Lee-side airflow pattern characterisation under offshore winds at a beach-dune system using high resolution three-dimensional anemometers and computational fluid dynamics modelling

Studies of the role of lee-side airflow effects demonstrate the importance of offshore flows in dune growth and maintenance. Turbulent processes at the lee side of aeolian dunes have previously been only qualitatively described. However, appropriate numerical characterisation has been not possible to date due to the inherent complexity of lee-side airflow behaviour. The recent use of ultrasonic anemometers (UAs), capable of measuring the three components of the wind vector, allows quantification of flow patterns in complex areas such as the lee side of dunes. Additionally, three-dimensional modelling using computational fluid dynamics (CFD) tools provides valuable insight into the behaviour of turbulent zones and associated potential sediment transport. Using combined data from an extensive array of high-frequency, three-dimensional UAs and CFD simulations, quantitative results from a three year NERC-funded study are presented on airflow patterns and turbulent zones in the lee-side of a coastal foredune. Field data was collected during an experiment carried out at Magilligan Strand, Northern Ireland, where prevailing winds from the SW (offshore) dominate its aeolian system and previous studies have reported significant secondary airflow effects. Data were collected using an array of twenty four UAs deployed in April-May 2010 in a 60 x 90 m grid over the beach surface. Field locations of sensors were guided by a preliminary CFD run indicating the extent of turbulent areas. Simulations were carried out using the open-source CFD software OpenFOAM which solves the system of partial differential equations representing the fluid dynamics on a three-dimensional computational grid. The one-equation eddy (k) Large Eddy Simulations (LES) were conducted because of to their ability to resolve turbulent wind flow characteristics Simulations included a constant aerodynamic roughness length to account for terrain roughness. The findings confirm some of the trends observed by researchers working in wind tunnels and desert environments, particularly with regard to the role played by different wind speeds, directions and dune morphology. Airflow reversal at the beach surface was observed under perpendicular offshore winds as low as 1-4 m s-1. Once formed, the location of turbulent zones remained constant under increasing wind speed at the dune crest, and for incident wind angles of +/-30° from offshore perpendicular. The distance to re-attachment was approximately four times the dune crest height, and increased alongshore with increasing aspect ratio. Observed thresholds for changes in the reversed and re-attached zones are discussed, as are the existence of complex vortices at the beach surface as well as the potential for onshore sediment transport under offshore winds.