Mean flow and turbulence responses in airflow over foredunes: New insights from recent research

This paper reviews recent progress in quantifying airflow over foredunes using traditional velocity profiles, location-specific time-averaged flow properties, vectors of non-uniform flow, and turbulence measurements. Though velocity profiles remain essential for characterizing flow structure, their use for estimating shear stress is limited as near-surface airflow is often non-uniform and unsteady. Topographic forcing and roughness effects produce distinct inflection points, slope segments, and shear layers. Further limitations result from the bulkiness of conventional instruments that are unable to measure within the thin constant stress region over dunes. Populations of time-averaged flow properties and flow vectors reveal location-specific flow responses to changing incident wind conditions and resulting topographic forcing and steering effects. For instance, streamwise velocity and steadiness are sensitive to topographic forcing on the stoss slope, but not at the dune toe due to the dominance of flow stagnation. Vertical velocity is most responsive at the crest and to changes in incident flow angle. Flow vectors are deflected onshore and resulting transport pathways have greater effective fetch than those identified by the regional wind. This response also varies with height above the surface and for incident angle and speed. Continued field research with turbulence instruments is needed to validate simulated and wind tunnel findings of turbulent Reynolds stresses over dunes and assess implications for sand transport, such as the role of micro-turbulent events in grain entrainment and transport.