Environment: Data for JONSWAP and ISSC spectra

Setting the data

The ISSC spectrum is defined by giving $\Hs$ and one of the linked parameters $\{\Tz,\Tp,\fm\}$. The remaining spectral parameters are not editable: OrcaFlex calculates them automatically.

For the JONSWAP spectrum you have a number of options for setting the spectral parameters.

Automatic. You define $\Hs$ and $\Tz$, and OrcaFlex calculates all the other parameters.
Partially specified. You define $\Hs$, one of the linked parameters $\{\Tz,\Tp,\fm\}$, and $\gamma$, and OrcaFlex calculates the remaining parameters.
Fully specified. You define all the spectral parameters ($\gamma$, $\alpha$, $\sigma_1$, $\sigma_2$, and either $\Tp$ or $\fm$); OrcaFlex calculates $\Hs$ and $\Tz$.

Note:

For the partially specified option, when $\gamma$ is modified OrcaFlex does not change $\Tz$ but updates $\Tp$ (and $\fm = 1/\Tp$). This means that if you wish to specify the spectrum using $\gamma$ and $\Tp$, you must first specify $\gamma$, and then $\Tp$.

Spectral parameters

The various spectral parameters are as follows.

H_s, T_z, f_m, T_p

$\Hs$ is the significant wave height. $\Tz$ is the zero crossing period. $\Tp$ and $\fm\ ({=}1/\Tp)$ are the spectral peak period and peak frequency, i.e. those with largest spectral energy.

For the ISSC spectrum, $\Tz,\Tp,\text{ and }\fm$ are interdependent: setting the value of any one of them sets the other two to match.

For the JONSWAP spectrum,

automatic parameters mean that you set $\Hs$ and $\Tz$, and OrcaFlex calculates $\Tp\text{ and }\fm$.
with partially-specified parameters, $\Tz,\Tp,\text{ and }\fm$ are interdependent, so setting any one of them sets the other two accordingly. You must also set the (independent) value of $\Hs$.
fully-specified parameters require that you set one of the interdependent values $\Tp\text{ and }\fm$, from which OrcaFlex calculates $\Hs\text{ and }\Tz$.

Peak enhancement factor (γ)

For the ISSC spectrum, $\gamma{=}1$ always.

For the JONSWAP spectrum, if automatic parameters are chosen, $\gamma$ is calculated using formulae given by Isherwood, 1987; otherwise you can choose the value yourself.

If you use partially-specified parameters, you must take care with the order in which you set parameters. The relationship between $\Tz\text{ and }\Tp$ (or $\fm$) depends on $\gamma$, so if you change the value of $\gamma$ OrcaFlex must update either $\Tz$ or both of $\{\Tp,\fm\}$. We choose the latter, keeping the value of $\Tz$ constant. Consequently, you must set $\gamma$ before setting either $\Tp\text{ or }\fm$.

Spectral width parameters (σ₁, σ₂)

These only apply to the JONSWAP spectrum. With the fully-specified parameters option, you can choose these values; otherwise, they are fixed at the standard values of $\sigma_1{=}0.07$, $\sigma_2{=}0.09$.

Spectral energy parameter (α)

For the ISSC spectrum $\alpha$ is calculated to give a sea state with the given $\Hs$ and $\Tz$.

For the JONSWAP spectrum, with the fully-specified parameters option, you can define $\alpha$ and OrcaFlex will report the corresponding $\Hs$; otherwise, OrcaFlex calculates $\alpha$ to give a sea state with the specified $\Hs$ and $\Tz$.