Air flow and sand transport over sand-dunes
Developments in the modelling of turbulent wind over hills and sand dunes of different shapes by Hunt et al. , Carruthers et al.  are briefly described, and compared with earlier studies of Jackson and Hunt  and Walmsley et al. . A new model (FLOWSTAR) is described; it has a more accurate description of airflow close to the surface, which is not in general logarithmic at typical measurement heights. Comparisons are made between the new model and the results of non-linear models using higher-order turbulence schemes, especially for surface shear stress.
The widely predicted and observed drop in velocity and shear stress at the base of a dune is confirmed by FLOWSTAR. It is clear that common models for the saltation flux based only on µ* are not appropriate at the toe of the dune where they predict a piling-up of sand.
Comparisons of the wind speed are made between the model and different sets of measurements over a dune, by Howard and Walmsley , and by our group in recent field measurements over a dune in Oman, and in a new wind-tunnel study of Howard’s dune. It is found that the FLOWSTAR calculations agree well with these sets of measurements upwind of the brink. Since the profile is not logarithmic over the dune at the measurement heights, estimates of µ* from wind measurements over dunes are likely to be less accurate than the FLOWSTAR computation of µ*. The saltation flux was measured over the Oman dune and increases in proportion to computed value of µ* 3 over the dune. This supports the use of the Lettau and Lettau version of Bagnold’s flux formula for modelling sand transport over the most of the upwind slopes of sand dunes.
KeywordsSand Dune Normalise Wind Speed Sand Transport Saltation Flux Surface Shear Stress
Unable to display preview. Download preview PDF.
- Carruthers, D. J., Hunt, J. C. R., Weng, W. S.: A Computational model of stratified turbulent air flow over hills-FLOWSTAR I. Proc. of Envirosoft. In: Computer techniques in environmental studies (ed. Zanetti) Berlin, Heidelberg, New York: Springer, pp. 481–492 (1988).Google Scholar
- Howard, A. D., Walmsley, J. L.: Simulation model of isolated dune sculpture by wind. In: Proc. Inter. Workshop on the Physics of Blown Sand, Memoirs No.8. Department of Theoretical Statistics, University of Aarhus, Aarhus, 377–391 (1985).Google Scholar
- Knott, P.: The structure and pattern of dune-forming winds. Ph. D thesis, University of London (1979).Google Scholar
- Livingstone, I.: Geomorphological significance of wind flow patterns over a Namib linear dune. In: Aeolian geomorphology. Binghampton Symposia in Geomorphology, International Series, 17, (ed. Nickling, W. G.) Boston: Allen and Unwin, pp. 97–112 (1986).Google Scholar
- Weng, W. S., Richards, K. J., Carruthers, D. J.: Some numerical studies of turbulent wake over hills. In: Advances in turbulence 2. (eds. Fernholz, H.-H., Fiedler, H. E., Berlin, Heidelberg: Springer, pp. 123–456 (1989).Google Scholar
- Zeman, O., Jensen, N. O.: Progress report on modelling permanent form sand dunes. Risø National Laboratory, Roskilde, Denmark (1988).Google Scholar
- Newley, T. M. J.: Turbulent air flow over hills. Ph. D thesis, University of Cambridge (1986).Google Scholar
- Taylor, P. A., Teunissen, H. M.: The Askervein hill project: report on the September October 1983 main field experiment. Internal report MSRB-84-6, Atmos. Envir. Service, Downsview, Ontario, Canada (1985).Google Scholar
- Weng, W. S.: Turbulent air flow and fluxes over low hills. Ph. D thesis, University of Cambridge (1989).Google Scholar
- Bagnold, R. A.: The physics of blown sand and desert dunes. London: Chapman and Hall (1941).Google Scholar
- Greeley, R., Iversen, I. D.: Wind as a geological process. Cambridge University Press. (1985).Google Scholar
- Lettau, K., Lettau, H. H.: Experimental and micrometeorological field studies of dune migration In: Exploring the world’s driest climate. (eds. Lettau, H. H., Lettau, K.) University of Wisconsin, Madison, pp. 123–456 (1978).Google Scholar
- Press, W. R., Flannery, B. P., Teukolsky, S. A., Vetterling, W. T.: Numerical recipes, p.381. Cambridge University Press (1986).Google Scholar
- Cook, N. J.: Wind tunnel simulation of the adiabatic atmospheric boundary layer by roughness, barrier and mixing device methods. Building Res. Est. Paper 6, 157–176 (1977).Google Scholar