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Sand shadows commonly form downwind of obstacles in arid and semi-arid regions. Understanding their dynamics requires insights into the airflow patterns behind the obstacles under different wind regimes. Here, we studied models of cuboid obstacles to characterize the three-dimensional responses of airflow behind obstacles with different shape ratios to variations in the incident flow in a wind-tunnel simulation. Wind velocity was measured using particle image velocimetry (Ply). The flow patterns behind cuboid obstacles were complicated by changes in the incidence angle of the approaching flow and in the obstacle’s shape ratio. The flows separated both horizontally and vertically, creating reverse-flow cells downwind of the obstacles. The horizontal cells were characterized by two asymmetrical, opposing reverse vortices at flow incidence angles greater than 60 to 65, whereas the vertical cells formed a single vortex at all angles. The cells shifted position in response to changes in the incidence angle and shape ratio, reflecting the interaction between outer and reverse flows. The parameters of the horizontal and vertical reverse cells depended on the shape ratios and incidence angles but not on the wind velocity. The sizes of the horizontal and vertical reverse cells were correlated, reflecting their interactions. The low-velocity "shadow" behind the obstacle caused sand deposition and determined the sand shadow’s evolution. Field experiments will be required to relate these airflow patterns to sand shadow development. (C) 2011 Elsevier B.V. All rights reserved.