Elastodyn damping and Equivalent Blade-Pitch-Actuator

1

Dear all,

In Elastodyn, we need to set damping for tower and blade, please see below:

          1   TwrFADmp(1) - Tower 1st fore-aft mode structural damping ratio (%)
          1   TwrFADmp(2) - Tower 2nd fore-aft mode structural damping ratio (%)
          1   TwrSSDmp(1) - Tower 1st side-to-side mode structural damping ratio (%)
          1   TwrSSDmp(2) - Tower 2nd side-to-side mode structural damping ratio (%)

and

     1   BldFlDmp(1) - Blade flap mode #1 structural damping in percent of critical (%)
     1   BldFlDmp(2) - Blade flap mode #2 structural damping in percent of critical (%)
     1   BldEdDmp(1) - Blade edge mode #1 structural damping in percent of critical (%)

It unclear for me：

  1. whether both tower damping and blade damping defined here are structural damping in percent of critical.

  2. I simulated the fixed wind turbine without wind and wave, and turn off servodyn.

          1   CompElast       - Compute structural dynamics (switch) {1=ElastoDyn; 2=ElastoDyn + BeamDyn for blades}
          0   CompInflow      - Compute inflow wind velocities (switch) {0=still air; 1=InflowWind; 2=external from OpenFOAM}
          0   CompAero        - Compute aerodynamic loads (switch) {0=None; 1=AeroDyn v14; 2=AeroDyn v15}
          0   CompServo       - Compute control and electrical-drive dynamics (switch) {0=None; 1=ServoDyn}
          0   CompHydro       - Compute hydrodynamic loads (switch) {0=None; 1=HydroDyn}
          0   CompSub         - Compute sub-structural dynamics (switch) {0=None; 1=SubDyn; 2=External Platform MCKF}
          0   CompMooring     - Compute mooring system (switch) {0=None; 1=MAP++; 2=FEAMooring; 3=MoorDyn; 4=OrcaFlex}
          0   CompIce         - Compute ice loads (switch) {0=None; 1=IceFloe; 2=IceDyn}
          0   MHK             - MHK turbine type (switch) {0=Not an MHK turbine; 1=Fixed MHK turbine; 2=Floating MHK turbine}

Then I set a initial displacement at tower top. During simulation, displacement of tower top will decay if there are some damping for blade and tower. After set both tower damping and blade damping to 0.001%, I found the displacement of tower top will not decay.

In my understanding, openfast adopts some integration methods (i.e. RK4, AB4, ABM4) to do structural calculation. If there are damping in these integration methods. My displacement of tower top should decay, even if the blade damping and tower damping are 0%. So I want to confirm whether no other damping is inherent in elastodyn and the only damping are blade damping and tower damping (when we only turn on elastodyn, and turn off all other modules).

  1. In the document entitled “Definition of a 5-MW Reference Wind Turbine for Offshore System Development”, it mentioned the Equivalent Blade-Pitch-Actuator Linear-Spring Constant and Linear-Damping Constant. But I can not find where to defined these two parameters in servodyn file.

Regards,
Ran

2

Dear @Ran.Tu,

Here are my answers to your questions:

  1. Yes, the ElastoDyn tower and blade damping ratios are specified in percent critical. That said, they are specified for the isolated tower and blade, respectively, without consideration of coupling to the rest of the turbine. The tower damping, in particular, is discussed my post dated Oct 08, 2013 in the following forum topic: Natural frequency and damping ratio calculation.

  2. The numerical integration schemes used by ElastoDyn (as you highlighted) do not include any numerical damping. The only damping inherent in the solution is structural (and aerodynamic/hydrodynamic, when enabled). So with tower and blade damping sent to 0.001% critical and only ElastoDyn enabled, the damping will be very minimal.

  3. ServoDyn does not currently include a blade-pitch actuator model (these would have to implemented by the user in their user-specified controller, if needed), so, the “equivalent blade-pitch-actuator line-spring constant and linear-damping constant” for the NREL 5-MW baseline wind turbine are not specified within ServoDyn.

Best regards,