
Entries Arranged Alphabetically
175
Blow-up near porosity layers
The reason for the blow-up may be application of too small porosity values.
Most often the porosity should be larger than 0.2. If you still have a blow-up,
you can try to use a larger value or increase the number of porosity layers in
front of the structure.
Blow-up near areas with large bathymetry gradients
Strictly speaking, the Boussinesq equations are derived assuming a slowly
varying bathymetry (bottom slope less 1:3). If the gradient of the water depth
is large (e.g. bottom slope of 1:1 or 1:2) instabilities may appear. You can try
reduce the bottom slope by editing the bathymetry.
Alternatively, you can try to use another numerical scheme for the convective
terms (see also Section 5.2.4). It is recommended to first apply the “Central
differencing with simple upwinding at steep gradients and near land”. In case
of a new blow-up you can try the quadratic upwinding scheme. If the instabil-
ity still persists please try the simple upwind scheme.
Blow-up near wave breaking
When there is a build-up of unrealistic waves in shallow water, or in areas
where there is a focusing of wave energy, the instability may be caused by
breaking waves. In that case you should absorb the breaking wave energy by
introducing sponge layers.
You may also reconsider you model setup and include wave breaking. As you
have to resolve the individual breaking waves, you would probably need to
reduce the grid spacing (to 1-2 m).
Blow-up near open boundaries/generation lines
When the build up of unrealistic waves appear near an open boundary/gener-
ation line this may be caused by an inconsistency. Please check the bound-
ary/generation specification, and the input data files.
2DH simulations with wave breaking and moving shoreline
In most 2DH applications including wave breaking and moving shoreline a
time extrapolation factor of slightly less than one is recommended for numeri-
cal stability. Time extrapolation factors within 0.8-0.9 (for all water depths)
have successfully been used in a number of wave breaking applications. If a
blow-up occurs (typically seen as high-frequency noise as shown in
Figure 5.55) you may try to reduce the time-extrapolation factor to 0.7.
For stability reasons it is recommended to use the scheme ‘Simple upwinding
at steep gradients and near land’ for the space discretisation of the convec-
tive terms in connection with simulations including wave breaking (and mov-
ing shoreline). If a blow-up occurs in the breaking zone you may try to reduce
the time step or use the scheme ‘Simple upwinding’. Please note this scheme
is quite dissipative if the waves are not resolved properly.
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Страница 1: ...MIKE 2017 MIKE 21 BW Boussinesq Waves Module User Guide...
Страница 2: ...2...
Страница 4: ...4 MIKE 21 BW DHI...
Страница 13: ...General Description 13 Figure 2 4 Simulation of wave penetration into Frederikshavn harbour Denmark...
Страница 16: ...Introduction 16 MIKE 21 BW DHI...
Страница 75: ...2DH Boussinesq Wave Module Examples 75 Figure 4 45 Visualisation 2D of instantaneous surface elevation...
Страница 185: ...Entries Arranged Alphabetically 185 Figure 5 58 Application of different time extrapolation factors...
Страница 190: ...Reference Manual 190 MIKE 21 BW DHI...
Страница 192: ...Scientific Documentation 192 MIKE 21 BW DHI...
Страница 193: ...193 INDEX...