Mode interpretation of blade number effects on wake dynamics of small-scale horizontal axis wind turbine

Abstract

The blade number of a wind turbine is a critical parameter that significantly affects its near-wake dynamics. In this study, we focused on the effect of the blade number on the wake of a small-scale horizontal-axis wind turbine (SHAWT) with a diameter of 0.18 m. An airfoil with a high lift coefficient was designed for a SHAWT model operating at a low Reynolds number. Large eddy simulation (LES) and optimal mode decomposition (OMD) were performed to identify the underlying coherence mode and to elucidate the wake dynamics. The LES results indicate that an increase in the blade number causes the double-peak velocity distribution to prematurely form a single peak shape. The effects of the blade number and tip speed ratio on the width and velocity deficit of the wake were also determined. The OMD results show that an increase in the number of blades results in the appearance of interharmonic modes with energies lower than that of the mean flow mode. Meanwhile, the increase in blade number causes early instability of the tip and hub/root vortices, leading to the break down and formation of patterns of streamwise vortices in the far wake. This leads to earlier wake recovery and influences the energy extraction and fatigue loads of the downstream turbines.