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Oscillating Wind Speeds on a Darrieus VAWT
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Hello Jake,
I think in your case the issue is that you are simulating a “free spinning” VAWT.
If there is no counteracting torque from a generator, or if the turbine is not held at a fixed rotational rate, it will accelerate until it reaches its no-load runaway condition. At this point the mean aerodynamic torque generated by the rotor is zero and the rotor can no longer accelerate, see the attached image.
This is not a meaningful power-producing operating point. The rotor is close to the TSR where the aerodynamic torque changes sign, so the averaged Cp can become very small or even negative, especially in unsteady inflow.
For a meaningful simulation you need to limit the rotor’s rotational rate. If you do not have a controller or generator torque model, you should use the “fixed rpm” setting for this purpose.
For the variable wind speed case I would also suggest manually calculating Cp, based on the rotor power production and a suitable reference velocity.
Best regards,
David
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