# DRNodes round off

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 ï¿½optï¿½ 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 ï¿½4.698ï¿½ to ï¿½4.7ï¿½ (in red). However FAST has NOT rounded off the rest of the values, such us the 3.002.
This conversion from ï¿½4.698ï¿½ to ï¿½4.7ï¿½ 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?

Regards,

Mikel

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( inputs, or too little precision in the specified RNodes( and/or DRNodes( values.

Both FAST and AeroDyn need to have the analysis nodes located at the centers of the elements. Thus, HubRad, TipRad, RNodes(:), and DRNodes( are not independent inputs, but must be correlated properly. For example, given HubRad and DRNodes(:), TipRad and RNodes( 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( if it is not. When trying to fix DRNodes( in your case, FAST is changing some of the DRNodes( 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,