Transient Adjoint Based Aerodynamic Optimization of Vertical Axis Wind Turbines Using RANS-based CFD

Vertical-axis wind turbines (VAWTs) have attracted project and policy attention worldwide
due to their unique advantages. Compared to horizontal-axis wind turbines (HAWTs),
VAWT onshore and offshore wind farms offer advantages such as light weight, high
stability, and low noise levels. Since VAWT technology is still in its early stages of
development, a significant research gap remains between its theoretical maximum efficiency and actual efficiency, presenting significant research potential.
This thesis proposes a novel fully transient adjoint optimization technique, applied for the
first time to the fluid dynamics of VAWTs. This technique is more accurate than currently
studied multi-snapshot steady-state (also known as semi-transient) adjoint optimization
methods. The challenges in this VAWT airfoil design optimization primarily lie in the
complexity of defining the Darrieus pitching motion and the flow separation that occurs
when the blade moves at high angles of attack. The results show that the transient adjoint optimization algorithm significantly improves the airfoil efficiency and can be effectively applied to the unsteady flow problems encountered in the design of vertical-axis wind turbine blades.
This research first applies transient adjoint to oscillation airfoils representative of H-Darrieus operation, explicitly accounting for wake effects in the objective and sensitivity evaluation. Then it extends transient adjoint to a 2D VAWT model, formulating power coefficient objectives and enforcing geometric and mesh-quality constraints across tip-speed-ratio scenarios. Finally it validates in 3D CFD, confirming that the optimized airfoil maintains 2D gains when spanwise induction are present.
The fully transient adjoint optimization technique is an innovative technology in the VAWT
field. It is implemented using the unsteady adjoint optimization algorithm of the open-source CFD software SU2, making it easy to be adopted by the VAWT design community and providing an important engineering application reference for all other unsteady mechanical structure designs.