A computational fluid dynamic analysis of the airflow over the keystone plant species, Azorella selago, on sub-antarctic Marion Island
Combrinck, Madeleine Lelon
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Research conducted on Marion Island aims at predicting the consequence of climate change in the Subantarctic region, as well as for other terrestrial ecosystems. The island has the ideal ecosystem, due to its size, isolation and relative simplicity, to be studied as a prediction model for the consequence of climate change and the interaction between various climate related parameters. The cushion plant, Azorella selago, is the focal point of this project due to the important functional roles it fulfils as well as its wide spread distribution over the island. A. selago grows in three different shapes; hemispherical, elliptical and crescent. The changes in airflow due to varying plant size are investigated as well as the exertion of force on the plant. The grass species Agrostis magellanica has the tendency to grow on top of A. selago. In addition it has been observed that the grass species grows more vibrantly on the leeward side of the cushion plant. In the light of this observation the particle deposition of grass seeds on A. selago is also investigated. Computational fluid dynamic analyses are conducted for various sizes of each shape of the plant. These studies are augmented by wind tunnel and in situ measurements and observation and experimental determining of particle drag coefficients. Time independent, incompressible, turbulent flow is modelled by means of a high Reynolds number turbulence model with a modified Law-of-the- Wall to accommodate for the significant surface roughness. Nine different dimensions over the various shapes were identified. Each shape is analysed and the patterns that emerged discussed. The windward pattern for all shapes display similar qualities. On the leeward side the shapes display distinctly different airflow patterns. The hemisphere shows two trailing lines typically associated with the horseshoe vortex phenomenon. The ellipse displays one distinctive trailing line. The most interesting flow patterns are found when analysing the crescent shape. Three trailing lines are observed, the side lines quickly dissipates with increasing height while the middle line remains distinctive. The complex recirculation patterns that emerge are further visualized by means of air particle tracks. The furthermost number of light particle deposition of A. magellanica seeds on A. selago are found at the windward location while physical evidence clearly indicates growth primarily on the leeward position. The leeward location on A. selago is protected from the harsh environmental conditions; wind speeds are minimal in this area. It will therefore be a more suitable site for the fostering of a vulnerable seedling that on the exposed windward side. A force analysis done on the plants reveals that the total force is two orders of magnitude higher that the shear force. Furthermore, the magnitude of the total force is directly proportional to the size of the plant. Three critical areas on the plant are common irrespective of shape or size: the stagnation point at the windward side, the apex region at the top and the leeward side of the plant.