Fully Automated Wind Tunnel Experiments for Performance Prediction of a Double Rotor Sail

Flettner rotors are increasingly explored as a sustainable hybrid propulsion technology in maritime applications, yet predictive tools for multi-rotor installations remain limited, particularly when rotors are independently controlled. This study presents an experimental investigation of a double Flettner rotor system, employing a fully automated wind tunnel setup with independent spin ratios. Two identical rotors (𝐿/𝐷 = 6, 𝐷𝑒/𝐷 = 2, 𝐷 = 0.09 m) were tested over 𝜆1, 𝜆2 ∈ [0,5], inflow angles 𝜑 ∈ [0°,90°], spacings 𝑆/𝐷 ∈ {2.2,3,4}, and three free stream velocities (𝑅𝑒 ≈ 5×104−1×105). The sense of rotation was used only to cover angles of 𝜑 > 90°. The fully automated setup acquired 10164 configurations, each repeated three times. We map projected thrust and side force coefficients (𝑐𝑥, 𝑐𝑦) across (𝜆1, 𝜆2, 𝜑, 𝑆/𝐷) and distill design-relevant trends. Within the tested ranges: (i) increasing spacing from raises 𝑐𝑥 for all operative points; (ii) optimal operating points are frequently asymmetric, with the windward rotor commanded at up to ~50% lower spin ratio than the leeward; (iii) the downstream rotor in the wake can exhibit up to ~11% higher lift than the upstream rotor at comparable operating points; (iv) a lateral deck alignment achieves ~10% higher peak 𝑐𝑥 compared to a longitudinal alignment at 𝑆/𝐷 = 4 when each is operated at its optimal (𝜆1,𝜆2) across 𝜑. We present compact 𝑐𝑥-𝑐𝑦 polars and lookup tables over the measured grid to enable simple performance prediction and control scheduling. Keywords flettner rotors; double rotor system; Magnus effect; performance prediction; sailing polar; wake effect