[MITgcm-support] Surface waves relaxation without viscosity terms

[Martin Losch]

Sergey,

for your first question, you might want to consider turning off the implicit free surface, which will have a damping effect, and use an explicit Crank-Nicholson scheme instead. Unfortunately, here: <http://mitgcm.org/public/r2_manual/latest/online_documents/node41.html>
it says, that this is not yet possible with non-hydrostatic code, so you'd have to try to implement that.
Without any dissipation, the code will blow up sooner or later

question 2: I cannot look at all the details, but from comparing obcs_calc.F in both setups I see that your "wn" is twice as large as (and has a differen sign) in your second case.

Martin


On Jan 22, 2013, at 6:07 PM, fancer lancer <fancer.lancer at gmail.com> wrote:

> 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
> <1.zip><01_U.jpg><01_W.jpg><2.zip><02_U.jpg><02_W.jpg>_______________________________________________
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