2nd Order Waves- proper upper cut-off frequency for sum-frequency terms (WvHiCOffS)

Wind & Water Computer-Aided Engineering Software Tools
1

Hello,

I would like to include 2nd- order wave kinematics into my simulation.

I would choose the lower and upper cut-off frequencies for the difference- frequency terms as recommended in the hydrodyn manual (WvLowCOffD=0, WvHiCOffD=2 π/Tp). Is that correct?

For the second-order sum- frequency terms the WvLowCOffS is advised to be set to 2 π/Tp. It further says that setting a proper upper cut-off frequency is very important. Since I have no experience in applying 2nd order wave kinematics to FAST-simulations, I don’t know how to figure out what would be a proper WvHiCOffS. Does anyone have any suggestions how to determine the upper cut-off frequency?

Thank you and best regards,

Manuela

2

Dear Manuela,

Your understanding is correct.

I don’t have a simple formula for setting WvHiCOffS. But as suggested by the draft HydroDyn User’s Guide and Theory Manual (wind.nrel.gov/nwtc/docs/HydroDyn_Manual.pdf), you’ll want to set WvHiCOffS high enough to ensure convergence of the second-order summations and set WvHiCOffS low enough to minimize computational expense and to avoid unphysical “bumps” in the wave troughs, prevent nonphysical effects when approaching of the breaking-wave limit, and avoid nonphysical wave forces at high frequencies (i.e., at short wavelengths) when using a strip-theory solution.

Best regards,

3

Good morning,

I have a doubt regarding cut-off frequency for 2nd-order wave kinematics.
According to HydroDyn’s user guide, the values of these frequencies should be in the following range: WvHiCoffD<= 2*pi/Tp ; WvLowCoffS>= 2*pi/Tp. However, when I check the baseline input files for HydroDyn (Test19, ‘NRELOffshrBsline5MW_OC3Monopile_HydroDyn.dat’, wp=2*pi/10=0.6283), the values for these inputs are:
WvHiCoffD=3.04292 (4.84 times the value of wp)
WvLowCoffS=0.314159 (0.5 times the value of wp)

I would like to know why these values don’t follow the recommendation of the user guide. There is any additional analysis to perform to properly set these cut-off frequencies?

Thanks in advance for your kind reply.

DBL

4

Dear David,

I don’t recall why WvHiCoffD and WvLowCoffS were set as such in Test19, but the CertTest is more used to test functionality rather than to represent best practice. I would suggest following the modeling guidance rather than using the CertTest files as good examples. You may want to to rerun Test19 with WvHiCoffD and WvLowCoffS set according to the modeling guidance; my guess is that you would not see much change in the results, but the solution should be a bit less computationally expensive.

Best regards,