Page 205 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea

194 Fundamentals of Ocean Renewable Energy

8.1.1 Horizontal Mesh Type
There is a vast array of ocean models, and a wide range of ways in which they
can be categorized. There are many places we could begin this classification
but, because it is of much interest to wave and tidal resource characterization,
we begin by differentiating between structured and unstructured meshes.
In a structured grid, all grid lines are orientated regularly so that, in a
2D case, the coordinate transformations of curvilinear lines result in a square
(Fig. 8.1A). By contrast, this restriction does not apply to unstructured grids (or
mesh) (Fig. 8.1B); however, this grid is more complex to deal with numerically.
The advantage of a structured grid is its simplicity and ease of preprocessing
and postprocessing; however, to achieve a desired resolution (e.g. to resolve the
curved geometry shown in Fig. 8.1A), it is necessary to use a constant and high-
resolution grid throughout the computational domain. Clearly, this resolution is
easier to achieve by spatially varying the grid resolution, as in an unstructured
grid (Fig. 8.1B), with the added advantage of lower grid storage (e.g. the circle
in the centre of Fig. 8.1, which could represent an island, requires no storage
using an unstructured grid).
Within the context of ocean energy, it is clear that due to its ability
to accommodate a wide range of scales within a single-model domain, for
example, a regional model which incorporates a tidal energy array within a
narrow strait, unstructured meshes tend to be favoured for resource assessment.
However, many ocean models, particularly 3D models, which simulate a wide
range of physical processes, are based on a structured mesh. Under such
circumstances, a sequence of nested models, with increasing grid resolution
from outer to inner nests, will be required. In general, structured meshes tend to
be favoured for larger-scale ocean modelling applications.

FIG. 8.1 (A) Structured and (B) unstructured horizontal meshes.