Flow Dynamics with Jet-induced Vortex Control in a Slotted Vertical-axis Wind Turbine

Abstract

This study investigates the aerodynamic impact of passive blade slots on the unsteady flow dynamics of a Darrieus vertical-axis wind turbine (VAWT) operating at low tip speed ratios. A two-dimensional URANS-based numerical investigation using the k–ω SST turbulence model was performed to analyze the creation of slot-induced internal jets and their interaction with the surrounding flow field. An energy-based Jet Strength Index (JSI) was introduced to quantify the momentum injection produced by the slot. The results revealed that the pressure difference between the pressure and suction sides of the blade naturally generated internal jets that injected momentum into the boundary layer and significantly modified the vortex formation. These jets developed primarily on blades 1 and 4, where they weakened the separation-induced vortices identified using the λ₂ vortex criterion and contributed to delaying the dynamic stall. The optimized slotted configuration led to a substantial aerodynamic improvement, with the torque coefficient increasing by up to 89% within the low tip-speed ratio range of λ = 1–2 for a free-stream velocity of 10 m/s. Spectral analysis of the unsteady loads indicated a rotation-driven vortex detachment characterized by a Strouhal number of St ≈ 0.07 and a reduced frequency of k ≈ 0.217 for λ = 1,25. Furthermore, near-wake analysis at λ = 2 primarily weakens separation-induced vortices and reorganizes their spatial distribution in the near wake, leading to reduced vortex strength, delayed breakdown, and more stable wake development, providing new insights into passive flow control mechanisms for enhancing VAWT performance.