DRNodes round off

Wind & Water Computer-Aided Engineering Software Tools
1

Hello everyone:

We are trying to run a FAST model of a 2MW wind turbine. We use 12 blade nodes in our analysis. The values of the DRNodes in our AeroDyn input file are shown for each node in the following column:

DRNodes
1.938
2.382
[color=red]4.698
3.002
[color=red]4.698
3.002
[color=red]4.698
3.002
[color=red]4.698
3.002
0.848
1.501

After trying to perform the simulation, the following values for the DRNodes can be found in the �opt� output file:

DR(m)
1.938
2.382
[color=red]4.7
3.002
[color=red]4.7
3.002
[color=red]4.7
3.002
[color=red]4.7
3.002
0.848
1.501

These values are slightly different from our input data in the AeroDyn file. It seems that Fast has rounded off our input value �4.698� to �4.7� (in red). However FAST has NOT rounded off the rest of the values, such us the 3.002.
This conversion from �4.698� to �4.7� means an increase in the total sum of the DRNodes. We suspect that this is the reason why FAST aborts with the following message when trying to run our model:

HubRad + SUM( DRNodes( : ) ) is not equal to TipRad

Why is FAST rounding off our AeroDyn input values?

Thanks in advance.
Regards,

Mikel

2

Dear Mike,

The error, “HubRad + SUM( DRNodes( : ) ) is not equal to TipRad” is generally caused by a miscorrelation between the HubRad, TipRad, RNodes(:), and DRNodes(:slight_smile: inputs, or too little precision in the specified RNodes(:slight_smile: and/or DRNodes(:slight_smile: values.

Both FAST and AeroDyn need to have the analysis nodes located at the centers of the elements. Thus, HubRad, TipRad, RNodes(:), and DRNodes(:slight_smile: are not independent inputs, but must be correlated properly. For example, given HubRad and DRNodes(:), TipRad and RNodes(:slight_smile: can be determined uniquely. FAST checks to ensure that HubRad, TipRad, RNodes(:), and DRNodes are correlated correctly (or, at least, checks to ensure that it is accurate within enough precision), and tries to fix DRNodes(:slight_smile: if it is not. When trying to fix DRNodes(:slight_smile: in your case, FAST is changing some of the DRNodes(:slight_smile: values, and triggering the error you are receiving in the process.

When I build a model, I generally start with HubRad and TipRad, then separate the blade into a number of element spacings in order to determine DRNodes(:). Then using this information, I find the centers of the elements in order to determine RNodes(:). I suspect if you perform an equivalent set of calculations, respecify the inputs, and rerun FAST, your problem will be fixed.

Please let me know if this works.

Best regards,