I have a bowup problem.
I probably need some basic help in the chioce of boundary conditions.
I have a fairly regular bathymetry 300 m x 2040 m, with DX=5 m and DY=10 m
with the coast at SOUTH (beach in Y=0, the rear land reaching level + 5 m in Y=40 m)
the open sea at NORTH (Y=2000m, depth of 10 m),
a smooth profile at WEST (X=0) and EAST (X=300m).
The tide Z(t) and the longshore velocity U(t) are known
in form of sinusoidal functions with max 1.2 m and 0.5 m/sec
starting from zero.
Activated options are
UV_VIS2
MIX_S_UV
WET_DRY (minimum depth=0.5)
UV_ADV
TS_MPDATA
DJ_GRADPS
MASKING plus ANA_MASK
SOLVE3D plus (default activated options)
SPLINES
UV_QDRAG
SEDIMENT plus ..
GLS_MIXING plus ..
All the rest is zero
DT=1 sec, NDTFAST=20
Boundary conditions are:
SOUTHERN_WALL
EAST_FSGRADIENT
EAST_M2CLAMPED > U_BAR=U(t) in file ana_m2obc.h
EAST_M3GRADIENT
WEST_FSCLAMPED > Z=Z(t) in file ..
WEST_M2GRADIENT
WEST_M3GRADIENT
NORTH_FSGRADIENT
NORTH_M2GRADIENT
NORTH_M3GRADIENT
The application blows up after approx. 4000 time steps.
I checked the generated dirichlet conditions of U(t) and Z(t):
they are the expected ones.
Some disturbance of the velocity arise in the domain and
a localised disturbance develops from mid boundary, at WEST (i.e.
where zero gradient velocity is prescribed; bottom here is very regular, depth is around 8 m).
Before blowup,
velocity is smooth, around 0.02 m/s,
except at some peaks where it is 0.2 m/s.
Water level is flat.
I believe there is a problem with boundary conditions
Indeed a similar application, with prescribed sinusoidal
water level at NORTH, radiation stress (derived from SWAN) in the domain and with the boundary condition EW_PERIODIC, runs smoothly.
Could anybody suggest the proper boundary conditions (for the 3 open sides) when tides and longshore currents are known?
Thanks in advance.
Michele.
Blowup. Boundary condtition problem?

 Posts: 22
 Joined: Fri Jul 08, 2005 5:42 pm
 Location: Kyoto University
jcwarner wrote:perhaps try the M2FLATHER or M2REDUCED type BC's on the east and west. If the longshore tranposrt is to the west, then try east_m2reduced and west_m2flather. You may need to try a few different options to see what works best for your application.
From my experience, I recommend use following combination.
FS > Chapman or Clamped
M2 > Flather
You may also use M2REDUCED for only free surface given condition as jcwarner suggested.
Thanks john and thanks nobu!
I have not solved the problem yet...
The longshore tidal current U(t) must be prescribed and can not be derived by the surface gradient. Therefore I cannot use M2reduced.
Further, I do not have a main current direction.. since the current is sinusoidal... so no main longshore transport.
My feeling is that I need to prescribe at one of the 2 sides, e.g. at east:
A) the longshore velocity ubar_east=U(t), vbar_east=0 in ANA_M2OBC, with condition M2_Clamped and
B) a pressure gradient which is related to the (time derivative of the) prescribed velocity. A sort of FS_reduced.. (not to be confused with FSOBC_reduced, which is similar to M2_reduced)
... or is it totally wrong?
I have not solved the problem yet...
The longshore tidal current U(t) must be prescribed and can not be derived by the surface gradient. Therefore I cannot use M2reduced.
Further, I do not have a main current direction.. since the current is sinusoidal... so no main longshore transport.
My feeling is that I need to prescribe at one of the 2 sides, e.g. at east:
A) the longshore velocity ubar_east=U(t), vbar_east=0 in ANA_M2OBC, with condition M2_Clamped and
B) a pressure gradient which is related to the (time derivative of the) prescribed velocity. A sort of FS_reduced.. (not to be confused with FSOBC_reduced, which is similar to M2_reduced)
... or is it totally wrong?
I have tried with
Fsclamped and m2reduced
both at the eastern and western boundaries.
At the beginning, elevation grows. the velocity computed by m2_reduced is positive at west, negative at east. Almost symmetric.
(water enters from both sides....)
As always, the blowup occurs when tidal elevation is still a few centimeters.
As usual: the greater instability is localised close to the western boundary and it spreads toward north (reaching the offshore boundary).
I also tried DG_GRADP rather than DG_GRADPS. There is a hole in the center of the bathymetry but it does not justify any problems with regard to the pressure gradient algorithm and as expected the change had no effect. Also, I remind you that with EW_periodic (and an additional external forcing) the application runs.
Any additional suggestion?
Fsclamped and m2reduced
both at the eastern and western boundaries.
At the beginning, elevation grows. the velocity computed by m2_reduced is positive at west, negative at east. Almost symmetric.
(water enters from both sides....)
As always, the blowup occurs when tidal elevation is still a few centimeters.
As usual: the greater instability is localised close to the western boundary and it spreads toward north (reaching the offshore boundary).
I also tried DG_GRADP rather than DG_GRADPS. There is a hole in the center of the bathymetry but it does not justify any problems with regard to the pressure gradient algorithm and as expected the change had no effect. Also, I remind you that with EW_periodic (and an additional external forcing) the application runs.
Any additional suggestion?