Blade Count and Rotational Speed Sensitivity of Vertical-Axis Turbines Under Guided Vehicle-Generated Transient Wakes

Abstract

Vehicle wakes generate short but energetic gusts that can be used by small roadside turbines, yet the operating conditions that allow a rotor to take full advantage of these events are still not fully clear. Flow-guiding structures that were previously shown to improve wake redirection and energy capture are employed in this study. However, the combined influence of blade count and prescribed rotational speed under true transient vehicle-induced inflow has not been examined in a systematic way. In the present study, vertical-axis turbines with two to six blades were evaluated numerically while a car traveling at 32 m/s produced the unsteady wake. The rotor speed was increased from the nominal value ω₀ = 6.677 rad/s (63.76 rpm, corresponding to a baseline tip-speed ratio of approximately 0.10) up to 2.2ω₀. The results show a strong dependence of performance on both blade number and rotational speed. The two-blade configuration provided the most consistent energy extraction and reached a maximum of 158.5 J at 2.1ω₀, representing a 24.5% increase relative to baseline speed. In contrast, higher-solidity rotors experienced significant performance deterioration at elevated speeds, and several cases produced negative net energy. This behavior was linked to repeated interception of the wake core by returning blades at unfavorable angular positions, leading to transient torque reversal. The findings suggest that, under short-duration vehicle wakes, low-solidity rotors operated at carefully selected rotational speeds are more reliable than higher-blade-count configurations.