hydrodynamic analysis of multi body platform

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Joshua.Cutler
Posts: 11
Joined: Wed Jun 03, 2020 9:23 am
Organization: Liverpool John Moores University
Location: UK

Re: hydrodynamic analysis of multi body platform

Postby Joshua.Cutler » Tue Apr 06, 2021 1:28 pm

Dear Jason,

It does indeed start at zero frequency, the maximum frequency due to limitations of AQWA mesh is 4rad/s and the data does converge to infinite frequency I believe. My only suspicion is that for a couple of high frequencies there is some negative damping terms, could this be the source of the instability even though they are very small in magnitude?

I have attached the radiation damping graphs for my model.

I will look into the kernel.

Kind regards,
Attachments
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captr 1.PNG (63.64 KiB) Viewed 96 times

Joshua.Cutler
Posts: 11
Joined: Wed Jun 03, 2020 9:23 am
Organization: Liverpool John Moores University
Location: UK

Re: hydrodynamic analysis of multi body platform

Postby Joshua.Cutler » Tue Apr 06, 2021 2:53 pm

Dear Jason,

Please see attached. For all rigid body modes they all converge to zero albeit it some take longer than others. I will increase RdtnTMax in Hydrodyn to see if this solves the problem.

Kind regards,

Josh
Attachments
IRF sway.PNG
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IRF Surge.PNG
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Jason.Jonkman
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Location: Boulder, CO
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Re: hydrodynamic analysis of multi body platform

Postby Jason.Jonkman » Tue Apr 06, 2021 3:38 pm

Dear Josh,

Regarding the damping plots, I see the damping converging to 0 at low frequency. Most curves also converge to zero at high frequency, but this is not the case for B(6,6) (yaw-yaw), at least by 4 rad/s. I'm also a bit surprised by the lack of smoothness in the data and very distinct peaks, but perhaps this is a result of the discretization you used or something you are expecting for your catamaran? It may help to manually smooth the data, e.g., to ensure it tapers to zero at high frequency.

Regarding the kernel plots, are these the values computed within HydroDyn or are these something you calculated yourself? I don't see the results decaying to zero by 100 s, which looks a bit odd to me (we often set RdnTMax = 60 s, but that assumes the kernel has reached zero by then). Does the kernel change if the damping data is smoothed forced down to zero at high frequency?

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Joshua.Cutler
Posts: 11
Joined: Wed Jun 03, 2020 9:23 am
Organization: Liverpool John Moores University
Location: UK

Re: hydrodynamic analysis of multi body platform

Postby Joshua.Cutler » Tue Apr 06, 2021 3:54 pm

Dear Jason,

By increasing the RdtnTMax the simulation can now complete and the instability has gone.

The catamaran shape is very basic to say the least and has not been hydrodynamically optimized. I will put your comments across to my supervisor. And thank you for suggesting manually smoothing, I did wonder this at one point but did not know if that was a "thing".

Regarding the kernel plots, I actually extracted them from WEC-Sim . I used this a while back and it was a fast way for a rough representation. I increased the RdntTMax to 100s.

I will manually smooth the damping coefficients and see the effect. Will keep you posted.

Many thanks for your help.

Kind regards,

Josh


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