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Maintaining Constant TSR in Unsteady BEM & Power Coefficient Exceeding Betz Limit
Hello QBlade team,
I am currently investigating a HAWT turbine using the unsteady BEM analysis to evaluate the power coefficient (Cp). I would like to set up the simulation so that the turbine maintains a constant TSR while adjusting its rotational speed according to the varying wind field. However, the “RPM Prescribe Type” field is grayed out, and I am unable to change it. Is there a way to implement this in QBlade? Or is this not possible within the software’s current capabilities?
Additionally, I have encountered an issue regarding the power coefficient in turbulent wind conditions.
When using steady BEM (uniform wind), the simulation converges to trustworthy Cp values based on the prescribed wind speed and the corresponding RPM for my chosen TSR.
However, when I switch the wind input type to a turbulent wind field while keeping the RPM fixed, the TSR increases, and my simulation outputs Cp values exceeding the Betz limit.
I am struggling to understand why this happens. At 11.4 m/s and a TSR of 8, my turbine should operate at 6.7 RPM. Unfortunately,
Any insights would be greatly appreciated. Thank you!
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Hello Paul,
when you simulate an aerodynamic-only definition of a wind turbine it is not possible to integrate the simulation with a wind turbine controller. Instead the rotational speed of the turbine has to be “prescribed” by the user. If you want to prescribe a varying RPM you can use a Simulation Input File, to prescribe the RPM over time.
Furthermore, its possible to prescibe the rotational speed through the SIL interface API, via Python, Matlab or similar scripts.
If you want to switch to a generator torque and blade pitch based control of the turbine you also need to include a structural model so that the inertia forces of the rotor can be accounted for correctly.
Regarding the Cp value: By definition the Cp value is based on a single reference wind speed. That said, the instantaneous power coefficient quickly looses its meaning when the turbine is operating in a turbulent windfield that is varying in space, across the rotor disc, and in time. In this case we are using the instantaneous “rotor averaged” velocity as a reference for the evaluation of Cp. In this sense it is also possible that the Cp value may exceed the Betz imit for short periods of time, e.g. when a gust is impacting on the rotor disc. The usage of this “rotor averaged velocity” as a Cp reference has been introduced in the most recent version of QBlade. So I would recommend upgrading your version to the latest release.
BR,
David
