Dear all,
In Elastodyn, we need to set damping for tower and blade, please see below:
1 TwrFADmp(1)  Tower 1st foreaft mode structural damping ratio (%)
1 TwrFADmp(2)  Tower 2nd foreaft mode structural damping ratio (%)
1 TwrSSDmp(1)  Tower 1st sidetoside mode structural damping ratio (%)
1 TwrSSDmp(2)  Tower 2nd sidetoside 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：

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

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 electricaldrive dynamics (switch) {0=None; 1=ServoDyn}
0 CompHydro  Compute hydrodynamic loads (switch) {0=None; 1=HydroDyn}
0 CompSub  Compute substructural 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).
 In the document entitled “Definition of a 5MW Reference Wind Turbine for Offshore System Development”, it mentioned the Equivalent BladePitchActuator LinearSpring Constant and LinearDamping Constant. But I can not find where to defined these two parameters in servodyn file.
Regards,
Ran