effective wind speed output?

Have looked through the FAST manual, but can’t find anything…
Is there any way (possibly from AeroDyn) to output the effective wind speed from a blade span location, or even from the tip of a blade?

Dear Monika,

Please clarify what you mean by an “effective wind speed.”

Best regards,

Hi Jason,
There’s almost certainly a technical term, but I don’t know it.

What I am after is the actual incoming wind speed at a given point on a blade (preferably) or a fixed point on the rotor disk if thats an option (I only seem to be able to get hub height information from FAST) and, ideally, the effective wind speed at a blade point which takes into account the blade’s movement.

What I am trying to do is show the weakness in relying on a nacelle anemometer for control system scheduling, so need to show how the wind at a point varies from the hub height value. I am using TurbSim generated wind files, so if there is a way to extract a fixed point from those that would suffice. Ideally I would like velocity and direction.


Dear Monika,

I think the term you are looking for is “relative velocity”, which is the vector sum of the local wind speed and structural motion (including rotor rotation and vibrations). In the current version of FAST, the AeroDyn aerodynamics module has a PRINT/NOPRINT option whereby you can access many aerodynamic values local to each analysis node. While the relative velocity is not directly available, the local dynamic pressure and local Reynolds number are, and the local relative velocity can easily derived from those.

Best regards,

Dear Jason

I am using Fast 8 and AeroDyn for NREL 5MW land based turbine. AeroDyn outputs Dynamic pressure ‘DynPres01’ at various gage points. But I wan tto know the location and orientation of this pressure on the airfoil. If you see the below image, should we consider the ‘DynPress01’ as normal to the airfoil and what are the (x,y) coordinates of the point of application of this pressure?. This information is required to model the loads in ANSYS for FEA.

Also if we know the Cp values from XFOIL as a function of chord distance (x) then in the below equation is the value of “p” same as ‘Dynpres01’ from AeroDyn ??. If yes then what is the value of freestream pressure and freestream velocity that we provide in the FAST input files?

See FAT output in Test18_AD.out


Dear Pravin,

AeroDyn does not model the details of the airfoil, so the dynamic pressure output from AeroDyn is not for a specific location on the airfoil. Instead, AeroDyn’s output of dynamic pressure is calculated as 1/2AirDensV_rel^2, where V_rel is the wind speed relative to the airfoil, including the effects of undisturbed/freestream wind, induced velocities, and structural motion (including rotor rotation). Static pressure is not even considered within AeroDyn because it doesn’t create any net force on the turbine.

If you know the airfoil Cp distribution (from XFOIL or the like), you could back out “p” by the equation in your post, but V_inf should be replaced with V_rel. Because rotors typically operate at high tip-speed ratio, V_rel is normally much larger than the freestream wind speed. But please be aware that if you’ve applied rotational augmentation or an unsteady airfoil aerodynamics model, than the instantaneous Cp distribution will be different than what XFOIL or the like would provide.

Best regards,

Thanks Jason