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Adding blade flexibility without doing structural analysis?
Hi,
Once you move away from idealized, steady-state, rigid-rotor BEM simulations, the power coefficient (Cp) starts to lose its practical meaning. This is especially true when the turbine operates in a non-uniform wind field and the rotor and shaft are tilted.
In that kind of situation, for an aeroelastic turbine you should evaluate performance using the “Structural Time Graph” outputs. Those channels represent the “measured” structural response (e.g., torque, thrust, bending moments) as seen by the drivetrain and support structure.
By contrast, the values in the “Aerodynamic Time Graph” are derived quantities. They are obtained by projecting aerodynamic forces onto an assumed rigid rotor, so they are not always representative of the actual loads and power transfer in an aeroelastic, tilted, sheared, or yawed inflow case.
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Regarding the tower: how you scale it depends on your objective, but I generally would not recommend using full-scale tower properties for a model-scale turbine. Full-scale properties will typically make the tower unrealistically stiff at model scale, which can distort modal behavior and may negatively affect the simulation results. To obtain a scaled tower, you have two common options.
One option is to construct the tower geometry similarly to a blade, using cylindrical (or tapered) profiles for your target tower dimensions, and then generate the corresponding sectional properties.
Another option is to apply established scaling laws to the tower’s mass and stiffness distributions (mass, EI, GJ, etc.) based on your chosen scale factor.
For the tower diameter entries in the structural tower properties table: these are typically used only for visualization and for computing aerodynamic tower drag. You can manually change those diameter values to match your design without affecting the structural dynamics, as long as the mass and stiffness distributions in the properties table remain consistent with the tower you intend to model.
In other words, the simulation’s structural response is driven by the tower’s mass and stiffness distributions, while the diameter field mainly affects how the tower looks and how much tower drag is applied aerodynamically.
Best regards,
David
Hi!
Thank you for your previous answer.
We have some questions about the “structural time graph” outputs. We are not going too deep with structural analysis since we do not have enough knowledge and our thesis’s main focus is how different parameters effects the power coefficient and power. However we are still wondering how to interpret the graphs obtained in the structural time graph. For example the graphs showing torque vs time and aero. power coefficient vs time are oscillating. We are not sure how to interpret this so could you explain? (also is aero. power coefficient the same as power coefficient?)
Another question we have is about the “rotational speed settings”. Previously we have used the always fixed option. However when we tried with the free rotation, it barely rotated. Are these options about the rpm during the ramp up or is it about during the actual simulation?
Best regards
Emma, Vanessa, Belinda, Bjørnar
Hi Emma,
First, just a quick note: the forum is intended for software-related questions and is not meant to serve as a general introduction to wind energy topics. Before posting, we kindly ask that you take a moment to consult the extensive documentation, which also includes a search function to help you quickly find specific terms or topics. You can find the forum and community rules here: https://qblade.org/qblade-forum/topic/forum-and-community-rules/
Now, regarding your questions:
In aero-elastic simulations, oscillations in torque, rotor speed, power, and power coefficient are normal and physically expected. They arise from unsteady aerodynamics (sheared inflow, tilted rotor, etc.) combined with structural inertia, elasticity, and damping. The Structural Time Graphs represent the system response measured at the structural turbine model and are the relevant outputs for this type of simulation.
The “Aero. Power Coefficient” in the structural graph is the measured power coefficient, derived from measured rotor torque and speed. In steady rigid simulations, it converges to a constant value, but in unsteady simulations Cp starts to loose its meaning quickly and should rather be interpreted using time-averaged values.
Regarding the RPM settings please checkout the docs here:
https://docs.qblade.org/src/user/simulation/simulation.html#rotational-speed-settings
With “free rotation,” the rotor speed evolves dynamically based on aerodynamic torque, inertia, damping, and generator or controller settings (if included). Due to the inertia of the rotor and the low aerodynamic torque produced at low RPM the startup of a rotor can take quite a while – thats why the “rampup-only” RPM option exists.
BR,
David
Hi!
We have a question about the downscaling of a fullscale wind turbine. The scaling factor used was 1/140. This was used to scale down with the turbine blades and the turbine tower. All of the tower data was scaled down except the following parameters: EIx_[N.m^2] EIy_[N.m^2] EA_[N] GJ_[N.m^2] GA_[N]. This was done to ensure sufficient tower stiffnes – due to several problems with the tower “melting” during simulation.
Unfortunately the graph data was not displaying similar trends as the fullscale graph data. We have achieved a Cp and Ct value that is corresponding with earlier fullscale simulations.
Is there something else we should be aware about when doing the downscale? What kind of data should we expect with this scaling factor?
Would you also comment whether if this is a good way of scaling down the wind turbine in Qblade?
Attachment that shows the simulation data:
Best Regards, Belinda.
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