
Basic Parameters
121
Time discretisation of cross-Boussinesq terms
The representation of the
Boussinesq cross terms
(i.e. Q
xyt
, Q
yt
and Q
xt
in
the x-sweep and P
xyt
, P
xt
and P
yt
in the y-sweep) has required special atten-
tion.
Time-extrapolation factor of 1
In order to obtain the correct time-centering, we have used linear time-extrap-
olation of these terms. A straightforward numerical representation leads to a
backward centering of these terms and will result in artificial dissipation of
waves propagating with an angle to the grid. This corresponds to a time-
extrapolation factor of 1 (default value).
Time-extrapolation factor of 0
A straightforward numerical representation leads to a backward centering of
these terms and will result in artificial dissipation of waves propagating with
an angle to the grid. This corresponds to a time-extrapolation factor of 0.
Solution of the enhanced Boussinesq equations (deep-water terms
included)
Even though approximately 35 time steps resolve the minimum wave period,
the time-integration of the enhanced Boussinesq equations may sometimes
result in instabilities, which eventually may cause a model blow-up. Often the
instability (high frequency noise) appears in the computational domain with
the largest water depth (typically near an internal generation line or at an
open boundary). As explained in “
Blow-up
after some time steps” (see link
below) the time step should be reduced to avoid this instability. Alternatively,
you can use a depth-dependent time-extrapolation factor.
Recommendations
If instability appears include 'Depth-dependent time-extrapolation' and set the
time-extrapolation factor to 0.8 in the area (determined by the water depth)
where the high-frequency noise appears. If this still does not help reduce the
time-extrapolation factor to 0.5. If problems still occur you can set time-
extrapolation factor to zero (for a local area).
It is important to limit the area where a time-extrapolation factor different from
one is used. Application of a time-extrapolation factor of 0 in the entire com-
putational domain should only be used as a last resort.
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.
5.2.5
Boundary
In most cases boundary positions and number of boundaries detected by the
model can be used. But in special cases you might want to define the posi-
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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...