# The form of aerodynamic load on blade

Dear Dr. Jason.

In the file NRELOffshrBsline5MW_AeroDyn_blade.dat, the Number of blade nodes used in the analysis in 19, then there are 19 B1NβFx or B1NβFy, from 0 to 61.4999, so I want to know between B1Nβ and B1N(β+1), the aerodynamic load is linear between the two points or the load is
B1NβFx from the point B1Nβ to B1N(β+1).
Another problem is alike this, the blade is divided into 49 parts, so there are 49 blade properties, and there are 49 BMassDen, FlpStff and EdgStff, I want to know between two points the property is liner or in other forms.

Thanks for your help.
Best Regards

Dear Yuanzhao,

Regarding the AeroDyn discretization, “yes” FAST currently uses a linear distribution of aerodynamic loads between nodes in the mesh-to-mesh mapping functionality that transfers these aerodynamic loads from AeroDyn to loads applied to the structural module (ElastoDyn or BeamDyn).

Regarding the distributed structural properties of ElastoDyn, “yes” ElastoDyn will linearly interpolate the mass and stiffness data from the specified input stations to the ElastoDyn analysis nodes.

I hope that helps.

Best regards,

Dear Dr. Jason.

Thank you for your help.
I want to use the structural-damping ahead of the velocity item in the equation of motion.
In Definition of a 5-MW Reference Wind Turbine for Offshore System Development,
I find the structural-damping ratio is 0.477465% critical in all modes of the isolated blade and 1% critical in all modes of the tower,
In DOWEC 6 MWPRE-DESIGN Aero-elastic modelling of the DOWEC 6 MWpre-design in PHATAS, it seesms that the critical damping is
zeta=c / cc = delda / (2 pi) and the logarithmic decrement (delta) = 3 %,
that is to say zeta=0.005, and the structural-damping is 0.0050.477465% and 0.0051%?
I feel it is two small.
so I want to know if it is the values of structural damping, if it isn’t the value, what is the specific value?

Thanks for your help.
Best Regards

Dear Yuanzhao,

I’m not sure I understand your question. 3%/(2*pi) = 0.477465%, as used for the blade in the NREL 5-MW turbine.

Best regards,

Dear Dr. Jason.

I’m sorry I didn’t make it clear。
I want to use the structural-damping ahead of the velocity item in the equation of motion.
I know the structural-damping ratio is 0.477465% critical in all modes of the isolated blade and 1% critical in all modes of the tower,
So what is the critical damping of blade and tower?
Is there a number which is the critical damping？

Thanks for your help.
Best Regards

Dear Yuanzhao,

The critical damping is related to the specified mass and stiffness distribution and mode shapes of the isolated blades and tower i.e.

c_critical = 2k/omega_n = k/(pif_n)

where,
c_critical = critical structural damping
k = generalized stiffness
omega_n = natural frequency in rad/s
f_n = natural frequency in Hz

Internally within FAST, k and f_n are calculated based on the specified mass and stiffness distribution and mode shapes of the isolated blades and tower. These data are not written directly to an output file, but you can find the associated variables in the FAST source code if need be.

I hope that helps.

Best regards,

Dear Dr. Jason.

Sorry to disturbe you.
I know FAST currently uses a linear distribution of aerodynamic loads, mass and stiffness between nodes in blade, is it same in tower?
Another problem is that in the simulation, I run the test25 in which the wind speed is 11.4m/s, Hs=2.44m，Tp=8.15s,
the out-of-plane deflection of blade, which is also called flapwise(If I’m not mistaken), is between 5-6m,
and the in-plane deflection of blade, which is also called edgewise, is between -1-0m.
I want to know the motion is in its own coordinate system or it considers surge or pitch of the platform.

Thanks for your help.
Best Regards,

Dear Yuanzhao,

Yes, FAST uses a linear distribution of aerodynamic loads, mass, and stiffness between nodes in the tower.

“Out-of-plane” and “flapwise” blade deflection are a bit different in that the flapwise blade deflection is expressed in a coordinate system that follows the blade-pitch angle. Likewise, “in-plane” and “edgewise” blade deflection are a bit different in that the edgewise blade deflection is expressed in a coordinate system that follows the blade-pitch angle.

The blade deflections output by FAST do not include the rigid-body motion of the platform; the values you report are deflections relative to the pitch axis.

Best regards,