FAST: Tower Drag and Tail vane inertia

Hello there,

I have a couple of questions for however is kind enough to answer them :slight_smile:

  1. I am guessing that the tail vane (tail fin) inertia about the yaw axis is calculated internally in FAST. Am I correct? (= (TFinCMyn^2 + TFinCMxn^2) * tail fin mass)
  2. The aerodynamic drag forces on the tower elements are not calculated in FAST. Do you think this leads to an acceptable approximation of the loading on the tower base in parked 50 year storm cases? (I am guessing that GL thinks this is OK as they have certified the FAST code, but I was looking after your thoughts on this)



Jason is on vacation for a couple of days, so I’ll attempt to answer your questions.

I would be stunned if something such as this were not the case.

None of our simulators currently include tower (or nacelle) aerodynamics. For the 50-year extreme event, you can easily calculate such loads outside FAST and add it to FAST’s loads, although these tower loads will not have an effect on tower bending.

We are not satisfied with this situation and we plan to add tower, nacelle, and hub aerodynamics to our next version of AeroDyn, and, once that is done, to FAST and ADAMS. Our initial plan to rewrite AeroDyn this year has been put off by a redirection to concentrate on offshore turbines. It looks like the rewrite will not happen until 2007.


Dear Thanos,

When the tail-furl angle is zero, then yes, the effective inertia of the tail vane about the yaw axis is equal to ( TFinCMxn^2 + TFinCMyn^2 )*TFinMass. The key point in this answer is the phrase, when the tail-furl angle is zero.

Please remember that the tail in FAST is modeled as two rigid bodies: (a) tail boom and (b) tail vane (fin). These two rigid bodies are fixed to each other so that they act as a single rigid body (let’s call the tail assembly). There is a revolute joint between this tail assembly and the structure connecting the yaw and tail-furl bearings (this structure is called the nacelle in FAST). The tail-furl revolute joint lies along the tail-furl axis, which is positioned by inputs TFrlPntxn, TFrlPntyn, TFrlPntzn, TFrlSkew, and TFrlTilt. (Please see the FAST User’s Guide for a detailed description of these and other inputs.) Thi tail-furl revolute joint DOF is enabled with the tail-furl DOF, TFrlDOF. Because of this DOF, the tail assembly is allowed to move relative to the yaw axis, so that its inertia about the yaw axis is time-varying–or more precisely, its inertia about the yaw axis is a function of the instantaneous tail-furl angle.

I hope that helps!

Best regards,

Hello Jason,
picking up this old thread about tower drag I have a question as well. I am using FAST 8 with AeroDyn v14.02.00c where the tower drag is activated as usual in the NewTower module in the following lines:

NEWTOWER TwrShad - Tower shadow velicity deficit or "NEWTOWER" to request the new tower model. True TwrPotent - Calculate tower potential flow (flag) True TwrShadow - Calculate tower shadow (flag) "..\towers\AeroDyn__TwrDrag.dat" TwrFile - AeroDyn tower file name (quoted string) True CalcTwrAero - Calculate aerodynamic drag of the tower at the ElastoDyn nodes.

and the corresponding file:

NREL 5.0 MW offshore baseline aerodynamic tower CD input properties. Used with AeroDyn 13.00.00 hidden tower influence feature. 61 NTwrHt - Number of tower input height stations listed (-) 16 NTwrRe - Number of tower Re values (-) 1 NTwrCD - Number of tower CD columns (-) Note: For current versions, this MUST be 1. 0.0 Tower_Wake_Constant - Tower wake constant (-) {0.0: full potential flow, 0.1: Bak model} ---------------------- DISTRIBUTED TOWER PROPERTIES ---------------------------- TwrHtFr TwrWid NTwrCDCol 0.000000 4.300000 1 0.000623 4.300000 1 ... 1.000000 3.094000 1 ---------------------- Re v CD PROPERTIES -------------------------------------- TwrRe TwrCD1 TwrCD2 TwrCD2 ... 0.010 0 0.020 0 ...

Which velocity profile is used for the drag loads on the tower?

Since the wind field (by TurbSim) I use is only generated for a square area in the height of the rotor plane I wonder what the undisturbed wind velocity in front of the tower is and especially if the wind shear is considered (e.g. by a logarithmic profile) for u_inf.

Best regards
Niklas Theilig

Dear Niklas,

TurbSim has an option (WrADTWR) to create a “lollypop” wind field with points for the tower. You must use the WrADFF option to generate a binary TurbSim file (*.bts) so the tower points will be included for AeroDyn to use.

See Figure 4 on Page 7 of the TurbSim draft manual.


One other point I’d like to make about FAST v8.03, is that InflowWind didn’t properly catch errors that should have been thrown when the program asked for wind speeds outside the grid. So, if you were using rectangular grids that covered only the rotor, it would have used 0.0 as the wind speed on the tower. This issue has been fixed in FAST v8.08, so you will need either the lollypop grids Marshall recommended or just very large rectangular grids that start near the ground (so that all the tower points are inside the grid).

Hello Bonnie and Marshall,
a late thank you for your reply.