Normal aerodynamic forces on the tower

Hi all,

I’m Lorenzo Montanari, a PhD student of the University of Parma (Italy).

I’m checking the wind turbine model, i’m writing in MATLAB language, with FAST. I’m considering the data of the 1.5 MW wind turbine you used to verify FAST.

I don’t find the data file to insert the drag coefficient and the geometrical parameters of the tower’s elements that need to calculate the normal aerodynamic forces along the tower’s height due to the wind!

How does FAST evaluate the normal aerodynamic forces of the wind on the tower ? Which drag coefficient and calculation area FAST consider ?

I thank you for your support. Best regards,

Lorenzo Montanari


The current versions of FAST and AeroDyn do not have the ability to calculate wind loads on a tower. We have plans to add it to the next major release, but that will be a while.



Dear Marshall Buhl,

     could you send me the geometrical and the mechanical data of the tower of the 1,5 MW wind turbine you modelled ? I'd need to know the radius at the nodal sections (if the section is circular) or the geometrical data (if the section has a polygonal shape)  

Usually, which drag coefficient could have a circular or polygonal section ? 0,5 ?

Do you have a free archive with geometrical and mechanical data of wind generator (blade, hub, nacelle, tower, … ) ?

I thank you for your help, best regards

Lorenzo Montanari


I did not model the 1.5 MW WindPACT turbine. Our subcontractors did it long ago. I don’t know how realistic it is, but look in the CertTest folder of the FAST archive for files with the word “baseline” in the name. Specifically, Baseline_ADAMS.dat and Baseline_Tower.dat. You can also look through the documentation of the WP 1.5 here: and search for “windpact”.

I don’t have drag data for a polygon, but I do have it for a circular one. The drag is dependent on Reynolds Number. The following data came from Roshko, “Experiments on the Flow Past a Circular Cylinder at Very High Reynolds Numbers,” Fluid Mechanics 10.

Re Cd
0.01 1.11
0.02 1.2
0.122 1.2
0.2 1.17
0.3 0.9
0.4 0.54
0.5 0.31
1 0.38
1.5 0.46
2 0.53
2.5 0.57
3 0.61
3.5 0.64
4 0.67
5 0.7
10 0.7

Dear Marshall Buhl

   i'm really grateful for your support, best regards

  Lorenzo Montanari


In upwind conditions, do you know how modelling the influence of the blades passing in the wind loading acting on the tower ?

I didn’t find papers in literature. Should i use the same theory law that i consider to simulate the “tower shadow” effect on the blades (in upwind conditions still) ?

Thank you again


Dear Lorenzo,

AeroDyn v12.58 and earlier had only one tower-influence model. This is a tower-shadow model that only creates a wake deficit behind the tower, so it only influences the aerodynamics of a rotor operating downwind of the tower.

AeroDyn v13.00.00-bjj has two tower-influence models. The default model is the same tower-shadow model from v12.58, as described above. The second tower-influence model includes a tower dam model for influence on an upwind rotor, as described in the AeroDyn Theory Manual: … Theory.pdf.

AeroDyn v12.58 and earlier have only one tower-influence model. This is a tower-shadow model that only creates a wake deficit behind the tower, so it only influences the aerodynamics of a rotor operating downwind of the tower.

See the forum post found here for information on how the upwind tower-influence model can be enabled in your model:

I hope that helps.

Best regards,

Jason’s reply was about the influence of the tower on the blades rather than the other way round. Our simulators do not currently include wind loads on the tower. We hope to add them in the future, as we’ve gotten quite a few requests for such a feature. Undisturbed wind loads are straightforward, but I personally know of no models that will account for blade passage on the wind loading of the tower. I suspect that the effect is pretty minor compared to the primary mechanical loading of the rotor/nacelle and the secondary mean wind loading on the tower. It might be interesting to come up with some sort of model to see just how much effect their might be.

I don’t know of an experiment that has tried to measure the effect. Lee Jay Fingersh took a video of a smoke test in the giant Ames wind tunnel that shows the tip vortex crashing on the tower. You may find that interesting. You can find the 22 MB video here:

Hi, thanks for your helps!

