Distributed blade property

Dear all,

I am confused by the NRELOffshrBsline5MW_Blade.dat

---------------------- BLADE ADJUSTMENT FACTORS --------------------------------
          1   FlStTunr(1) - Blade flapwise modal stiffness tuner, 1st mode (-)
          1   FlStTunr(2) - Blade flapwise modal stiffness tuner, 2nd mode (-)
    1.04536   AdjBlMs     - Factor to adjust blade mass density (-)  !bjj: value for AD14=1.04536; value for AD15=1.057344 (it would be nice to enter the requested blade mass instead of a factor here)
          1   AdjFlSt     - Factor to adjust blade flap stiffness (-)
          1   AdjEdSt     - Factor to adjust blade edge stiffness (-)
---------------------- DISTRIBUTED BLADE PROPERTIES ----------------------------
    BlFract      PitchAxis      StrcTwst       BMassDen        FlpStff        EdgStff
      (-)           (-)          (deg)          (kg/m)         (Nm^2)         (Nm^2)
0.0000000E+00  2.5000000E-01  1.3308000E+01  6.7893500E+02  1.8110000E+10  1.8113600E+10
3.2500000E-03  2.5000000E-01  1.3308000E+01  6.7893500E+02  1.8110000E+10  1.8113600E+10
1.9510000E-02  2.5049000E-01  1.3308000E+01  7.7336300E+02  1.9424900E+10  1.9558600E+10
3.5770000E-02  2.5490000E-01  1.3308000E+01  7.4055000E+02  1.7455900E+10  1.9497800E+10
  1. what does AdjBlMs mean? Will BMassDen be multiplied by the coefficient AdjBlMs during calculation?

  2. Now I am using aerodyn15. Do I need to change AdjBlMs to 1.057344?

  3. Is there any documentation describing the meaning of parameters in NRELOffshrBsline5MW_Blade.dat. What are PitchAxis and StrcTwst?

  4. Could anyone please share me the input file of BModes for this 5MW blade? I want to do a simulation to check whether I can get the same mode shape coefficients for blade.

Regards,
Ran

Dear @Ran.Tu,

Here are my responses:

  1. Yes, AdjBlMs is a factor to (linearly) scale BMassDen. It is typically used if the integration of BMassDen along the blade yields a total blade mass that is slightly different than the expected value (due to discretization, integration error, or inexact properties).

  2. If the total blade mass is different than you expect, you can adjust AdjBlMs accordingly.

  3. PitchAxis is not used when AeroDyn v15 is enabled, e.g., as discussed in my post dated Jul 13, 2022 in the following forum topic: Stress Analysis of Blades and a Tower. The structural twist (StrcTwst) defines the orientation of the principle axes of bending

  4. The mode shapes for the NREL 5-MW baseline blade were not originally developed using BModes. See the following forum topic for a related discussion: Blade mode shapes with BModes.

Best regards,

Thank you @Jason.Jonkman .

  1. Is StrcTwst is actually the same definition and value with BlTwist in NRELOffshrBsline5MW_AeroDyn_blade.dat?

  2. Below figure is the cross-section of blade. It explained the BlTwist (it is the orientation of the local chord about the vector normal to the plane of the airfoil). I want to know whether the origin of these axis is the aerodynamic center (reference point for the airfoil lift and drag forces)?

  3. It seems that Aerodyn15 defines all blade data on aerodynamic centers of cross-section. For example, BlCrvAC specifies the local out-of-plane offset of the aerodynamic center normal to the blade-pitch axis. For Elastodyn, I am guessing whether BlFract and StrcTwst are also defined based on aerodynamic centers. And FlpStff is the bending stiffness around local blade chordline with origin at aerodynamic centers. EdgStffin is the bending stiffness around the in-plane axis perpendicular to local blade chordline with origin at aerodynamic centers

Regards,
Ran

Dear @Ran.Tu,

Here are my answers:

  1. The structural (StrcTwst) and aerodynamic twist (BlTwist) may be independent in general, but they are the same in the NREL 5-MW baseline blade.

  2. Yes, the origin shown in this figure is the aerodynamic center. See also the AeroDyn Blade Geometry here: 4.2.1.2. Input Files — OpenFAST v3.2.0 documentation.

  3. The structural properties set in ElastoDyn are defined relative to the pitch axis, not the aerodynamic center. (ElastoDyn assumes the blade is straight, with no offsets in structural properties from the pitch axis.)

Best regards,

Hey @Jason.Jonkman ,

  1. Pitch axis is what I am going to figure out. In Elastodyn, blade is assumed straight. Pitch axis will have an intersection point with cross-section. I want to understand what this intersection point is. Mass center of blade cross-section? Geometric center of blade cross-section?

  2. As Elastodyn needs to define FlpStff and EdgStff for blade, I guess FlpStff is the bending stiffness about local blade chordline (please see above figure where local blade chordline is demonstrated). But I don’t know about which axis EdgStff is defined. The axis for EdgStff should be perpendicular to the axis for FlpStff. These two axes should have an intersection point. The intersection point is mass center of blade cross-section or geometric center of blade cross-section?
    If I know where is the intersection point. Then I will be able to locate the axis for EdgStff and calculate the EdgStff by myself.

Regards,

Dear @Ran.Tu,

Here are my answers.

  1. In ElastoDyn, there are no offsets of the mass and elastic centers from the pitch axis. So, the center of mass and tension center lie along the pitch axis.

  2. The FlpStff and EdgeStiff should be specified about the principal axes of bending, which are orthogonal to each other. The axes for flapwise stiffness need not be aligned with the chordline (in general, StrcTwst and BlTwist may differ), although this is the case for the NREL 5-MW baseline wind turbine (StrcTwst = BlTwist at the same radial location for this turbine). The intersection point is the tension center, which again in ElastoDyn must also be aligned with pitch axis and mass center as well.

Best regards,

Thanks for your reply :grinning:

Dear @Jason.Jonkman ,

I found that aerodynamic center of blade cross-section is defined twice.

In NRELOffshrBsline5MW_AeroDyn_blade.dat, aerodynamic center is defined by BlCrvAC and BlSwpAC. But in Airfoils folder, it is also defined in “NumCoords”. The first point in “NumCoords” must always locate the aerodynamic center.

Although above two aerodynamic centers are defined in different reference frame, I am confused that these two aerodynamic centers must be located at the same position?

If they are different, which one is really used to calculate the aerodynamic load in Aerodyn15?

Or “NumCoords” in airfoils folder actually is only for surface visualization, and will not affect the calculation results?
Regards,
Ran Tu

Dear @Ran.Tu,

The values of BlCrvAC and BlSwpAC define the aerodynamic center relative to the pitch axis and are used in the aerodynamic calculations (e.g., offsets from the pitch axis will create pitching moments). Within the airfoil data file, the shape of the airfoil is defined relative to the aerodynamic center, not the pitch axis, but these data are only used for visualization, not in any calculation.

Best regards,