Induced velocities due to blade deflection

Dear all,

From comparing my hand calculations using the inductions factors computed by WT_Perf, I was not able to get a consistent Airflow angles reported by the WT_Perf.

My calculation as follows:
a =0
a’ = 0.209
r = 2.8667 m
U = 9m/s, Omega = 10.296 rpm
Precone and shaft tilt: 0deg
I’m currently using the equation below to calculate the Airflow angle:
Airflow angle = ATAN(U(1-a)/Omega*r(1+a’)) - which calculates to be 67.45deg

However WT_Perf computes the airflow angle to be 71.05deg. Based on the AeroDYN theory guide, if the blade motion is significant, we must include the local induced velocities due to the blade deflections (please see the attached fig - indicated by a red circle). Hence the deficiency in my flow angle is possibly due to not including these term in my calculation.

Could anyone please help me on how WT_Perf computes these induced velocities of the elements from blade deflections (ve-op and ve-ip)?

Any help is greatly appreciated,
Kind regards,
Thank you :slight_smile:

WT_Perf does not have blade deflections. If you need that, I suggest you use AeroDyn with FAST and look at the element files that result when you have PRINT at the end of the lines specifying the analysis nodes.


Many thanks for your response Marshall.

This did occur to be as I have not input any structural properties of the turbine, and from my understanding WT_Perf is a pure BEM method that does not account for the aeroelastic behaviour of the turbine. Hence could you please suggest a reason why there is a difference between the flow angle that I’m computing and the angles computed by WT_Perf?

A snapshot of the results from the .bed file has been attached to this post for your attention. Yellow column shows the node location inputs from the .wtp file.

Kind regards,

Dear Kisorthman,

I’m afraid I cannot figure it out without knowing all your inputs. Precone, wind shear, shaft tilt, yaw, and skewed-wake corrections all impact the calculation. If you would like me to run it in the debugger to find out why the hand calculation differs, please send me your input files.


Dear Marshal,

Thank you very much for your resent response, it was very helpful. After some WT_Perf runs, I think I have found the cause of the problem. I have been using the WT Perf version 3.04.00c-mlb, where I think the reported Airflow angles are for zero tangential induction factor because the reported AlphaDs are consistent with my hand calculation.

In comparison with the WT Perf v3.10 my hand calculated airflow angles matches very closely indeed. Hence I think there may have been a bug in the older version that was fixed in this updated version of WT Perf, also noticed that different tangential induction factors were solved for these two different versions, I presume different skewed wake and swirl corrections were used.

Sincere apologies for any inconvenience caused.
Kind regards,

Dear Kisorthman,

First off, I should tell you that we changed our version numbering scheme some years ago. The WT_Perf v3.10 used the old scheme. Under the new scheme, that would now be called v3.01.00, so it is much older than the current alpha version 3.04.00c-mlb. It has been so many years, I really don’t remember all the changes that have been done since then. But, I am pretty sure we added the shewed-wake correction since v3.10. Feel free to examine the latest alpha change log, which you can find here: