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hello

In tomawac, the value of peak factor is about 3.3 . In the books, I red that for a big oceanic swell, the value of 7 is better. Is someone can help me about this choice ??

Thanks a lot

Hello,

In TOMAWAC, the peak enhancement factor is a parameter you can modify in the steering file, via the keyword BOUNDARY PEAK FACTOR and INITIAL PEAK FACTOR. It is used to set the JONSWAP spectrum at the boundaries and for the inital conditions.

The value of 3.3 for the peak enhancement factor is a mean value, which is generally used ad default value. In the literature, generally, the peak enhancement factor varies between 1 and 7.
As far as I know this "gamma" factor is used to take into account the fetch limited wind sea : the higher the peak factor is, the "younger" the sea-state is. With gamma=1 you get the Pierson-Moscowitz spectrum, which represents a fully developed sea-state.

If you are looking for a mathematical formulation of "gamma", you can refer to :
Donelan, M. A., J. Hamilton and W. H. Hui (1985): Directional spectra of wind-generated waves. Phil. Trans. R. Soc. Lond., A315, 509–562.
In this model, to calculate "gamma" you need to provide the wind speed U10 and the fetch length.

Hope it helps,

Giovanni

hello

thanks for your answer ! I am working with Julien Dujardin on this subjet, one of your former student. He says hello.

I find the formula for gamma, but what I don't understand is that in Tomawac you have to specify the value of the fetch and the value of U10. Why gamma isin't calculated automatically ? Is there a difference between the fetch value to specify and the fectch in the formula of gamma ??

Thanks a lot !

Hello,

in TOMAWAC the gamma factor is set directly via the steering file and it is not calculated as a function of fetch and wind speed.

You can choose among 6 options to specify the JONSWAP spectrum in TOMAWAC (via the keywords TYPE OF INITIAL DIRECTIONAL SPECTRUM and TYPE OF BOUNDARY DIRECTIONAL SPECTRUM). For more details, see chapter 8.4-8.5 of the TOMAWAC manual.
In some of those options you have to specify the fetch and the wind speed in order to calculate the Phillips constant and the peak frequency (options 1 and 2).
With other options (e.g. option 6), you just need to set directly the Phillips constant and the peak frequency, without specifying fetch and wind speed.
The calculation of the JONSWAP spectrum depending on the options chosen, is coded in the subroutines SPEINI.f and SPEJON.f.

You can always modify the subroutines in order to define also the gamma factor as function of fetch and wind speed.

My greetings to Julien !

Kind regards,

Giovanni