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

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

Dear Manuela,

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,

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?