A computational fluid dynamic analysis of the airflow over the keystone plant species, Azorella selago, on sub-antarctic Marion Island
Date
2009-03
Authors
Combrinck, Madeleine Lelon
Journal Title
Journal ISSN
Volume Title
Publisher
Stellenbosch : University of Stellenbosch
Abstract
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.
Description
MScEng
Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2009.
Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2009.
Keywords
Plant aerodynamics, Particle deposition, Dissertations -- Mechanical engineering, Theses -- Mechanical engineering