upwind and downwind turbines

Hi everybody,

I would appreciate it if any member could let me know how in FAST primary input file, we can determine the modeled turbine is upwind or downwind.

Best Regards

Dear Sara,

In the FAST primary input file, the only difference in the specifications of an upwind or downwind rotor are in the signs of the OverHang, ShftTilt, PreCone, and ShftGagL inputs (and perhaps HubCM). All but ShftGagL should be negative for an upwind rotor; reversed for a downwind rotor. All coordinate systems, as well as the azimuth angle and pitch angle remain the same between upwind and downwind systems (e.g, the x-axis always points nominally downwind regardless of whether the rotor is upwind or downwind of the tower). See Figures 14 and 16 from the FAST User’s Guide for more information.

I hope that helps.

Best regards,

First of all, thanks for this great tool and thanks for all the discussions and answers in the forum!
I have a question concerning the parameter NacCMxn for upwind and downwind turbines. According to the previous post the sign of NacCMxn does not need to be changed when going from an upwind to a downwind configuration. This agrees with the theory manual, where NacCMxn is positive for the downwind turbine (Figure 14) and also positive for the upwind turbine (Figure 16). However, when looking at the files in the CertTest folder the upwind turbines WP 1.5 MW and SWRT have negative NacCMxn and the downwind turbine AWT-27CR2 has positive NacCMxn.
Should NacCMxn be generally positive for downwind turbines and negative for upwind turbines?
Thank you.

Dear Marianne,

In FAST, NacCMxn is defined positively downwind for both upwind and downwind turbines. The actual value for NacCMxn, though, will depend on the turbine (likely the location of the generator within the nacelle).

Best regards,

Dear Marianne,

Just to clarify for what I think you are asking, if you take an existing upwind turbine and convert it to a downwind turbine without changing where the CM is within the rotating nacelle coordinate system, you would need to change the sign of the CM. So, if you had an upwind system with the CM behind the tower due to a heavy generator, that generator would be upwind when you yawed it downwind and the CM would go upwind with it. As the input parameter does not care if you are modeling an upwind or downwind turbine (it’s always positive downwind of the turbine), you should change the sign on that parameter.

Dear Jason and dear Marshall,

thanks a lot for you answer! It is very clear now.

Kind regards,

When I look at the results, I see the same sign for the GenSpeed and Rotspeed? Shouldn’t they have opposite in signs (+ , - or -, +) in any simulation?


Dear Kumara,

In the ElastoDyn module of FAST v8, the convention is that GenSpeed and RotSpeed have the same sign if the gearbox ratio is positive valued (GBRatio > 0).

Best regards,

Dear all,

I would like to know if something has changed since 2012 in order to simulate a downwind turbine, or is it still enough to change the overhang, tilt and coning signs.
On top of that, i want to turn the rotor anti-clockwise, but i could not find any parameter referring to this. Should i change the sign of the gearbox ratio (this is how it is done in other softwares). Besides this change, would it be necessary to modify the airfoil polars?

Kind regards


Dear @Roberto.Echeverria.

The way of specifying a downwind rotor has not changed…it is still done by changing the sign of OverHang, etc.

FAST / OpenFAST can only model wind turbines rotating in the clockwise direction (when looking downwind). For more information, see my response to a similar question in my post dated Feb '13 in the following forum topic: Coordinate System in FAST - #9 by Neelabh.Gupta.

Best regards,