About Calculate 1st Fore-Aft Bending Mode

Wind & Water Structural Analysis
1

Dear everyone,
I’m trying to calculate the 1st Fore-Aft Bending Mode using openfast, and I found that if I turn off the pitch direction degrees of freedom during the calculation the result looks like this.

5cf4eaab9b96847d39324ee523d72df
5cf4eaab9b96847d39324ee523d72df817×665 9.97 KB

If I turn on the pitch degrees of freedom the result is this, which is closer to the value defined by the task,

284e82bbf9febd81da04b7521d3f33c
284e82bbf9febd81da04b7521d3f33c705×536 13 KB

Here is the information from my elastdyn:
---------------------- DEGREES OF FREEDOM --------------------------------------
False FlapDOF1 - First flapwise blade mode DOF (flag)
False FlapDOF2 - Second flapwise blade mode DOF (flag)
False EdgeDOF - First edgewise blade mode DOF (flag)
False TeetDOF - Rotor-teeter DOF (flag) [unused for 3 blades]
False DrTrDOF - Drivetrain rotational-flexibility DOF (flag)
False GenDOF - Generator DOF (flag)
False YawDOF - Yaw DOF (flag)
True TwFADOF1 - First fore-aft tower bending-mode DOF (flag)
True TwFADOF2 - Second fore-aft tower bending-mode DOF (flag)
False TwSSDOF1 - First side-to-side tower bending-mode DOF (flag)
False TwSSDOF2 - Second side-to-side tower bending-mode DOF (flag)
False PtfmSgDOF - Platform horizontal surge translation DOF (flag)
False PtfmSwDOF - Platform horizontal sway translation DOF (flag)
False PtfmHvDOF - Platform vertical heave translation DOF (flag)
False PtfmRDOF - Platform roll tilt rotation DOF (flag)
True PtfmPDOF - Platform pitch tilt rotation DOF (flag)
False PtfmYDOF - Platform yaw rotation DOF (flag)
---------------------- INITIAL CONDITIONS --------------------------------------
0 OoPDefl - Initial out-of-plane blade-tip displacement (meters)
0 IPDefl - Initial in-plane blade-tip deflection (meters)
90 BlPitch(1) - Blade 1 initial pitch (degrees)
90 BlPitch(2) - Blade 2 initial pitch (degrees)
90 BlPitch(3) - Blade 3 initial pitch (degrees) [unused for 2 blades]
0 TeetDefl - Initial or fixed teeter angle (degrees) [unused for 3 blades]
0 Azimuth - Initial azimuth angle for blade 1 (degrees)
0 RotSpeed - Initial or fixed rotor speed (rpm)
0 NacYaw - Initial or fixed nacelle-yaw angle (degrees)
5 TTDspFA - Initial fore-aft tower-top displacement (meters)
0 TTDspSS - Initial side-to-side tower-top displacement (meters)
0 PtfmSurge - Initial or fixed horizontal surge translational displacement of platform (meters)
0 PtfmSway - Initial or fixed horizontal sway translational displacement of platform (meters)
0 PtfmHeave - Initial or fixed vertical heave translational displacement of platform (meters)
0 PtfmRoll - Initial or fixed roll tilt rotational displacement of platform (degrees)
0 PtfmPitch - Initial or fixed pitch tilt rotational displacement of platform (degrees)
0 PtfmYaw - Initial or fixed yaw rotational displacement of platform (degrees)
Which approach should I take and how should I correct it if both are incorrect?
Best regards,

2

Dear @Yushun.Fu,

I’m not sure I really understand your question. The platform degrees of freedom of ElastoDyn should be enabled if you have SubDyn and/or HydroDyn enabled; do you? The platform degrees of freedom of ElastoDyn will also determine the base boundary condition of the tower, and so, will naturally impact the tower natural frequencies.

Best regards,

3

Dear Jason，
I would like to ask when calculating the tower fore-aft intrinsic frequency, is it true that when calculating the tower fore-aft intrinsic frequency, is it true that in elastdyn, you only need to turn on the First fore-aft tower bending-mode DOF, and make sure that the rest of the platform degrees of freedom and TwFADOF2, TwSSDOF1, TwSSDOF2, and all of these are turned off?

Best regards，

4

Dear @Yushun.Fu,

Again, the tower natural frequencies will be impacted by the tower-base boundary condition. If your model has SubDyn and/or HydroDyn enabled, you should enable the platform DOFs in ElastoDyn when calculating the tower natural frequencies. If your model does not have SubDyn and/or HydroDyn enabled (e.g., for a land-based wind turbine with fixed foundation), you should have the platform DOFs disabled in ElastoDyn.

Best regards,

5

Dear Jason，

Thank you very much. I understand now.

Best regards，