Computational Fluid Dynamic modelling of Three-Dimensional airflow over dune blowouts

Blowout enlargement is primarily driven by aeolian transport, where high velocity winds entrain and remove sediment from the landform. However patterns of deflation in blowouts are poorly understood as near surface airflow in a blowout is complex. In this study three-dimensional airflow during a light (3.93 m/s) and moderate (8.98 m/s) onshore wind event was calculated using the two-equation Renormalized Group k-epilson airflow turbulence model, by the Computational Fluid Dynamics software OpenFOAM. The saucer blowout located on the coastal foredune in the Belmullet Peninsula, Western Ireland, measures 100 metres long, 60 metres wide and 13 metres deep. Results show that during both wind events flow separates and reverses as it enters the blowout over the foredune crest, as it reattaches it is steered, diverging from the centre of the blowout and before
topographically accelerating over the rim crest. Flow which enters the throat of the blowout accelerates and remains directionally unchanged before accelerating up the deposition lobe slope, where flow at the crest was simulated as being 100% faster at the crest than velocity simulated on the beach. In lee of the blowout at the depositional lobe airflow decelerated and a large zone of flow separation and reversal was created. Whilst relative change of velocity and direction at 1 metre above the surface of the blowout displayed similar behaviour during both wind events, flow in the lee during the light wind event did not appear to fully reattach and recover to velocity levels on the beach.