# Ploting Spectrum

Dear Sir,
When we are doing coupled analysis in FAST using WAMIT files . we have the option of choosing a regular as well as irregular waves. and for irregular waves we can use spectrum’s. My doubt is that if we want to plot that input spectrum then how can we do that .
Thank you.

Dear Anuj Gupta,

In a new version of HydroDyn that we are developing, we’ve added the ability for the wave spectrum (and other data) to be written to a summary file when HydroDyn is run. Until this version is integrated with FAST and released, you have two ways of plotting the spectrum. First, you can (quite easily) modify the source code yourself so that the wave spectrum used within HydroDyn is written to a file (or to the screen) when HydroDyn is run. Second, you can take the time-series wave elevation output from the FAST/HydroDyn simulation and calculate/plot the power-spectral density (PSD) of this time series.

Best regards,

Dear Jonkman,
actually i want to know the range of frequencies in the spectrum plot.
Because when we change the starting and finish frequency of the spectrum the response changes , so what FAST is taking i want to know.
In your technical report 2007 (Dynamics Modeling and Loads Analysis of an Offshore Floating Wind Turbine ) a theory given about spectrum but with my Hs and Tp the total energy is not coming. so i want to know the range of frequency in FAST.

Dear Anuj Gupta,

Inside FAST’s HydroDyn module, the wave-elevation spectrum is calculated at a range of frequencies and an inverse Discrete Fourier Transform (DFT) is applied to derive the wave-elevation time series from this spectrum. The time series is calculated for the range of 0 to WaveTMax-WaveDT in equal steps of WaveDT. The frequencies of the wave-elevation spectrum are calculated for the range of -WaveOmegaMax+WaveDOmega to WaveOmegaMax in steps of WaveDOmega. In this range, WaveOmegaMax = pi/WaveDT and WaveDOmega=2*pi/WaveTMax (for frequencies with the units of rad/s). Of course, because the wave-elevation time series is real-valued, the DFT is such that the values of the DFT for negative-valued frequencies are equal to the complex conjugate of the values of the DFT for positive-valued frequencies, so, only the positive-valued frequenices are “unique”.

Please note that there is also a wave cut-off frequency hard-coded into the current version of HydroDyn. A wave spectrum is often zeroed-out above some cut-off frequency in order to avoid nonphysical wave forces at high frequency (i.e., at short wavelengths). In reality, waves that have wavelengths smaller than the member diameter (high frequency) do not contribute to the net force because regions of positive and negative velocity/acceleration are experienced by the member at the same time and cancel out. To include this effect in HydroDyn, the method proposed by Massel* is implemented, in which the cut-off frequency is chosen to be proportional to the peak-spectral frequency. HydroDyn currently uses a hard-coded proportionality of 3.0 in all simulations. In the next release of HydroDyn, we plan to give the user control over the cut-off frequency (or to disable it).

*Massel, S. R., Ocean Surface Waves: Their Physics and Prediction, Advanced Series on Ocean Engineering - Vol. 11, World Scientific Publishing, Singapore - New Jersey - London - Hong Kong, 1996.

Best regards,