I have a question regarding specifying the encounter frequency in Regular Wave Surface. From "Analysis < Calculate Wave Surface < Regular Waves" I want to manually input the encounter frequency but couldn't find such option. There is a drop down list specifying only a handful of fixed frequencies. How do I manually input encounter frequencies other than those?Also how do I activate the "User defined frequency range"?
You've almost managed to answer your own question here :-)
We can only calculate the wave surface at frequencies where we have calculated the vessel response. So you would need to select the nearest encounter frequency to the frequency you need.
In your second question, I notice that you are only calculating 5 frequencies. The default number is 91 frequencies. You won't get a good representation of the response spectrum with such few frequencies. If you calculate more frequencies, then you'll be able to display the wave surface at a frequency closer to the one you want.
And finally, you can only set a user defined spectrum when using the Panel Method (not strip theory)
I would appreciate some help on the following issues:
1) In following seas (e.g. negative wave encounter frequencies), roll RAO (deg/m) may result higher than 360 degree. How these values should be interpreted?
2) Maxsurf Motions requires to set at least one wave spectrum to calculate CG RAOs. I would expect a mandatory wave spectra settings only when a response spectra (C.G Spectra) calculation is requested by the user. Could you please to comment on this?
3) Based on the reasoning poimted out in 2), I would appreciate some clarifications about why a wave encounter frequency range cannot be set by the users when only RAOs (e.g. no response spectra) are requested.
Thank you in advance
1) Thats a really high value! Remember that Strip theory is a linear theory, so if the motions take the vessel a huge way away from its neutral position then its not going to be accurate. If your vessel is doing barrel rolls in a 1m sea state then your results aren't going to be valid.
- I assume the you have Data | Non-dimensionalisation set to be using Wave Amplitude
- You need to be checking your model inputs and make sure your strip theory sections are formed correctly, mass distribution is realistic etc.
- If it is the remote location RAO then you could check that the remote location position is valid (although roll won't be affected much by location)
2) You don't have to use the Spectra if you don't need it. From a computational point of view, calculating the RAO is the slow part, calculating the spectra is quick and easy, so it isn't really costing you anything. Just include one as a dummy if you don't need it.
From a software point of view, it is probably necessary so that we have some data to populate tables with.
3) Unfortunately - we don't only calculate the RAO. So I think this question becomes a moot point?
What is your actual objective? Why is it important to have specific wave encounter frequencies?
Your reply is really appreciated.
It is correct, strip approach is a linear theory validated for moderate seas, slender bodies, etc. Following seas usually result in a no linear roll but the strip theory still may provide some decent results in realistics negative encounter wave frecuencies. Running a model from the Maxsurf library in following seas, as you said, may result in barrel rolls even for small wave amplitudes. I believe that this could be avoided if a wave frequency range could be set by the user since this could filter unrealistic wave frequencies as shown below
I have some doubts wether these extreme roll RAOs are the result of a mathematical problem in the software due to negative wave frequency or a limitation from strip theory (non lineal problem). Any additional comments on this would be great.