[MITgcm-support] Surface waves relaxation without viscosity terms

Wed Jan 23 14:54:18 EST 2013

Martin, Jean-Michel,

Thank you very much for the helping me with such specific issues.

Concerning the 1st question.
I corrected my setup of the model to use the Crank-Nicholson scheme.
I have now:
implicSurfPress=0.5,
implicDiv2Dflow=0.5,
By default, implicitNHPress = implicSurfPress.
So I hope it help. I shall write about the results of the simulation here.

Concerning the 2nd question.
Martin, you are absolutely right. Since the environment in the
experiments is a bit different, they have the different dispersion
relations:
1st experiment: w^2 = k*g*tanh(k*h)
2nd experiment: (w - k*U) = k*g*tanh(k*h)
As you can see if frequency (w) is the same in the both experiments,
the wave-number (k) is different (if the 1st exp. does have the
non-zero background flow (U)). It doesn't matter what sign of the
wave-number is specified in the first experiment, because the graphic
is symmetric relatively OY axis. I setup the following parameters for
the wave-maker:
1st experiment: frequency  w=2*PI*1.5, wave-number k=2*PI*1.515
2nd experiment: frequency  w=2*PI*1.5, wave-number k=2*PI*3.271

In the first experiment it gives appropriate surface sinusoidal wave,
but in the second case we have the high-frequency patters on the
background of the vertical velocity. In addition the space Fourier
specter of the surface wave doesn't fit the initial wave-number at all
for the second experiment. I setup k/(2*PI)=3.271 1/m, but the Fourier
analysis gives k/(2*PI) = 1.9 1/m. Any suggestion what else can be
wrong?

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

On Wed, Jan 23, 2013 at 8:03 PM, Jean-Michel Campin <jmc at ocean.mit.edu> wrote:
> Hi Sergey,
>
> Just a comment regarding this:
>> it says, that this is not yet possible with non-hydrostatic code, so you'd have to try to implement that.
> I think the documentation is not up to date. Crank-Nicholson type time
> stepping has also been implemented for the non-hydrostatic pressure gradient
> (parameter: implicitNHPress, which default to implicSurfPress)
> and is tested in the verification experiment "short_surf_wave".
>
> Cheers,
> Jean-Michel
>
> On Wed, Jan 23, 2013 at 11:19:00AM +0100, Martin Losch wrote:
>> 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>_______________________________________________
>> > MITgcm-support mailing list
>> > MITgcm-support at mitgcm.org
>> > http://mitgcm.org/mailman/listinfo/mitgcm-support
>>
>>
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