Aerodynamic Optimization of H-Darrieus Vertical Axis Wind Turbine (VAWT) for Improved Efficiency under Variable Wind Conditions

Driven by the global energy shift and emissions reduction goals, wind energy has emerged as a very promising renewable energy source, distinguished by its abundant resources, established technology, and declining levelized cost of electricity. In comparison to horizontal-axis wind turbines (HAWTs), Darrieus-type vertical axis wind turbines (VAWTs) have several advantages such as no need of yaw requirements, a compact design, reduced noise levels, and simplified maintenance at ground level. They are especially appropriate for intricate urban airflows and distributed contexts. However, their overall efficiency and self-starting capabilities are limited by unstable aerodynamic phenomena, such as rapid variations in angle-of-attack (AOA), dynamic stall, upstream-downstream interference, and wake stagnation. This research examines the aerodynamic optimization of H-Type Darrieus VAWTs in the context of dynamic wind speed conditions. It proposes an active deformation approach and implements a passive flow-control device to address existing limitations. Initially, the implementation of a deformable airfoil strategy aims to expand the efficient operating range. Subsequently, the use of Gurney flaps (GF) is intended to improve mean torque and start-up capability within the suboptimal tip speed ratio (TSR) region.The study establishes a two-tier framework that utilizes the Double Multiple Stream Tube (DMST) model for performance evaluation and selection of suitable airfoils across varying wind speeds. Subsequently, a 2D unsteady URANS, SST k-ω model in ANSYS Fluent is employed to conduct a comprehensive aerodynamic analysis across the operating range of 0.5 to 3.5 TSRs. The numerical workflow rigorously implements mesh independence, time step independence, and rotor-revolution convergence examination.Motivated by biomimetic principles, this study examines the aerodynamic efficiency of VAWTs, with an emphasis on an innovative Dual-airfoil morphing approach designed for H-type VAWT. The design exploits the aerodynamic advantages of two unique airfoil profiles, enabling the turbine to adjust to varying wind speeds and maintain optimal performance across wide conditions. The ability to transition between airfoil profiles instantaneously expand the operational range of the turbine and maximizes its energy capture efficiency. The approach incorporates the selection of airfoils and the aerodynamic analysis using the DMST model, QBlade software (QBladeCE_2.0.8_win) was employed. QBlade serves as an open-source platform for the design and aerodynamic analysis of wind turbines. The numerical model was validated against established benchmark data, confirming its accuracy. Key findings demonstrate that among all tested airfoils, the NACA 64(2)-415 airfoil exhibits the higher power coefficient at lower wind speeds, whereas the FX 84-W-127 airfoil demonstrates optimal performance at elevated wind speeds. A strategy and mechanism for morphing are proposed to ease a smooth transition among these two profiles and allow extensive operational adaptability. By improving functionality and efficiency, the proposed design have has the potential to significantly contribute to the advancement of renewable wind energy technologies, positioning VAWTs as a more viable choice for large-scale energy production.The study on passive flow control approach investigates low TSR start-up and low-speed torque by implementing various sizes of GFs, which are installed perpendicularly at a 90° angle to the chord line (with a chord length of 0.25 m) on the trailing edge of the specified airfoil (NACA 64(2)-415). A 2D computational domain was created to perform a parametric URANS study, investigating the effects of GF height (defined as a percentage of chord length) and operating parameters. Numerical simulations compare the performance of a baseline airfoil with configurations featuring GFs of 0.5%c, 1%c, and 1.5%c chord lengths over various TSRs. The findings indicate that the 0.5% GF represents the optimal configuration, delivering consistent power enhancement across all tested conditions, compared to the taller flaps, which exhibited inconsistent or negative effects. The optimal configuration attained a peak power coefficient (Cp) of 0.366 at a TSR of 2.0, representing a 3.73% enhancement over the baseline. Additionally, it improved low-speed power by 6.30% at TSR=0.5, thereby enhancing the turbine's self-starting capability. Flow field analysis demonstrates a dual-benefit mechanism contributing to enhanced performance: at low TSRs, the GF delays flow separation during the upwind pass, thereby increasing lift; conversely, at higher TSRs, it efficiently manages the wake during the downwind pass, resulting in reduced drag and diminished negative torque. The study concludes that a 0.5% GF achieves an optimal balance between lift augmentation and drag reduction.The key contributions can be stated as follows: ● A unified workflow combining DMST rapid screening with URANS detailed analysis, validated against established literature benchmarks. ● A dynamic approach and mechanism that improves the aerodynamic efficiency of VAWTs by transitioning between airfoils designed for variable wind conditions. ● A passive GF mechanism enhances low-TSR torque and start-up through modifying trailing-edge pressure recovery and near-wall vortices, incurring minimal penalty at mid-to-high TSR. ● A comprehensive 2D analysis of unsteady VAWT aerodynamics (performance metrics, azimuthal torque, velocity/pressure fields), establishing a basis for further 3D effects, fluid-structure interaction, and coordinated variable-pitch/morphing control.This work addresses engineering application challenges by proposing and evaluating two complementary aerodynamic enhancement strategies: active deformation to broaden the efficient operating range and improve overall aerodynamic efficiency, and passive GF to substantially enhance torque and start-up performance in low TSR regions. These methodologies together enhance the aerodynamic efficiency of H-type VAWTs. Extend a feasible technological approach and mathematical foundation for using wind energy in dispersed and urban environments.