BLAST 4 : Computational Fluid Dynamics (CFD) simulation of airflow over flow-transverse dunes

The role of secondary flow structures in the morphodynamics and maintenance of sand dunes has received much attention in aeolian geomorphology, largely via event-based field research (e.g., Lancaster et al. 1996, Frank & Kocurek 1996, McKenna Neuman et al. 2000). More recently, wind tunnel simulations have offered a higher-resolution picture of flow properties in a more controlled, yet dynamically constrained setting (e.g., Wiggs et al. 1996, Walker 2000, Walker & Nickling 2002). Whilst field and laboratory studies have extended our knowledge of the relationships between dune form, flow properties and sediment transport, very little quantitative information exists on the properties and sedimentological significance of secondary airflow patterns from micro-turbulent bursting to coherent macro-turbulent structures (e.g., flow separation and reversal). In part, this relates to a paucity of field and laboratory results for a range of dune forms and incident flow conditions, but also to the limited availability (until recently) of appropriate turbulence instrumentation to measure turbulence in natural airflows.

Numerical simulation of airflow over dunes using computational fluid dynamics (CFD) software that solve the Navier-Stokes equations for complex flow is now possible. Though such software has been in use for engineering applications for many years, its debut in geomorphology is recent via fluvial research (e.g., Bates & Lane 1998). CFD allows precise control and simulation of flow for a variety of morphological scenarios and provides a new, rapid method for examining spatially & temporally rich data on complex flow field properties & processes that would otherwise be difficult to measure directly. This research applies a CFD model (PHOENICS 3.4) to model flow over idealized transverse aeolian dunes and describes the sensitivity of different elements of the flow field to variations in geomorphic parameters. The model appears capable of simulating the highly turbulent reverse flow vortex in the lee and provides an acceptable solution of the downwind distance to flow re-attachment given variations in dune height, windward slope length and thus, aspect ratio.

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