Dear Jason,
I have some questions while I run the FAST simulation with test 13.
I was trying to simulate 1.5MW Turbine with my own generated wind field (1Hz). When I try to run the case with dt = 0.0125sec, it shows that " Supersonic" related comments ( it was initially set to 0.005sec). When I run the case with dt = 0.005sec, it runs. But when I checked the hubheight wind, it modifies the wind field too much(Not even the high frequency contents, but also the low frequency contents).
Therefore, I was trying to run the case with longer time step. While I checked the forum, I saw the post that the Test13 is not physically correct (NREL 1.5 MW - #26 by Jason.Jonkman, Am I understood correctly?). Also I saw that you done some calculation on tip inertia(?) and others. But I wasn’t sure which file and which part to change.
Do you have a file for a correct WindPACT 1.5MW model ?
If not, could you explain which part of the fst file and model file I should change?
Thanks,
JaeSang Moon
Dear JaeSang,
The structural time step (DT) required in FAST is dictated by numerical stability requirements. The structural time step of 0.005 s is suitable for the WindPACT 1.5-MW turbine with all relevant degrees-of-freedom (DOF) enabled. You cannot increase this time step without disabling relevant DOFs. For more information, see the following forum topic for more information: FAST Integrator, step size & precision.
If you wish, you can always output results at a lower resolution then the structural time step via input DecFact in FAST v7 or DT_Out in FAST v8.
I’m not sure what you are referring to when you say the time step is modifying the wind field. If the wind data and FAST time step are not equivalent, the wind data will be linearly interpolated.
I’m not sure what you mean when you say that “Test13 is not physically correct” and I don’t know what corrections you want.
Best regards,
Dear Jason,
I will try the DecFact.
What I mean by the wind field modification is that it seems like not simply interpolate. Attached figure has the time series of 1Hz (top), time series from FAST output(middle), and PSD of FAST output wind field(bottom), It seems like not just interpolate but put some characterized noise (Plz see the fluctuation of the PSD).
For the WindPACT parameter, I saw the post : Tower fore-aft modes shapes. In here, you said about tower-top mass input data being wrong. If I understood wrong, just ignore it
Thanks for the quick reply.
Jae Sang Moon
Dear JaeSang,
Your plots refer to “LES”. Were these not generated by TurbSim? If not, how did you generate the wind data and how are you specifying the wind data in FAST (what wind file format)? Which FAST output are you referring to when you say, “u, FAST, 5D”?
The post you linked to refers to a BModes model of the WindPACT 1.5-MW baseline, which didn’t have the correct tower-top mass/inertia properties set correctly. However, the toewr-top mass/inertia properties of the WindPACT 1.5-MW turbine in Test13 of the FAST CertTest are correct.
Best regards,
Dear Jason,
“LES” data is the wake turbulence captured from LES results (worked with Dr.Churchfield and Dr.Lee). I used the captured turbulence as input. I wrote my own userwind.f90 file and compiled FAST. u,v,w in the title of the plots are longitudinal, lateral vertical component. 5D is the downstream distance.
The another reason that I thought the FAST adds noise is that, When I filtered high-frequency contents, around 80%( not sure but in eyes) of the original shape was restored.
Thanks for the quick reply.
Dear JaeSang,
Routine UserWind() takes a position in space (x,y,z) and a given time (t) to calculate a local undisturbed wind velocity (u,v,w). Have you debeloped your UserWind() routine to interpolate in 4D to ensure that the u,v,w that are calculated/returned are smooth even when x,y,z and t are not coincident with points in your LES grid? Otherwise, your UserWind() file may be introducing the “noise”.
Best regards,
Dear Jason,
I attached a part of the file you mentions (4D generation). I’m not sure this is a new algorithm. But I received this part from other students in our group.
Thanks
Jae Sang Moon
Getwind.txt (19.5 KB)
Dear Jae Sang Moon,
I have not reviewed your routine carefully, but it looks to be doing interpolation. I suggest that you debug the routine to ensure that the interpolated values are as you expect. FAST itself will not modify the undisturbed wind speeds calculated by this routine, although–depending on the output selected–FAST may augment the wind speeds to calculate the influence of the tower on the flow or the induced velocities from the rotor.
Best regards,