible for sand transport and its robust, simple, and cost-effective design shows promise for validating these findings in natural dune settings.