Thank you very much for the answer and for the article. Your article explained me a lot about pretwist beam and made me find the axis orientation error in my model.
I checked the equation (21) of your article and noticed that it is in the local coordinate. When I transformed the whole equation to the global coordinate, it equals the equation I used (in my last post). The term that adds the influence of the twist on the beam is the second area product of inertia, it was exactly in that term where the error was. So you hit it right when you said that my model was not represented the coupling correctly. Many thanks for your guidance.
Now the only difference between my response and the FAST response is in the tower displacement in the plane of rotation. I am attaching an image with the answers. In the following forum topic (Why do tower top side-to-side deflections have non-zero average? - #4 by Jason.Jonkman) you comment on the application of torque at the top of the tower. I think that this error in the response occurs by the lack of the addition of this torque on the top of the tower. I did not add the rotor torque on my model, I just considered the forces. Am I right in suspecting the lack of torque at the top of the tower? How does FAST add that torque to the tower, considering only the following DOFs (FlapDOF1; EdgeDOF1; TwFADOF1; TwSSDOF1)?
Yes, I would assume that the absence of torque in your model is the cause of this difference.
The tower DOFs are related to bending, so, a bending moment (induced by rotor torque) is natural to induce bending of the tower. Regardless, FAST considers all loads (both forces and moments) in its equations of motion.
Forgive me for the stupid mistakes. I added the momentum induced by the rotor at the top of the tower. As we suspected, the model response approached the FAST response, but has a slightly higher frequency. What would cause this behavior (attached image)? I have already added aerodynamic and structural damping and the influence of gravity on the stiffness.
Could you explain a bit more what was the error in the product moment of inertia? Are the equations in the above post (Fri Nov 18, 2016 6:00 pm) correct to obtain the product inertia (in/out of plane)?
Regarding to know Cp, Ct, power and thrust load in each element of the airfoil, thought I set “Print” (PrnElm) in AeroDyn input file, the fast does not make element.plt file, I was wondering to know what would be the probable solution or is there any other way to know lift, drag, extracted power for each blade element in FAST.
In FAST v7.02, if you used enabled “PRINT” for one or more aerodynamic elements in AeroDyn, then you should get an output file with a *.elm extension that contains the AeroDyn output. Do you get that file?
I’m Gabriel a PhD student from Edinburgh and I’m investigating the effects of unsteady hydrodynamics on tidal turbine blades.
I have implemented the BEMT model used in Aerodyn v15, exactly as described by Ning et al. 2015, AIAA, (nrel.gov/docs/fy15osti/63217.pdf). I have attempted to validate my implementation with CP and CT values generated using AeroDyn v15. It is a simple steady-state case, the blade comprises of constant thickness NREL S814 profiles, zero yaw, zero pitch, default AeroDyn settings.
For CP agreement is good for tip-speed ratios (TSR) up to about 5. However, above this, my implementation is underpredicting as shown in the attached figure. For CT agreement is worse, I am overpredicting from 3.5 - 9, then underpredicting. I encounter negative CL and CD values above TSR = 8 due to high flow angles. I wonder if anyone has any insight into where I am going wrong? I have read the AeroDyn change log and believe my implementation in terms of theory and convergence method is the same. I suspect it is a case of numerical implementation.
Ning et al’s AIAA SciTech 2015 paper formed a solid basis for the implementation of BEMT within AeroDyn v15; however, there were several modifications (especially in newer versions of AeroDyn v15) that were made to ensure the robustness of the implementation within the aero-elastic solution of FAST. Unfortunately, we have not had the time/funding to publish an update to the BEMT algorithm. It is difficult for me to guess what specifically would lead to differences between your and AeroDyn’s solutions.
I would suggest simplifying the model as mush as possible (e.g. fewer analysis nodes, no skewed flow, steady inflow) and looking at the details of the solution (e.g. calculated values of the inflow angle, inductions, and lift and drag coefficients) to identify the sources of the differences. You could also look at the AeroDyn source code to see how the BEMT has actually been implemented within AeroDyn v15.
Are these relations still true for the latest version of AeroDyn v15:
Vrel_n = (Vwind_n + Vstruct_n)*(1-a)?
Vrel_t = (Vwind_t + Vstruct_t)*(1+ap)?
Where Vwind_n/t ia the local undisturbed wind speed normal/tangential to the rotor plane and Vstruct_n/t is the structural velocity with the same nomenclature.
Also, in the momentum part of the BEM equation for the thrust and torque (For thrust: dT = 4pirrhoUinf^2(1-a)adr), is Uinf still the undisturbed wind velocity at a given blade section or does it take into account the structural velocity of the section (under the assumption that the platform motion adds energy to the system)?
In AeroDyn v15, the BEM formulation does not distinguish between the wind velocity and structural velocity. The relative velocity (wind minus structure) is used in all blade-element and momentum equations.
