Question Regarding NREL Phase VI FAST Model

Dear Jason,
I’ve few questions regarding the test10(NREL PHASE VI) model provided with FAST V8, I haven’t made any changes to model other than TMax to 1000 and few output requests.
1).Is there is any reason why the magnitude of the aerodynamic(normal) forces(~230N) is much lesser than the experimental(~765N-from Modelling of UAE turbine for FAST_AD) and why

it doesn’t look like what we expect for a normal force, but the rotor power for(test10) seems right, can we compare it directly with experiment like this or there is any mistake in my understanding.
2). Spn(i)MLy(i) seems odd and can I use Spn(i)FLxb(i) and Spn(i)FLyb(i) directly as the local edgewise and flap wise point force’s for FEM modelling.

Best Regards,
Anbazhagan

Dear Anbazhagan,

I’m not sure I can answer your first question. Can you explain where the -765 N you are referring to comes from?

Many years ago Marshall Buhl of NREL did some calibration of the airfoil data used in Test09 and Test10 to better match the UAE data, but I’m not sure of the details and I don’t know if he compared anything locally along the blade span. I doubt the airfoil data is valid for all blade stations and all wind speeds. If you plan to compare FAST against the UAE Phase VI dataset, I suggest that you review the airfoil data carefully and make your own modifications as necessary. Because the UAE rotor operates well into stall for all wind speeds above 10 m/s, when I use the UAE data, I tend to focus on wind speeds between 5 and 10 m/s where BEM tends to fair better without heavy calibration of the airfoil data.

I’m not sure what you think looks odd with the Spn(I)MLyb1 (I would expect the bending moment to increase towards the blade root), but yes, Spn(i)FLxb(i) and Spn(i)FLyb(i) can be used directly within FEM software for post-processing the local stress/strains, as summarized in the following forum topic: http://forums.nrel.gov/t/stress-analysis-of-blades-and-a-tower/489/1.

Best regards,

Dear Jason,
Thanks for the quick response,
1). The normal force(~765N) comes from Modeling of the UAE Wind Turbine for Refinement of FAST_AD document, I got the normal force coefficient at 30%span(figure 7.11) and multiplied together with dynamic pressure(figure 7.21) and chord@30%. can I take it his way right?. I’ll review the airfoil coordinates.
2).yes you’re right on the second question, it’s a mistake on my part.

Regards,
Anbazhagan

Dear Anbazhagan,

I sense a couple problems. First, the normal force coefficient in Figure 7.11 from NREL/TP-500-34755 is normal to the chord, whereas ForcN output from AeroDyn v14 is the normal to the rotor plane (the coordinate systems differ by the sum of the blade pitch and aerodynamic twist angles). Second, densityCnchord represents a force per unit length (N/m) whereas ForcN from AeroDyn v14 is force across an element (N); the latter can be converted to a force per unit length by dividing ForcN by the corresponding value of DRNodes.

I hope that helps.

Best regards,

Dear Jason

For taking the output from FAST to FEA tools for blade stress analysis which approach should we use?. For example for Test18, should we use

either a) Use the ‘ForcN’, ‘ForcT’ and ‘Pmomt’ from Test18.AD.out results as per Anbazhagan’s post below
OR b) Use the Spn1FLxb1, Spn1FLyb1 combined with Spn1MLxb1, Spn1MLyb1 and Spn1MLzb1 pitching moments from Test18.out

Approach (a) does not have Root forces and moments though.

This is also a follow-up to your reply at http://forums.nrel.gov/t/application-of-fast-output-for-static-and-fatigue-fea-run/1194/1

-Best Regards
Pravin

Dear Pravin,

The AeroDyn outputs (ForcN, etc. from AeroDyn v14) are the applied nodal aerodynamic loads, whereas the ElastoDyn outputs (Spn1MLxb1, etc.) are the reaction loads, accounting for both the applied aerodynamic loads and inertial loads integrated from the tip to the node. I would think you’d want to use the latter in your FEA tool so as to not have to compute dynamics in the FEA tool, but you shouldn’t use the ElastoDyn outputs as applied loads, because they are integrated from the tip to the node.

Best regards,

Hi Jason

Thanks for the reply. SInce in FEA tool, I want to do fatigue analysis of blade due to time varying wind load, I think from your reply that since ForcN, ForcT are applied nodal aerodynamic loads, they should be the ones I need to use as input loads in FEA. In FEA tool, I plan to distribute these forces on the airfoil nodes as there is no mesh at the aerodynamic center of airfoil

However I am getting confused by your reply to Anbazhagan in the same thread that “Spn(i)FLxb(i) and Spn(i)FLyb(i) can be used directly within FEM software for post-processing the local stress/strains

Please guide me as per your advice how I should use ElastDyn output as applied loads but “not as they are” because they are integrated from the tip to the node. How do I modify these forces to make them valid applied wind loads?

Or should I use DynPres (dynamic pressure) from FAST output and calculate lift and drag forces from Cl, Cd and use them as applied wind loads?
Sorry for the repeat query on this topic but I am blocked due to these multiple options.

-Best Regards
Pravin

Dear Pravin,

There are multiple ways to pass data from FAST to an FEA tool for stress/strain recovery. For example:

  • You can directly pass the sectional reaction loads (Spn1MLxb1 etc. from ElastoDyn) to a 2D sectional-analysis FEA tool such as VABS.
  • You can take the aerodynamic applied loads (ForcN etc. from AeroDyn v14) and apply them to a 3D FEA tool such as ANSYS, but then you’ll probably want to model spinning blade dynamics in your FEA tool.
  • You can take the sectional reaction loads (Spn1MLxb1 etc. from ElastoDyn) and derive equivalent applied loads to be applied to a 3D FEA tool such as ANSYS that result in the same reaction loads at each cross section of interest (via a static analysis).

I hope that helps.

Best regards,

Thanks a lot Jason !

-Regards
Pravin