To evaluate the along-wind wind loading, Frot/nac, acting on the hub/nacelle of a upwind wind turbine with rotor plane perpendicular to the wind direction, i think that can be used the formula:


where Cd_rot is the draf coefficient of the frontal rotor/nacelle surface, d_air is the air density, Umean and Ufluct, respectively the mean and the fluctuating velocities at the hub and A is the frontal invested surface.

Does FAST consider this force Frot/nac ? I immagine that is not so easy to define Cd_rot. What do you know about ?

Thanks for you help, best regards


Dear Lorenzo,

The formulation you propose sounds reasonable for considring the drag force on the nub/nacelle. This force is not currently calculated inside FAST/AeroDyn (it is on our list of features to add in the future). I agree that specifying the drag coefficient may be difficult.

Best regards,

Dear Jason, thanks for your support, best regards


I know this information is a bit late for this thread, but it might be of academic interest. The following publications were researched as part of the European JOULE project ROTOW on rotor/ tower interactions.

Graham,J.M.R. , Brown,C.J. , Giannakidis,G. , Aerodynamic interaction between wind-turbine rotor blades and the tower, European conference on Wind Energy; Wind energy for the new millennium, Copenhagen, WIP, 2001, Pages:359-361

“ROTOW: Investigation of the Aerodynamic Interaction Between Wind Turbine Rotor Blades and the Tower and its impact on Wind Turbine Design”, DGXII, JOULE- III programme, 1998-2000.

Kind Regards,


Jason and I exchanged e-mail messages recently and I was asked to post the conversation on this forum. Here goes.

  1. would you suggest we wait till the next release of FAST (we need to start load calculations at the end of August 2012)?
  2. in your posting on 26th February 2010, you mention “a hook for user-defined tower loading”. Can you point me to the documentation for this feature?
  3. would you advise against trying to include aerodynamic drag loads arising from a full-field turbulent wind file because you know the code-writing requirements are prohibitive (we would have to consider the tower aerodynamics after the FAST simulation, as additional static loads)?
  4. would you recommend we make the required changes to FAST ourselves, expecting the new release of FAST to duplicate our internal version when it’s available?
  5. would you find it helpful for us to make the required changes to FAST and return you a modified version, for your internal checking, QA and incorporation into the next release (we’d need guidance about variable-naming conventions, code structure, hierarchy, interconnectivity)?
  6. can you direct me to the location in AeroDyn where the tower drag calculation ought to be coded?


We do not plan to release un updated version of FAST/AeroDyn with tower aerodynamic (drag) loading in the next several months, so, I suggest you add the feature yourself if you need it now.

Unfortunately, the user-defined tower loading option of FAST is not currently documented in the FAST User’s Guide. Some information is given in my September 12, 2011 post in the following forum topic: When the user-defined tower loading option is enabled, FAST will call routine UserTwrLd separately for each tower node for each structural time step. Hopefully the sample source code of UserTwrLd that is supplied with the FAST archive is self-explanatory. Note that the routine specifically says that it is not meant for tower aerodynamics, but this is only because we plan to build tower aerodynamics into AeroDyn at some point. Until then, the routine can be used for this purpose.

From within UserTwrLd, you can call AeroDyn’s WindInf_GetVelocity function to get the wind speed at any given point in space at any given time. You can use this e.g. to access the data from hub-height wind files or turbulent wind files including at points along the tower. You can, of course, use this wind speed to calculate the wind drag force that will be applied to the tower from within UserTwrLd.

We’d be pleased if you send us your code, which may help us expedite our own code-development effort.

additional discussions about tower wind loading is also in this thread:-

In addition to the publications mentioned by Mark Spring in his post above (Aug 7 2012), I found the following paper:

“Study of blade-tower interaction using a 2D navier-stokes solver”, by Franck Bertagnolio.

which studies the unsteady flow caused by passing of the blade in front of the tower + influence vortex shedding of tower on blade + influence vortex sheddig of blade on tower + interaction of all this.

If I understood correctly the conclusion was that (quote) “the influence of the airfoil on the tower was quite high”, but I found the graphs a little difficult to understand myself, so I do not quite know what quite means, and even if the pressure variation is high, the load may be small compared to the load caused by rotor thrust as Marshall noted.