Reynolds number effects on horizontal axis wind turbine performance and optimal pitch angle

Modern wind turbines exceed 200 m in diameter and generate up to 15 MW power with their performance depending on the diameter-based Reynolds number (Re_D). A recent study by Miller et al. (2019) observed that the turbine performance becomes Reynolds number invariant beyond Re_D ∼ 10⁷. However, the effect of the optimal blade pitch angle on this Reynolds number trend is unknown.

Traditional wind tunnels cannot reach such values without increasing flow speed, which leads to compressibility effects and makes it difficult to match the tip speed ratio (TSR). To resolve this issue, we use the Variable Density Turbulence Tunnel (VDTT) at the MPI-DS, which enables 1 × 10⁴ ≤ Re_D ≤ 2 ×10⁷ by pressurizing sulfur hexafluoride (SF₆), allowing for high TSRs at realistic rotation rates. This setup bridges the gap between lab and field studies. A model turbine (MoWiTO 0.6) is used. We record power output and determine the optimal pitch angle. Results show that the power coefficient C_p becomes Reynolds number invariant above a chord-based Reynolds number of Re_c ∼ 1 × 10⁶. We also investigate hysteresis in the C_p data depending on blade pitch direction.