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Adding blade flexibility without doing structural analysis?
Hi!
We are investigating the opportinities for adding flexibility to our wind turbine blade in Qblade, without doing a structural analysis (without importing a structural input file). If this is possible, would you please share how to achieve this?
Thank you in advance.
Hi Vanessa,
Blade flexibility cannot be modeled in a physically meaningful way without providing structural input data.
Aeroelastic simulations require structural properties such as stiffness, mass distribution, and damping in order to compute blade deflections and their interaction with the aerodynamics. If no such data is provided, the blade is treated as rigid.
QBlade does include the QFEM module, which allows users to define simplified, isotropic structural properties directly within the software.
While this functionality is primarily intended for educational or conceptual studies and is not suitable for realistic or detailed blade flexibility analyses, particularly for full-scale wind turbine blades, it can still serve as a useful starting point and enables the generation of structural inputs, and thereby aeroelastic simulations, with relatively little effort.
Best regards,
David
Hi!
Thank you for your previous answer.
We have another question regarding how to obtain airfoil polars for a scaled‑down wind turbine. We are using the sample model NREL_5MW_Blade and have scaled it down to a lab‑scale model with a blade length of 0.45 m. We calculated the Reynolds numbers for each blade element in the scaled model, using the local relative velocity from the formula shown in the attached file.
We are wondering whether we should use the same freestream velocity in the lab scale as the one used in the full‑scale steady BEM analysis, considering our goal is to compare a full‑scale turbine to a lab‑scale model.
Would it also be reasonable to assume the theoretical values of a and a’ when calculating the Reynolds numbers for the lab‑scale model?
We calculated the angular velocity (ω) using the tip‑speed‑ratio formula. Should we use the same TSR value as the full‑scale turbine when the goal is comparison, or does a difference in TSR not matter significantly?
For the down scaling we did the above with values:
- Freestream velocity V1=12 m/s
- a=1/3
- a′=0
- ω=173.33 rad/s (calculated with TSR = 6.5)
This resulted in relatively small Reynolds numbers in the range 34,649–93,866. We then generated lift and drag polars for each blade element and extrapolated them to 360°. However, some polars did not contain enough datapoints or did not have at least five datapoints in the range 0∘<α<20∘. Could this be due to the low Reynolds numbers?
We proceeded to the Blade Design Module and replaced the original polars with the ones we calculated. For the airfoils where extrapolation was not possible, we kept the original full‑scale polars.
We then performed a steady‑state BEM analysis and obtained the Cp curve. We observed that the maximum power coefficient of the lab‑scale model was lower than that of the full‑scale turbine. Considering the assumptions we made when calculating the polars, and the fact that not all polars could be extrapolated, how reliable would you consider these results?
Best regards,
Vanessa, Emma, Bjørnar, Belinda
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Hi,
for comparing full-scale vs lab-scale, the key is to match the non-dimensional TSR. The wind speed at which you operate in full- and lab-scale depends on your lab setup. Using 12 m/s is a sensible choice.
You can estimate the Reynolds numbers manually (by assuming optimal induction), but you can also get the spanwise Re distribution directly from a BEM simulation in QBlade (check the “Aerodynamic Blade Graph”).
The Reynolds numbers you report seem plausible. In general, XFOIL can have issues generating polars at low Re, but convergence problems can also be related to a low quality airfoil discretization. One trick that often helps at low Re is to force transition at the leading edge (set forced transition on top and bottom to 0, instead of the default value of 1).
The lower Cp you get for the lab-scale model is consistent with theory – the airfoils are simply less efficient at lower Reynolds numbers.
Best regards,
David
Hi!
Thank you, David.
About our previous question regarding adding blade flexibility, we are doing a more conseptual study with blade flexibility rather than an realistic, so using QFEM will be sufficient for our study. We are however wondering how to use the data from QFEM in the turbine structural model. We observed that we can export data from QFEM from the Blade Structure menu with the option between 3 different formats. The QBlade format gave a “str-file” which is the type of file that is uploaded in the “structural modeling” documentation. We tried to load this file in the Turbine Structural model but got errors about Keywords not being found. Therefore, we were wondering how to export and use the data from QFEM to performe an aeroelastic simulation.
Best regards,
Vanessa, Emma, Bjørnar, Belinda
Hi,
to generate a structural model in QBlade a set of structural input files is needed (main file, blade files and tower file).
Checkout the documentation here: https://docs.qblade.org/src/user/turbine/structure.html#structural-modeling
In that section you also find examples for these input files.
Best regards,
David
Thanks,
We found the other download of the sample project NREL5MW with existing structural input files. We copied the content inside this file into a .dat file and changed it to a .str file. We also changed the files NREL5MW_Blade.str and NREL5MW_Tower.str to the file names we had obtained from QFEM. However, when we load this file in turbine structural model it says the file could not be interpreted with the keywords STIFFTUNER and MASSTUNER not found. It is included in the file since we directly copied the content from the existing file. We are wondering how we eventually can fix that.
Best regards
Vanessa, Emma, Bjørnar, Belinda
Hi,
you simply need to add the keywords to the files that contain the blade and tower structural data tables:
1.0 STIFFTUNER
1.0 MASSTUNER
After adding these keywords the the blade and tower files should be read correctly.
BR,
David
Thank you David,
we finally managed to load a main structural input file 😀
Best regards
Vanessa, Emma, Bjørnar, Belinda
Hi!
we performed some aeroelastic simulation using data obtained from QFEM changing shell and internal material. We used the tower data from QFEM, but it it had some issues with the diameter of the tower at the nacelle being really small, making it collapse during simulation. We decided to use the same tower data as the one from the sample project NREL5MW version 1.2. After the simulations we obtained that the power coefficient were slightly higher than the power coefficient obtained from steady BEM analysis. Is this expected? We noticed the same thing when we removed the turbine controller in the sample project NREL5MW version 1.2.
We are also wondering if it is possible to use the same tower data for the full scale model on the lab scale model. We attempted by changing the height of the tower data from NREL5MW version 1.2, but all other geometry remained the same (the tower top radius and tower bottom radius remained the same). We also attempted using the tower data from QFEM, which fits, but it has the same issue as described in the previous question with the diameter of the tower being really small at the nacelle and collapsing during simulation. So we are wondering if it is possible to use a scaled downed tower of the one used on the full scale model. If so could you explain how to scale it down?
Best regards
Emma, Vanessa, Bjørnar, Belinda
