Aerodynamic and aeroacoustic investigation of vertical axis wind turbines with different number of blades using mid-fidelity and high-fidelity methods

The aerodynamics and aeroacoustics of small-scale Darrieus vertical axis wind turbine (VAWT) are investigated at chord-based Reynolds number below 4e5. A statistical temporal and grid convergence study is conducted to analyse the thrust and torque coefficients. Four different VAWTs with different numbers of blades (1, 2, 3, and 4) are investigated using both the high-fidelity Lattice Boltzmann Method (LBM) and mid-fidelity Lifting Line Free Vortex Wake (LLFVW) method. The statistical temporal convergence is achieved much earlier for the 1-bladed rotor than for the 3-bladed rotor, using both methods. Power performance analysis reveals that VAWTs with more blades generate more power at lower Tip Speed Ratios (TSRs), while VAWTs with fewer blades generate more power at higher TSRs. The aerodynamic efficiency of each blade decreases as the number of blades increases, leading to a decreased amplitude of rotor loading variation in a single rotation. Both the mid-fidelity LLFVW and high-fidelity LBM capture these physical trends well. However, LLFVW predicts lower peak thrust and torque values in a single rotation and higher streamwise velocities in the wake, as compared to LBM. Moreover, the former predicts higher average power output than the latter, and the discrepancy increases as the number of blades increases. In terms of noise, at constant TSR, low-frequency BPF noise is found to be higher in VAWTs with fewer blades, while high-frequency noise is found to be higher in VAWTs with more blades. Overall Sound Pressure Level values revealed that overall noise increased with an increase in the number of blades except for the 4-bladed VAWT for which the noise decreased.