Rotor Design: Part 5

As previously mentioned, the Cl/Cd vs. Alpha curve for an airfoil is dependent on the Reynolds number. However, selecting a meaningful Reynolds number to create the model with can be a little bit tricky since it in turn varies with the relative wind speed and chord length of the blade. Micro scale turbines, such as the ones Minvayu is designing, have fairly low Reynolds numbers across the board. A good rule of thumb would be to use a Reynolds number of 250,000 for very small turbines.

However, using just one Reynolds number for the entire length of the blade is not the most accurate approach. For instance, the local TSR decreases from the tip of the blade to the root of the blade. The relative wind speed seen at the root of the blade is less than that of the tip; therefore, the Reynolds number varies with the radial position of the blade given that everything else is held constant. A more accurate model would use larger Reynolds numbers for the tip of the blade and smaller Reynolds numbers for the root of the blade. However, it’s up to the engineer to determine whether or not this is truly necessary.

I have a few final comments on the selection of Reynolds numbers. Generally, a higher Reynolds number will lead to a higher Cl/Cd. This can be used to artificially inflate the theoretical performance of a blade if an engineer is not careful. Also, the alpha at which Cl/Cd peaks shifts slightly with changes in Reynolds numbers. This is very important when trying to optimize for that perfect sweet spot or when designing stall regulated blades. Finally, QBlade calculates the Reynolds number versus the radial position of the blade as a function of wind speed. This can be used to check if the polars being used are in the right ball park or not.

FX 63_137 Polars

Reynolds

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