Numerical Modelling of Turbulent Flow Structures in a Trough Blowout

Blowouts are erosional landforms often formed on coastal sand dunes by the deflation of sediment by wind flow. Quantitative observations of wind flow within blowouts aided by the deployment of smoke-bombs, have noted that temporal and spatial variations in sediment transport occur with the presence of turbulent flow structures. However because of the discrete nature of anemometry data, the presence of flow structures has been difficult to quantify and our understanding remains largely conceptual. This study presents a detailed investigation of turbulent flow structures within a trough blowout using high resolution, three-dimensional computational fluid dynamic modelling. We show that when incident wind flow was parallel to the blowout axis, only limited flow steering took place but a well-defined near surface jet developed along the deflation basin. Conversely when incident wind flow was oblique to the axis of the blowout wind flow became steered along the axis of the blowout but no near surface jet was produced. During neither incident wind direction were corkscrew or helicoidal vortices produced. Our study concludes that the incident wind flow direction is critical to the effectiveness of a trough blowout as a corridor for wind-blown sediment. While wind from a range of oblique angles may be steered along the axis of blowout, its relative effectiveness of eroding and transporting sediment beyond the deflation basin and erosional walls is much reduced compared to axis parallel wind flows of the same incident wind speed.