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Title: The application of adaptive mesh techniques to convective processes in oceanography
Author: MacTavish, Flora Pamela
ISNI:       0000 0004 2741 8095
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2013
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Idealised numerical simulations of two oceanographic processes, salt finger formation and the restratification phase of open ocean deep convection, are considered. These processes are modelled using the Imperial College Ocean Model (Fluidity-ICOM). This is a finite element code with the novel capability to perform mesh adaptivity. The mesh is triangular/tetrahedral and can be unstructured. If mesh adaptivity is used, all the fields are periodically interpolated onto a new mesh that has been optimised from the previous mesh. The new mesh is designed to represent one or more of the solution fields as accurately as possible by putting more nodes in regions where the Hessians of the fields being adapted to are higher. The first process to be presented is the formation of salt fingers in double diffusive convection. A secondary instability is observed to form. Due to the unstructured mesh, the fingers start off slightly different lengths from each other and this difference is observed to grow with time. A new set of simulations are run in which the secondary instability is initialised from a perturbation in the initial condition. These results are used to compare between fixed and adaptive mesh results. Evidence is obtained to show that adaptive mesh is able to produce the same results as the fixed mesh with fewer computational nodes because the resolution is used in the regions where is it most needed. The second process is the restratification after open ocean deep convection. In order to run ocean scale simulations the model must be able to accurately represent geostrophic and hydrostatic balance on a high aspect ratio domain. In order to do this with an unstructured mesh it is shown that it is necessary to constrain the nodes to be aligned in the vertical. This type of mesh is known as a 2+1 mesh and it can be adapted in both the vertical and the horizontal in order to resolve the solution fields more accurately. The model is able to reproduce previous results for a simple restratification test case using mesh adaptivity. The representation of balance is investigated using different types of mesh and different finite element shape functions. A more complex restratification test case in which baroclinic eddies form is then examined. The results obtained are compared to other models with different numerical schemes. Fixed and adaptive results are compared. These results demonstrate that Fluidity-ICOM is able to represent balance and model relatively complex processes on ocean scale, high aspect ratio domains whilst using mesh adaptivity.
Supervisor: Cotter, Colin ; Piggott, Matthew Sponsor: Imperial College London
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available