open TELEMAC-MASCARET The mathematically superior suite of solvers

Hi,

I read the manual but i don't understand the difference between:
- Initial/ Boundary main direction 1 and main direction 2?
- Initial/Boundary directional spread 1 or 2 ?

Thanks in advance
Regards

Florian

As explained in the documentation "A parameterised spectrum with two directional peaks can also be generated. In such case, both main
propagation directions (theta1 and theta2) as well as the weighting factor (lambda) between the two power peaks must be specified."

But in most of the cases the main direction 1 (and main directional spread 1) are only used.

Elodie

Hello,

I have a question which could be related to Florian's one..
I would like to configure the directional spreading function.
Thus I set the BOUNDARY ANGULAR DISTRIBUTION FUNCTION= 3 (in order to use the Mitsuyasu formula). Then I'd like to set my boundary directional spreading parameter S. In the Mitsuyasu function, S is different from Smax the peak value of S. So am I configuring S or Smax ? (I think S is a function of Smax so it should be Smax but in the Tomawac manual we only talk about the directional spread parameter S, so I'm a bit confused...)

Regards,
Rémi.

Hello,

In Tomawac, the Mitsuyasu formula is coded with s=constant (given by BOUNDARY DIRECTIONAL SPREAD 1 in the parameter file) in the cos^2s((theta-theta0)/2) formula. If you want to parametrize s as in Mitsuyasu et l. (1975), you will have to code it.
The routine where the cos^2s((theta-theta0)/2) formula is programmed is fsprd3.f.

Regards,

Elodie

Hello Elodie,

Thank you for your answer and sorry for the delay of mine.
I checked the fsprd3.f file but I didn't understand everything... Especially the formula of the constant DELTA1 (or DELTA2), apparantly it is using Gamma function but I don't know where the formula is coming from ?
I've read a book from Yoshimi Goda and he is prescribing some values for Smax (the peak value of S):
Smax=10 wind waves
Smax=25 swell with short decay
Smax=75 swell with long decay
can I use these values for Tomawac to configure S (BOUNDARY DIRECTIONAL SPREAD 1)(even if you told me that we are not parametring Smax...) ?

Thank you,

Regards,
Rémi

Hello,

This is explained in page 65/144 of the Tomawac documentation (for initial conditions but it is the same for prescribing boundary conditions) : a parameterised spectrum with two directional peaks can be generated. In such case, both main propagation directions (theta1 and theta2) as well as the weighting factor (lambda) between the two power peaks must be specified.
I assume you have a single peak spectrum. In this case, the value of lambda is equal to 1 (default value) and you don't need to worry about that.

About the value of Smax given by Goda, you cannot use it directly as S (given by BOUNDARY DIRECTIONAL SPREAD 1 in the parameter file) is not equal to Smax in the formula included in Tomawac. Do you know the parametrisation of the S you want to use or only the values of Smax?
Nevertheless, you can still try to use the values of S equal to 10, 25 or 75 and see if the results suit you. But what exactly do you want to model?

Regards,

Elodie

Hello,

I would like to model a swell which is coming from around 400km (so with a medium decay...) and which is propagating on a bay. I tried different values of S (10,25,75), and as expected, it is impacted a lot on the "spreading" of the swell on the bay. But I don't know the range of values I can take for S in this case of a generating area situated at around 400km from the studied area ...

Thanks again for your time,

Regards,
Rémi

Hello,

I don't know which value of Sp between 25 and 75 I should advice you. It will also depend of the characteristics of your swell (peak frequency...). But I think you should programm the S parameterization depending of fp (s=sp(f/fp)^5 for f<fp and s=sp(f/fp)^(-2.5) for f>=fp), as in the Mitsuyasu formula, in the routine fsprd3.f.

Regards,

Elodie

To complete my answer, I think that the choice of the S value in your case is more a matter of expertise than a matter of assisting a Tomawac user. I can only advice you to refer to cited reference as the Goda (1997) book or may be this article could also help you :
A COMPARATIVE REVIEW ON THE FUNCTIONAL FORMS OF DIRECTIONAL WAVE SPECTRUM, YOSHIMI GODA, Coastal Engineering Journal 1999 41:01, 1-20

Regards, and good luck for your study!

Elodie

Ok,

Thank you for your answers.
You're right, I think I'm going to code a S function. It will be better to link it with the frequency of the spectra.
It would also allow me to use the different values of Smax that I found in the references.

Regards,

Rémi.