Flow deflection over a foredune

Flow deflection of surface winds is common across coastal foredunes and blowouts. Incident winds approaching obliquely to the dune toe and crestline tend to be deflected towards a more crest-normal orientation across the stoss slope of the foredune. This paper examines field measurements for obliquely incident winds, and compares them to computational fluid dynamics (CFD) modelling of flow deflection in 10 degrees increments from onshore (0 degrees) to alongshore (90 degrees) wind approach angles. The mechanics of flow deflection are discussed, followed by a comparative analysis of measured and modelled flow deflection data that shows strong agreement. CFD) modelling of the full range of onshore to alongshore incident winds reveals that deflection of the incident wind flow is minimal at 0 degrees and gradually increases as the incident wind turns towards 30 degrees to the dune crest. The greatest deflection occurs between 30 degrees and 70 degrees incident to the dune crest. The degree of flow deflection depends secondarily on height above the dune surface, with the greatest effect near the surface and toward the dune crest. Topographically forced flow acceleration (speed-up) across the stoss slope of the foredune is greatest for winds less than 30 degrees (i.e., roughly perpendicular) and declines significantly for winds with more oblique approach angles. There is less lateral uniformity in the wind field when the incident wind approaches from >60 degrees because the effect of aspect ratio on topographic forcing and streamline convergence is less pronounced. (C) 2014 Elsevier B.V. All rights reserved.