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Cp and Ct is larger than Bladed
Hi David,
I recently had a problem with the calculation of Cp and Ct values. The Cp value calculated with bladed is less than 0.4 and the Ct value is less than 0.9. while the Cp value calculated with Qblade is greater than 0.45 and the Ct value is greater than 1.2, which seems to be incorrect (see the picture 1~3). The bladed settings are shown in Figure 4 and the Qblade setting are shown in Figure 5. Could you tell me why there is such a big difference between the two software. Is it a result of incorrect parameter settings somewhere?
Looking forward to hearing your reply soon.
Best regards,
Carl
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Hi Carl,
the steady BEM calculations in both QBlade and Bladed should provide correct results, based on the blade geometry and its associated polars. The BEM routines are among the simplest and most thoroughly validated modules in both QBlade and Bladed. Both codes have also been compared directly – for example, see this NREL report: https://docs.nrel.gov/docs/fy24osti/88685.pdf.
So I would assume the reason for the discrepancies come from either a different geometrical definition of the blade, or differences in the underlying polar data. Pinpointing those differences can be tricky, but it is certainly possible if you have access to both the geometry files and the polar datasets.
BR,
David
Dear David,
Thank you for your reply.
I suspect that it is the interpolation of different Reynolds number airfoils that causes the Bladed and Qblade Cp values to be different. Could you tell me how to interpolate the airfoil aerodynamic data from two airfoils with different thicknesses and different Reynolds numbers to find out the airfoil aerodynamic data in other positions?
Looking forward to hearing your reply soon.
Best regards,
Carl
Hi Carl,
did you also verify that the hub‐radius is defined consistently between the two definitions?
Polars in QBlade can be interpolated in the 360 Polar Module:
In the top menu, go to 360 Polar → Interpolate a 360 Polar.
Select your two reference airfoils and specify the interpolation fraction.
QBlade will generate the intermediate polar for you.
Regarding Reynolds number: For the first comparison I would just compare the two blades at a fixed Reynolds number to simplify the comparison. Later you can assign a multi-polar blade in QBlade – this allows to define polars over a range of Reynlds numbers at each blade station. During the simulation a linear interpolation is then carried out to obtain the airfoil data at the current blade Reynolds number.
BR,
David
Dear David,
I set the Hub Radius parameter equal to the Root Length in Bladed (see the picture), is this the right setting? Could you tell me how the IP Offset, OOP Offset and PAxis parameters can be converted from a bladed blade model to a Qblade model? What are the conversion formulas for the two software in these parameters?
Your prompt reply would be greatly appreciated.
Best regards,
Carl
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Hi Carl,
from a lack of experience with Bladed I cannot guide you through the conversion of Bladed models into QBlade models. However if you know how Bladed defines the coordinate system in the blade setup this should be mostly straight foward.
You can find a description of these values in the QBlade blade design here: https://docs.qblade.org/src/user/blade/blade.html#advanced-hawt-blade-design
QBlade however is capable of converting blade models directly from a windIO definition. WindIO is a common descriptive format for wind turbine geometries. To my knowledge Bladed can also generate models from windIO – so this could be a suitable basis for a comparison.
BR,
David
