Dear @Iqbal.Mohammed,
I see a couple problems:
- Normally the frequency domain is shown as a power spectral density (PSD) rather than just a raw FFT
- Your frequency discretization is not computed correctly
You can use my script that calculates the PSD directly via FFT without windowing (but with optional binning to smooth the PSD). This script, Jason_PSD.m can be found in the following forum post: User defined spectrum from routine UserWaveSpctrm in HydroDYn - #7 by Jason.Jonkman.
Best regards,