[MITgcm-support] Surface waves relaxation without viscosity terms

[fancer lancer]

Hello dear MITgcm developers and users.

I am trying to create the MITgcm setup, which could describe the
following two laboratory experiment cases:
1) There is 2D tank with depth 0.196 m and length 12.4 m, bottom is
plain. Left boundary is solid, but the right boundary is opened. The
open boundary represents a wave-producer, which generates a sinusoidal
surface wave with frequency 1.5 Gz.
2) The same experiment as above, but the left boundary is also opened
and the background flow runs from left to right with velocity 0.232
m/s.

Both setups are attached to the e-mail. The modeling works pretty well
except the following issues:
1. I turned off all the dissipation (horizontal - viscAh=0, vertical -
viscAr=0, biharmonic - viscA4=0, even the viscosity term in the
momentum equation - momViscosity=.true.), but the surface waves keep
dissipating (picture 02_U.jpg, blue line under the 0 vertical level is
the surface elevation). Of course, the reason can be in the numerical
viscosity (diffusion), but the dissipation is too intensive. It can be
possible, that I miss something and there is another term of
dissipation, which I haven't tuned off. Can anybody check whether I
miss something or not?
If the reason still is the numerical viscosity what can I do to
decrease such the annoying effect?

2. The second issue is a bit more complex. We use the formulas of the
vertical and horizontal waves velocities in the basin with the finite
depth (see the obcs_calc.F file) to create the harmonic on the right
boundary (some kind of wave-maker). That formulas work pretty well in
case of the zero background flow (see 01_U.jpg, 01_W.jpg). But if the
background flow exists then the high-frequency patterns appear on the
picture of the vertical velocity (see 02_W.jpg, 02_U.jpg).

The attached files:
01_U.jpg - horizontal velocity and surface displacement (t ~ 17 s), no
background flow
01_W.jpg - vertical velocity and surface displacement (t ~ 17 s), no
background flow
02_U.jpg - horizontal velocity and surface displacement (t ~ 25 s),
with background flow
02_W.jpg - vertical velocity and surface displacement (t ~ 25 s), with
background flow

Does anybody have any suggestion what can be the reason of the
patterns? I am quite sure, that the formulas of the wave-maker are
correct.

In addition here are my projects:
1.zip - experiment without the background flow,
2.zip - experiment with the background flow.

Any help would be highly appreciated.

Best regards,
Sergey V. Semin
Post graduate course student
Department of Mathematics,
Nizhny Novgorod State Technical University n.a. R.E.Alekseeva
http://www.nntu.ru/
117-24 ulitsa Minina, Nizhny Novgorod, 603950, Russia
e-mail: fancer.lancer at gmail.com
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