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Hm0 is too high, and FPD does not capture non-linear effects
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TOPIC: Hm0 is too high, and FPD does not capture non-linear effects

Hm0 is too high, and FPD does not capture non-linear effects 8 years 5 months ago #22062

  • huyquangtran
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Dear All,

I simulate wave-current interaction by using a coupling model: TELEMAC2D-TOMAWAC, but I think the results do not present correctly actual phenomenon in my area. Particularly, the modelled significant wave height is too height, and the modelled peak period underestimates the measurement. Apparently, the modelled peak period does not capture sudden changes.

In my location, the current speeds are very strong (up to 3.5m/s), so in the model I have added both keywords: DISSIPATION BY STRONG CURRENT = 2 and NON-LINEAR TRANSFERS BETWEEN FREQUENCIES = 1; However, it does not help to improve the modelled results.

Does anyone suggest me how I can improve the results?

Thank you in advance!

Best Regards

Huy
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Hm0 is too high, and FPD does not capture non-linear effects 8 years 5 months ago #22064

  • cyamin
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Dear Huy,

Have you tried calibrating the model by changing the keywords
  • DISSIPATION COEFFICIENT FOR STRONG CURRENT
  • SATURATION THRESHOLD FOR THE DISSIPATION
that apply to the Westhuysen formulation (DISSIPATION BY STRONG CURRENT=2)?

Regards,
Costas
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Hm0 is too high, and FPD does not capture non-linear effects 8 years 5 months ago #22065

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Thanks Costas,

I have not tried these keywords as I analysed the measured data I found there is no blocking waves in opposing currents.

I am trying these keywords now, but is there any other keywords to think of?

Best Regards

Huy
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Hm0 is too high, and FPD does not capture non-linear effects 8 years 5 months ago #22066

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Make sure CONSIDERATION OF SOURCE TERMS is enabled. Otherwise, wave blocking (and a lot other effects) are not taken in account. You can post your cas files for other possible advice.

Regards,
Costas
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Hm0 is too high, and FPD does not capture non-linear effects 8 years 5 months ago #22067

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Thanks Costas,

Here are my steering files. Could you have a look?

Best Regards

Huy
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Hm0 is too high, and FPD does not capture non-linear effects 8 years 5 months ago #22068

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Since you are coupling TOMAWAC with TELEMAC2D, the timestep for TOMAWAC is defined in the TELEMAC2D cas file as:

TOMAWAC timestep = TELEMAC2D timestep * COUPLING PERIOD FOR TOMAWAC

From your cas files, the TOMAWAC timestep is 30*132480 which is way too large and probably not what you intended. You should set COUPLING PERIOD FOR TOMAWAC =10 to get a TOMAWAC timestep of 300s.

Also try to increase NUMBER OF ITERATIONS FOR THE SOURCE TERMS. 100s for one iteration is large. How coarse/fine is your mesh?

Regards,
Costas
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Hm0 is too high, and FPD does not capture non-linear effects 8 years 5 months ago #22069

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Here is a guideline that I have noted down from another TOMAWAC post in the forum:
As rule of thumb, you should set the time step in order to have a Courant number between 1 and 2.

The Courant number Cr is given by : Cr=Cg(fp)*Dt//Dx

Where Cg(fp) is the group velocity associated to
the peak frequency, Dt is the time step and Dx is the size of the
smallest element of the spatial mesh.

The group velocity can be calculated from the dispersion relation. In
the deep water case, the group velocity is approximately equal to 0.78/fp, while in shallow waters Cg=(g*d)0.5, where g is the gravity acceleration and d is the water depth.

Regards,
Costas
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Hm0 is too high, and FPD does not capture non-linear effects 8 years 5 months ago #22071

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Dear Costas,

Regarding time step, I have read the post you mentioned. However in my model waves propagating from the ocean (my domain extended 15km to the ocean), so I consider waves propagate into intermediate depth. Furthermore, I have calculated Cg based on measured data, Cg max ~ 11m/s; Cg min ~3m/s.

My question is: how can I select a time step for my case?

Best Regards

Huy
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Hm0 is too high, and FPD does not capture non-linear effects 8 years 5 months ago #22070

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Dear Costas,

Your suggestions are really helpful!

My grid ranges from 100m (in the area of interest) to 1500m in the others.

Regarding COUPLING PERIOD FOR TOMAWAC, I though if I want to get a coupling period of 46 days (3974400s), I set up total time from TELEMAC 2D = 30 x 132480 that is equal to 300x13248 in TOMAWAC. Therefore, the COUPLING PERIOD FOR TOMAWAC in TELEMAC steering file should be 132480. Is it wrong?

By the way, which iteration is suitable for my model?

Thanks & Best Regards

Huy
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Hm0 is too high, and FPD does not capture non-linear effects 8 years 5 months ago #22072

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Dear Huy,

What you did is not right. This is not how coupling works.

From the TOMAWAC manual:
COUPLING PERIOD FOR TOMAWAC (variable PERCOU_WAC), which specifies every how many TELEMAC time steps TOMAWAC is called:
o The TOMAWAC time step must be less or equal TELEMAC simulation time step multiplied by PERCOU_WAC.
o The ratio of the two time steps must be an integer.

So, I suppose that you don't define a TIMESTEP is TOMAWAC, this is taken as
I mentioned earlier:
TOMAWAC timestep = TELEMAC2D timestep * COUPLING PERIOD FOR TOMAWAC

Supposing that the minimum freq is 0.06 (from your cas file), then the max TOMAWAC timestep should be approx 15s.

So, as a starting point:

In TELEMAC2D cas:
I suggest that you use the keyword DURATION (instead NUMBER OF TIMESTEPS) in TELEMAC2D. Set DURATION = 3974400, TIMESTEP = 15 and COUPLING PERIOD FOR TOMAWAC = 1.

In TOMAWAC cas:
TIMESTEP = 15
NUMBER OF ITERATIONS FOR THE SOURCE TERMS = 1

If everything is OK, then you can try increasing TIMESTEP and COUPLING PERIOD FOR TOMAWAC for as long as you get acceptable results.

Regards,
Costas
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Hm0 is too high, and FPD does not capture non-linear effects

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