Physical and numerical modelling of particle settlement in a turbulent flow: implication for the settlement of algal propagules.
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
A fundamental stage in rocky-shore seaweed life history is the recruitment process involving external fertilisation and then settlement of the propagules on a suitable substrate. The ultimate step in this settlement stage is the crossing of the viscous sub-layer and attachment to the substrate. Given the extreme conditions met in the intertidal zone, propagules can be dislodged at any time before they secure a strong enough anchoring.
Flow conditions and propagule properties are key to this process. The settlement process under turbulent conditions was recreated within a stirred benthic chamber for five different species. Whereas propagule properties (size, density) vary with species, and propagules are adapted to the different conditions in the intertidal, they exhibit the same settlement behaviour. They nevertheless exhibit different settling velocities and settlement thresholds. Several methods of characterisation of the tank flow from particle tracking velocimetry and acoustic Doppler velocimetry data are reviewed, as well as an analytical model. Turbulent settling was found to be independent of the well-mixed tank bulk flow and to depend only on the boundary-layer mechanics. A model of settlement threshold is presented from which propagule mucilage adhesiveness estimates are derived, leading to good correlations between adult plant exposure and the stickiness of its propagules and to the conclusion that settlement can only occur in calm conditions.
To extend the work, computational fluid dynamic techniques are developed by extending the Gerris Flow Solver. A 2-D approach to tank modelling and a pilot study of expansion to 3-D is described. This extends the perspective given by the experiments, notably through output of the hydrodynamical forces experienced by the propagules. Finally, in the view of realizing direct numerical simulations of propagule behaviour in the viscous sub-layer, a new and unique 2-D/3-D fully conservative solid/fluid interaction model is developed and tested with success.