A computational fluid dynamic analysis of the airflow over the keystone plant species, Azorella selago, on sub-antarctic Marion Island

[摘要] Research conducted on Marion Island aims at predicting the consequence ofclimate change in the Subantarctic region, as well as for other terrestrialecosystems. The island has the ideal ecosystem, due to its size, isolation andrelative simplicity, to be studied as a prediction model for the consequence ofclimate change and the interaction between various climate related parameters.The cushion plant, Azorella selago, is the focal point of this project due to theimportant functional roles it fulfils as well as its wide spread distribution over theisland. A. selago grows in three different shapes; hemispherical, elliptical andcrescent. The changes in airflow due to varying plant size are investigated as wellas the exertion of force on the plant. The grass species Agrostis magellanica hasthe tendency to grow on top of A. selago. In addition it has been observed that thegrass species grows more vibrantly on the leeward side of the cushion plant. Inthe light of this observation the particle deposition of grass seeds on A. selago isalso investigated.Computational fluid dynamic analyses are conducted for various sizes of eachshape of the plant. These studies are augmented by wind tunnel and in situmeasurements and observation and experimental determining of particle dragcoefficients. Time independent, incompressible, turbulent flow is modelled bymeans 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 isanalysed and the patterns that emerged discussed. The windward pattern for allshapes display similar qualities. On the leeward side the shapes display distinctlydifferent airflow patterns. The hemisphere shows two trailing lines typicallyassociated with the horseshoe vortex phenomenon. The ellipse displays onedistinctive trailing line. The most interesting flow patterns are found whenanalysing the crescent shape. Three trailing lines are observed, the side linesquickly dissipates with increasing height while the middle line remainsdistinctive. The complex recirculation patterns that emerge are further visualizedby means of air particle tracks. The furthermost number of light particledeposition of A. magellanica seeds on A. selago are found at the windwardlocation while physical evidence clearly indicates growth primarily on the leewardposition. The leeward location on A. selago is protected from the harshenvironmental conditions; wind speeds are minimal in this area. It will thereforebe a more suitable site for the fostering of a vulnerable seedling that on theexposed windward side. A force analysis done on the plants reveals that the totalforce is two orders of magnitude higher that the shear force. Furthermore, themagnitude of the total force is directly proportional to the size of the plant. Threecritical areas on the plant are common irrespective of shape or size: the stagnationpoint at the windward side, the apex region at the top and the leeward side of theplant.

[发布机构] Stellenbosch University