
Calibration Parameters
135
you set up the Boussinesq wave simulation. The file must be a 2D grid data
file (dfs2).
You also have to choose between velocity (default) or flux based eddy viscos-
ity formulations.
Recommended values
It is recommended to use a Smagorinsky velocity-based formulation with con-
stant coefficient of 0.25-1.
It is recommended to use the velocity formulation as the flux formulation is
only correct at a constant depth and in general should be applied with great
care in order to avoid falsification of the flow pattern. The velocity based for-
mulation, which is more correct, is unfortunately also more difficult to imple-
ment in the numerical algorithm. This is because the model uses the fluxes
as the unknown parameters and not the velocities. Therefore the velocity-
based formulation is implemented by using the velocities from the previous
time step. This can, however, lead to stability problems when the eddy vis-
cosity coefficient becomes large. The eddy coefficient must fulfil the criterion
(5.5)
It is
not
recommended to use eddy viscosity to damp out numerical instabili-
ties in MIKE 21 BW simulations.
Remarks and hints
Eddy viscosity is mainly introduced in MIKE 21 BW for modelling of wave-cur-
rent interaction, where sub grid effects in the current field are not resolved.
Please also note that a wave-induced flow field can only exist when the forc-
ing by radiation stress is balanced by bottom friction and mixing processes.
Mixing processes are usually modelled by means of an eddy viscosity. In the
breaking zone, the viscous shear is to some extent taken into account by the
additional convective terms due to the surface roller. Outside the breaking
zone, the eddy viscosity is usually very small. Therefore eddy viscosity
should usually not be used in connection with modelling of wave-induced cur-
rents.
5.3.7
Filtering
An explicit numerical lowpass filter is introduced to remove high-frequency
instabilities during uprush and downrush and to dissipate the wave energy in
the area where the surface roller can not be resolved.
If wave breaking and moving shoreline is included in your simulation you
most often also have to specify a data file including filter coefficients. Near the
v
t
x
2
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2
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Содержание 21 BW
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...