Aeolian processes across transverse dunes. I: Modelling the air flow

This paper discusses a two-dimensional second-order closure model simulatin g air flow and turbulence across transverse dunes. Input parameters are upw ind wind speed, topography of the dune ridge and surface roughness distribu tion over the ridge. The most important output is the distribution of the f riction velocity over the surface. This model is dynamically linked to a mo del that calculates sand transport rates and the resulting changes in eleva tion. The sand transport model is discussed in a separate paper. The simulated wind speeds resemble patterns observed during field experimen ts. Despite the increased wind speed over the crest, the friction velocity at the crest of a bare dune is reduced compared to the upstream value, beca use of the effect of stream line curvature on turbulence. These curvature e ffects explain why desert dunes can grow in height. In order to obtain real istic predictions of friction velocity it was essential to include equation s for the turbulent variables in the model. In these equations streamline c urvature is an important parameter. The main flaw of the model is that it cannot deal with flow separation and the resulting recirculation vortex. As a result, the increase of the wind s peed and friction velocity after a steep dune or a slipface will be too clo se to the dune foot. In the sand transport model this was overcome by defin ing a separation zone. Copyright (C) 1999 John Wiley & Sons, Ltd.