Abstract
Airflow dynamics across dune surfaces are the primary agent of sediment transport and resulting dune migration movements. Using 3D computational fluid dynamic modelling, this study examined the behaviour of near surface airflow travelling over transverse (reversing) dunes on a beach system. Wind direction was modelled in two opposing directions (both perpendicular to dune crestline) to investigate surface alteration of flow on the dune topography. Surface shear stress, velocity streamlines and potential sediment flux were extracted from the modelling. The work shows that under SW winds the surface (under the configuration measured) underwent almost 10% more aeolian flux than with opposing NE winds of the same magnitude. Differences were also noted in the airflow behaviour with SW winds staying attached to the surface with less turbulence while NE winds had detached flow at dune crests with more localised turbulence. The work provides detailed insights into how 3D airflow behaviour is modified according to incident flow direction of reversing dune ridges and the resulting implications for their topographic modification. These dune types also provide interesting analogues for similarly scaled Transverse Aeolian Ridges found on Mars and the findings here provide important understanding of flow behaviour of such landforms and their potential movement.
Highlights
Airflow over terrestrial sand dunes is the fundamental forcing mechanism in dune landform dynamics
Acceleration of airflow over dune crests/brink is a key parameter in driving dune migration and slip face dynamics and an understanding of localised interaction of the dune landform with wind forcing is central in unravelling larger dune field-scale behaviour and dynamics
Several parameters extracted from the Computational Fluid Dynamic (CFD) simulations enabled a spatial examination of wind velocity, shear velocity as well as resulting aeolian flux over the multiple dune ridges under both wind direction scenarios
Summary
Airflow over terrestrial sand dunes is the fundamental forcing mechanism in dune landform dynamics. Unvegetated (arid/semi-arid climates) dune fields provide excellent opportunities to examine airflow dynamics over various types and scales of dune landforms and to explore how they are forced by wind action. The use of Computational Fluid Dynamic (CFD) modelling in recent years enables investigation of the 3D behaviour of airflow over complex terrain (Paterson and Holmes, 1993), helping to provide new insights into heterogeneous surface flow and aeolian transport on dune surfaces on a large (dunefield) scale. Recent developments in this modelling applied to dunes Field validation of a CFD modelling approach (Jackson et al, 2011) shows good agreement with in situ 3D measurements of wind flow and CFD proves to be an effective tool in examining wind flow over small (dune length) scales (Jackson et al, 2013b, 2015; Smith et al, 2017b) and even at larger dune field scales (Jackson et al, 2013b, 2015; Smith et al, 2017b)
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