15MW airfoil sections and aerodynamic reference points

Duncan.vanderHeul · March 14, 2022, 10:01am

Dear all.

In reference to the Aerodyn model and turbine definition data in the repository

and the definition document I would like to ask the following:

How are the geometry and the polars of the 50 aerofoil sections in the Aerodyn model derived from the 8 ‘mother’ aerofoil sections described in the definition document of the wind turbine?

And furthermore, why is the reference point for the polars not the same for all the sections?
I understand this is an arbitrary point but it still interests me to know why it is not chosen as e.g. 0.25 for all the aerofoil sections? Is the reference point the theoretical Aerodynamic Centre: the location with respect to which the pitching moment does not change for small changes in the angle of attack?

Thank you in advance,
Duncan

Pietro.Bortolotti · March 14, 2022, 7:01pm

Hello Duncan,
thanks for your email. The interpolation of airfoil polars and coordinates is performed by this WISDEM function

github.com

WISDEM/WISDEM/blob/77456353f8a183045b17e0ad1b8abe6791b5fa20/wisdem/glue_code/gc_WT_DataStruc.py#L979

      
        
                        outputs["ref_axis"][:, 1] = np.interp(outputs["s"], nd_span_orig, ref_axis_orig[:, 1])
                        outputs["ref_axis"][:, 2] = np.interp(outputs["s"], nd_span_orig, ref_axis_orig[:, 2])
                    else:
                        outputs["s"] = inputs["s_default"]
                        outputs["chord"] = inputs["chord_yaml"]
                        outputs["twist"] = inputs["twist_yaml"]
                        outputs["pitch_axis"] = inputs["pitch_axis_yaml"]
                        outputs["ref_axis"] = inputs["ref_axis_yaml"]
            
            

            
class Blade_Interp_Airfoils(om.ExplicitComponent):
                # Openmdao component to interpolate airfoil coordinates and airfoil polars along the span of the blade for a predefined set of airfoils coming from component Airfoils.
                # JPJ: can split this up into multiple components to ease derivative computation
                def initialize(self):
                    self.options.declare("rotorse_options")
            
            
    def setup(self):
                    rotorse_options = self.options["rotorse_options"]
                    self.n_af_span = n_af_span = rotorse_options["n_af_span"]
                    self.n_span = n_span = rotorse_options["n_span"]
                    self.n_af = n_af = rotorse_options["n_af"]  # Number of airfoils

As briefly described here Blade — windIO windIO documentation, in between the user-defined airfoils, the interpolation scheme for both coordinates and polars is the Piecewise Cubic Hermite Interpolating Polynomial (PCHIP), which is implemented in Matlab(Piecewise Cubic Hermite Interpolating Polynomial (PCHIP) - MATLAB pchip) and in the Python library SciPy(scipy.interpolate.PchipInterpolator — SciPy v1.8.0 Manual).
The chordwise position of the aerodynamic center is also interpolated with a chip and is 50% at blade root (cylinder) and 25% for the other airfoils.
I hope this helps.
Best regards,
Pietro Bortolotti

Duncan.vanderHeul · March 15, 2022, 7:34am

Dear Pietro,
Thank you for answering my questions.
If I understand correctly the thickness/chord is interpolated from the thickness/chord of the ‘parent’ sections at their appropriate span wise stations to a full span wise distribution of the thickness/chord.

Next the aerofoil properties are interpolated based on the local thickness/chord value at a particular span wise station.

Thank you again and best,
Duncan