Fig 1.
Workflow showing the multi-scale modelling approach used to generate localised Computational Fluid Dynamics results of surface airflow.
Fig 2.
Showing the MRAMS grid setup, the black mark in Grid 1 shows the location of Nili Patera.
Fig 3.
Showing the location of the study site.
(a) Top: MOLA Topography map showing the location of Syrtis Major Planum. Bottom: Detailed view of the caldera complex, with the ellipse surrounding the northern caldera of Nili Patera and the southern caldera of Meroe Patera. This image is extracted from Fawdon et al., (2015) [67]. (b) CTX image of the barchan dune field located within the south-east part of the Nili Patera caldera, the white box indicates the location of image c. (c) The white box contains the portion of the dune field containing the dunes of interest for the CFD simulations.
Fig 4.
Diagram showing the typical shape of a barchan dune, the Length (L) and Width (W) shown, as well as the unidirectional wind flow which causes the uniform shape in all barchan dunes.
Image from Hersen (2005) [73].
Fig 5.
Showing the CFD domain boundary setup.
Fig 6.
Wind rose of the 73 winds which exceed the friction velocity for sediment transport, which gave an average incident direction of 66.4.
Fig 7.
a: Wind speeds 1.9 m above the surface over a barchan dune at the study site, from coarse and fine mesh resolutions. Results converged at a 1.67 m mesh resolution; the 1.67 mesh line is behind the 1.11 m mesh line. b: Wind speeds for the 1.67 m and 1.11 m meshes, 1.9 m above the surface of the dune.
Fig 8.
Wind profile over the central barchan shown in the mesh independence check of Fig 6.
Fig 9.
Diagram adapted from Durán et al. (2010) [104] showing the separation bubble of recirculating flow in the lee of the dune.
Fig 10.
Showing time-averaged wind speeds 1.9 m above the surface of the STL over the study site in Nili Patera from the CFD simulation.
Inlet wind direction flows from 66° angle from upper right to lower left, shown in the wind streaks. The arrow across the central barchan shows the orientation and length of the domain probed for the mesh independence check (Fig 7a and 7b).
Fig 11.
Wind flow over the large central barchan (Dune 1 in Fig 10).
The morphology of the dune field distorts the speed and direction of the wind flow.
Fig 12.
Showing recirculation of wind flow in the lee slope of a barchan dune.
Wind flow is from left to right in this image. The wind accelerates to a high of 22 m s-1 at the crest of the dune, before rapidly decelerating in the lee of the slope.
Fig 13.
Showing the reverse image of the recirculating flow shown in Fig 10.
Further to the recirculating flow, there is also additional downwind distortion of wind flow which does not rejoin the main body of wind flow.