From dome dune to barchan dune: Airflow structure changes measured with particle image velocimetry in a wind tunnel

Abstract

The evolution from the initial sand accumulation to a mature barchan dune can be divided into five general stages. The first two stages are transient sand patches that disperse easily during downwind movement of the dune. In this study, we focused on the flow field changes around the dunes during the last three stages of dune evolution, namely the dome, protobarchan, and mature barchan stages. By means of particle image velocimetry, we measured the mean and turbulent flow fields around 3D-printed scale models of real dunes in a wind tunnel and discuss the significance of the airflow variation for dune evolution. We found that topographic forcing plays an important role in the changes of the mean and turbulent airflow structures. The airflow patterns on the windward slope were similar at all three stages, with deceleration at the windward toe and horizontal acceleration combined with vertical upward movement of the airflow along the windward slope. On the leeward sides of the dune during the three stages, the airflow structure evolved from airflow diffusion to airflow diffusion accompanied by transient flow separation, and then to complete airflow separation and reattachment. The turbulence intensity near the dune surface increased significantly as the dune matured and two zones with a high Reynolds shear stress appeared: in the middle of the windward slope and in the lee of the dune. Based on our results, we discuss the impact of changes in the airflow structure at different evolutionary stages on the evolution of dune morphology.