### Phases of the diffraction forces

Posted:

**Tue Jul 16, 2019 4:59 am**Hi all,

I'm running simulation with a semi-submersible, using a hybrid solution of potential flow diffraction forces and morison drag terms. The model is fixed.

I'm looking at total hydrodynamic forces and total wave-excitation loads from the diffraction in the frequency domain (singel sided amplitude spectra). As far as I understand, the remaining force is the drag force contribution. If I'm using two different WAMIT files (one with radiation, one without) I expected that the wave-excitation loads from diffraction change, and consequently the total hydrodynamic forces too. I thought that the difference, which is the drag force contribution, should remain equal. This is not the case, so I' trying to find an explanation. Does this happen because the diffraction force has a different phase than the drag force? As far as I understand, the information of the phase of the diffraction force is given in the WAMIT files, while the phase of the drag force coincides with the relative velocity. Is this correct?

Best Regards and many thanks,

Manuela

I'm running simulation with a semi-submersible, using a hybrid solution of potential flow diffraction forces and morison drag terms. The model is fixed.

I'm looking at total hydrodynamic forces and total wave-excitation loads from the diffraction in the frequency domain (singel sided amplitude spectra). As far as I understand, the remaining force is the drag force contribution. If I'm using two different WAMIT files (one with radiation, one without) I expected that the wave-excitation loads from diffraction change, and consequently the total hydrodynamic forces too. I thought that the difference, which is the drag force contribution, should remain equal. This is not the case, so I' trying to find an explanation. Does this happen because the diffraction force has a different phase than the drag force? As far as I understand, the information of the phase of the diffraction force is given in the WAMIT files, while the phase of the drag force coincides with the relative velocity. Is this correct?

Best Regards and many thanks,

Manuela