PrnElm

Wind & Water Computer-Aided Engineering Software Tools
1

Hi

In the Aerodyn file, you can get a lot of aerodynamic data by setting PrnElm to PRINT for each of the blade elements.

But you only get 1 file. Are all the readings in the 1 file for blade 1 or are they averaged over 3 blades?

Thanks

Alec

2

Dear Alec,

In the current version of AeroDyn, the aerodynamic data of blade #1 is output to the element data file. We plan to allow all blade data to be written to a file in a future release.

Best regards,

3

Thanks Jason

In the output file, there are lots of variables, but I’m particularly interested in FN and FT.

Here is the description of FN (FT is pretty similar) from the Aerodyn UG, V12.50, December 24, 2002.

The aerodynamic force on the element normal to the plane of rotation (or blade span), not the chord line (see Figure C2). Note this is the aerodynamic force only (no inertial forces), and is the force passed back to the dynamics routine by Aerodyn.

So that sounds like the FN and FT forces are referenced to the plane of rotation, which sounds like they are linked to the hub coordinate system (figure 7 from the FAST manual V3.01, August 2005). We’re not teetering, so I think the hub coordinate system = azimuth coordinate system.

I know that in the description it went as far as underlining that the forces were not on the chord line, but then Figure C2 shows the forces on the chord line and so If I believe figure C2 it is more like the coned coordinate system (figure 8 from the FAST manual). I had expected to see them floating in mid air, away from the blade, on the imaginary plane of rotation.

We’re doing some comparisons between FAST/Aerodyn and CFD and knowing if we should be including adjustments for the precone angle in our calculations or not would be helpful.

Thanks

Alec

4

Dear Alec,

I agree that the AeroDyn User’s Guide is not clear on this. I’m quite sure that this is because AeroDyn was originally developed within the old YawDyn code, which modeled the blades rigidly, and while the code was updated, the guide was not.

AeroDyn calculates the loads in the deflected state. The coordinate system that ForcN and ForcT are calculated in is a coordinate system that is fixed in a cross section of the blade (moving with the deflected/pitching blade) that is normal and tangential to the plane of rotation (“cone of rotation” for a blade with precone) when the blades are undeflected. For an unpitched and untwisted cross section, this coordinate system would have an x-axis that is nominally downwind, a y-axis aligned with the chordline and pointing towards the trailing edge, and a z-axis pointing towards the tip. For a cross section with pitch and twist, the coordinate system would be rotated from the chordline about the z-axis by the pitch plus twist angle. Please note that while this coordinate system is used internal to AeroDyn, in the AeroDyn v13.00.00 interface (and newer), the coordinate system where information is exchanged between the structural code and AeroDyn is the body-fixed coordinate system aligned with the chordline.

I hope that helps.

Best regards,