Difference Between Applying External Thrust Forces in HydroDyn and ServoDyn Without a Rotor Geometry

[YuYuJou]

Hi all,

I’m currently working on an OpenFAST model without providing any rotor geometry, but I’d still like to study how blade thrust affects the overall system response.

To do this, I tried applying external forces separately through HydroDyn and ServoDyn. However, I noticed that the simulation results differ quite significantly between the two approaches.

In both simulations, I have mean drift and drag effects enabled.

I’m wondering what causes this difference. From what I understand, HydroDyn doesn’t seem to allow specifying the exact point of force application. Therefore, in my current setup, we apply the external force and the corresponding moment at the center of mass (CoG) to represent the effect of thrust.

Could someone explain the conceptual or numerical differences between applying such external loads in HydroDyn vs. ServoDyn? And whether applying the force at the CoG is a reasonable approach in this case?

Thanks in advance for your insights!

Best regards,

[jjonkman]

Dear @YuYuJou,

Can you clarify how you are applying an external thrust force in each approach? Presumably in HydroDyn, you are applying an additional platform preload through AddF0? And in ServoDyn you are applying a force through a substructure-based StC with StC_DOF_MODE = 4? Can you clarify the details of both settings?

Best regards,

[YuYuJou]

Hi jjonkman,

Thanks for your reply!

Here are more details about my setup:

The first image shows my configuration in HydroDyn.

The second image shows the setup in ServoDyn, where I apply external forces at two points.

The .TXT file contains the input time series for the applied external forces.

Please let me know if you need more details about the parameters or input format.

Best regards,

[jjonkman]

Dear @YuYuJou,

I can only see a portion of your input files, but I don't see that the forces are specified identically and I can't confirm they are applied at the same location and orientation. To apply the forces at the same location, make sure the body reference point in HydroDyn (PtfmRefxt, PtfmRefyt, PtfmRefzt) matches (StC_P_X, StC_P_Y, StC_P_Z) in StC for a substructure-based StC.

Best regards,