USER MANUAL SWAN Cycle III version 40.72A

60 Chapter 4
set-up is zero in the deepest point in the computational grid. The user can modify
this constant by the value of [supcor]. The user can thus impose a set-up in any
one point (and only one) in the computational grid by first running SWAN, then
reading the set-up in that point and adding or subtracting the required value of
[supcor] (in m; positive if the set-up has to rise).
Default: [supcor]=0.
DIFFRACtion [idiffr] [smpar] [smnum] [cgmod]
CANNOT BE USED IN CASE OF UNSTRUCTURED GRIDS.
If this optional command is given, the diffraction is included in the wave computation. But
the diffraction approximation in SWAN does not properly handle diffraction in harbours or
in front of reflecting obstacles (see Scientific/Technical documentation). Behind breakwaters
with a down-wave beach, the SWAN results seem reasonable. The spatial resolution
near (the tip of) the diffraction obstacle should be 1/5 to 1/10 of the dominant wave length.
Without extra measures, the diffraction computations with SWAN often converge poorly
or not at all. Two measures can be taken:
1. (RECOMMENDED) The user can request under-relaxation. See command NUMERIC
parameter [alpha] and Scientific/Technical documentation (Eq. (3.31)). Very limited
experience suggests [alpha] = 0.01.
2. Alternatively, the user can request smoothing of the wave field for the computation
of the diffraction parameter (the wave field remains intact for all other computations
and output). This is done with a repeated convolution filtering. The mother filter is
E n i,j = E n−1
i,j
− a [E i−1,j + E i,j−1 − 4E i,j + E i+1,j + E i,j+1 ] n−1
For a = 0.2 (recommended), the final width of the filter is ε x = 1 2√
3n∆x (in
x−direction and similarly in y−direction) and n is the number of repetitions (see
Scientific/Technical documentation, Eq. (2.100)).
[idiffr]
[smpar]
[smnum]
indicates the use of diffraction. If [idiffr]=0 then no diffraction is taken
into account.
Default: [idiffr]=1.
smoothing parameter for the calculation of ∇ · √E
tot . During every
smoothing step all grid points exchange [smpar] times the energy with their
neighbours. Note that [smpar] is parameter a in the above text.
Default: [smpar] = 0.
number of smoothing steps (n in the above text). For a = 0.2, it should be