Dear Jason,
Recently I activated the blade gages in FAST to extract internal reactions in various blade sections for blade design issues. While reporting my results I noticed some unreasonable jumps in loading values between Blade Root and its local Spans. I suspect that this is due to the numerics of how the FAST calculations are performed when pitch angle is at its limiting values i.e. 0 and 90 degrees. Here I added a graph of internal shear forces with respect to time in which Fx and Fy are along flap and edge directions respectively. Note that when pitch angle is 0 the discrepancy is seen in Fx and when it is 90 the discrepancy is seen in Fy but during the change of pitch angle from 0 to 90 there is no large difference between the forces. Please let me know if there is an explanation for this issue. Thanks in advance.
Best Regards.
Dear Yahya,
The difference in the loads between the blade root and local span gages (blade analysis nodes) can be explained as follows. At the root of blade 1, FAST only permits output of loads in the “b1” and “c1” systems, so, neither are rotated by the structural twist. The only difference between the “b1” system and the “c1” system is the rotation of the pitch angle of blade 1. The “Lb1” system (local blade system) available at the blade analysis nodes is rotated additionally by the structural twist (and by the deflection of the blade). With a simple vector rotation using the structural twist angle at the root, you should be able to compare the root loads in the b1 system with the near-to-the-root loads in the Lb1 system.
I hope that helps,
Dear Jason,
Thank you very much for your reply. I tried to consider your points of view in my analysis so I set the structural twist for all blade nodes to zero (i.e. the AeroTwst column in the AeroDyn primary file). Also I disabled the blade flap- and edge-wise displacement DOFs. But the discrepancy is still there. The result of the simulation is depicted in the following figure.
Regards.
Dear Yahya,
There are two different blade twists specified in FAST. One is the aerodynamic twist (AeroTwst) defined in the AeroDyn input file; the other is the structural twist (StrctTwst) defined in the blade input file of FAST (for FAST v7) or ElastoDyn (for FAST v8). It is the later–the structural twist–that is the cause of this discrepancy.
Best regards,
Dear Jason,
Exact to the point! That resolved the problem totally. Thank you very much.
Best Regards.
Hi Yahya, Jason
I am using Fast V8, I want to know which parameter in the ElastoDyn file “NRELOffshrBsline5MW_Onshore_ElastoDyn.dat” I should change to make the strctural twist OFF?
We have below parameters in this file related to twist.
Also there is no flag to make them OFF/ON
“OoPDefl1” - Blade 1 out-of-plane and in-plane deflections and tip twist
“IPDefl1” - Blade 1 out-of-plane and in-plane deflections and tip twist
“TwstDefl1” - Blade 1 out-of-plane and in-plane deflections and tip twist
“TTDspFA” - Tower fore-aft and side-to-side displacements and top twist
“TTDspSS” - Tower fore-aft and side-to-side displacements and top twist
“TTDspTwst” - Tower fore-aft and side-to-side displacements and top twist
-Regards
Pravin
Dear Pravin,
The OopDefl1 etc. variables you list in your post are outputs of ElastoDyn – they do not impact how the model is set up or run.
To remove structural twist, you should set input StrcTwst in the ElastoDyn blade file to zero.
Best regards,
