Rotor Design: Part 2

XFOIL is a wonderful program developed by MIT that assists with the design and analysis of subsonic airfoils. Based on the airfoil geometry and flow specifications, XFOIL can generate polars (Cl/Cd vs. Alpha data) for a small range of alpha. Additionally, XFOIL allows me to easily blend airfoils and generate polars for these new geometries. This is especially useful for more accurately modeling the region of the blade that occurs between two pure airfoils.

Interpolated Geometry

This data is incredibly important when it comes to designing the right airfoil for the job. For instance, the area swept by the root of the blade is smaller than the area swept by the tip of the blade for an equally sized portion of the blade radius. This means that the region of the blade near the root is not able to generate as much energy as say the tip, because the total swept area is less. Additionally, the relative wind (tangential velocity of the blade combined with free stream wind velocity) is much less near the root than near the tip of the blade. Should chord length and airfoil geometry remain the same, the contribution of the root to the overall performance of the blade is much smaller than that of the primary and tip regions.

It is possible to mitigate the loss in performance of the root by increasing the chord length of the blade; however, given the same airfoil geometry, root chord lengths may have to be impractically large to achieve the same performance as some other part of the blade. Another way of getting around this is by modifying the airfoil geometry to have a larger Cl/Cd. By choosing an airfoil with greater lift, you can cut down on the chord lengths needed and increase the overall performance of the root.

The FX 63-137 airfoil exhibits a relatively gentle Cl/Cd vs. Alpha curve and a high peak Cl/Cd. These characteristics are desirable for several reasons. First, the high Cl/Cd helps to mitigate the lower performance expected near the root of the blade while maintaining practical chord lengths. Second, when the rotor begins to start up, the tip speed ratio (ratio of the tangential velocity of the tip of the blade to the free stream wind velocity) of the rotor is much less than the design tip speed ratio. As the TSR of the rotor speeds up to the design TSR, the alpha decreases.

FX 63_137 Polars

Airfoils with peak Cl/Cd at very specific alphas do not respond well to these changes. For instance, the airfoil may be operating around peak Cl/Cd during startup and then stall during normal operation, which hurts the annual energy production (AEP) of the turbine. Conversely, if the airfoil is designed to peak during normal operation, the rotor will be less effective at starting up leading to higher cut-in speeds. Airfoils with a significant Cl/Cd over a broader range of angles help with the startup of the rotor and prevent stalling at low wind speeds without significantly sacrificing the AEP.

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