ERDC/CHL CHETN-I-68
March 2004
Create Mesh. This is potentially a long process, depending on the size function and the extent of
your domain. It is a good idea to save your work first using the File|Save command. Then, select the
Feature Objects|Map to 2D Mesh command to begin the mesh generation.
Figure 10a and 10b illustrate the section of the created Tedious Creek CGWAVE mesh in the
vicinity of the public piers and north breakwater. It illustrates the finite element mesh based on the
T = 6 sec wave period and a size function defined by one tenth of the wavelength. The mesh contains
a total of 339,042 elements and 171,265 nodes.
The primary purpose of this Tedious Creek study was to compare existing and authorized breakwater
configurations. Figure 11 shows the two breakwater lengths relative to the channel outlines (red
outlines). To accomplish this, a different mesh, using the same size function definitions, with the
revised arcs representing the altered breakwater alignments, was created.
Check Mesh Quality. Element and mesh quality can be checked using the CGWAVE | Model
Check command in the Mesh module. A mesh should have certain properties to insure that it runs
efficiently during execution and does not cause instabilities in the solution. It should have good (a)
elemental properties, (b) smooth bathymetric contours, (c) gradual area change, and (d) mild
longitudinal depth changes. The elemental properties include aspect ratio, shape, and angle. An ideal
element has an aspect ratio with sides that are the same length, no thin triangles, and interior angles
greater than 10 deg. Adjacent elements should not have area or depth changes greater than 20
percent and should follow depth contours. If the Model Check does not return any serious errors,
proceed to assign CGWAVE model parameters.
Assign Model Parameters. Defining the CGWAVE model parameters is a two-step process:
(a) renumber nodestrings, and (b) input model control parameters.
Renumber nodestrings. Nodestrings are subsets of sequential nodes that allow specific
parameter assignments and operations. SMS automatically creates nodestrings from the feature arcs
when it generates the mesh. The nodestrings should retain the properties, such as reflection
coefficients, that were already assigned to the feature arcs so all you need to do is renumber them.
Renumbering gives new numbers to all the nodes in an orderly fashion to improve numerical
bookkeeping. It only has to be done once after all mesh editing is finished. Typically, the best
renumbering location is the ocean boundary nodestring. The mesh is renumbered by selecting the
Nodestrings | Renumber command from the Elements menu after having selected a nodestring. The
nodestring is used as a seed to start the renumbering process. The bandwidth option is the default.
Input model control parameters. Input the model control parameters by selecting the
CGWAVE | Model Control command in the Mesh module. These include sections on (a) incident
wave conditions, (b) nonlinear wave options, (c) open boundary, (d) 1-D wave transformation inputs,
and (e) numerical solver. Incident wave conditions are input along the open ocean boundary and
include wave period, direction, and amplitude. The amplitude is one half the wave height, and should
not be confused with the wave height. Nonlinear wave options include bottom friction, wave
breaking, and nonlinear dispersion. When first debugging a model, it is best to do the simple cases
first. Then, a sensitivity analysis can be performed with these wave options to determine which
should be exercised. Open boundary conditions should match those selected when generating the
domain in the Map module. These are semicircular in the Tedious Creek example.
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