CFD modelling of VAWT wake effects

Wake effects are important to wind turbine design and wind farm design, because they will affect the aerodynamic performance and structural loads of wind turbine operating in a wind farm. Wake effects were investigated extensively for horizontal axis wind turbine(HAWT) in the past, but there has been very limited work done for the vertical axis wind turbine(VAWT), whose wake effects are unique because the blades will go through their own wake region during the operation. The presented thesis aims to bridge this knowledge gap by modelling the VAWT wake effects using CFD. As for the general wind turbine wake effects study, four key aspects can be identified: wake models, aerodynamics, structural dynamics, and structural integrity. Relevant literature is reviewed in the thesis, and a comprehensive framework of studying the VAWT wake effects is proposed. The framework covers all the four key aspects of the wind turbine wake effects study, and two of them will be addressed in the presented thesis, wake models and wake aerodynamics. CFD modelling in the thesis is based on RANS method. The near wake modelling focuses on the aerodynamics prediction and the far wake modelling focuses on the wake structure prediction. As for the near wake study, wake effects of a circular cylinder at Re=140000 is studied and validated. the aerodynamic performance of NACA0015 airfoil at various angle of attack at Re=2000000 is modelled using different turbulence models, dynamic stall effects of the airfoil at three different regimes are investigated. They form the basis of analysing the aerodynamic performance of VAWT rotor. A 17m 2-bladed VAWT designed based on such geometries (circular cylinder and NACA0015 airfoil) is modelled thereafter, simulated aerodynamic performance under different tip-speed ratios are compared with experiment data. As for the far wake study, both rotor simplification using porous disk and full rotor simulation are presented. A persistent symmetric wake region is observed from the porous disk modelling while the full rotor modelling predicts an asymmetric wake region. The wake interaction is then studied in a two turbine VAWT array, the influence of wake effects on the performance of VAWT at 3 diameters downstream is investigated. Overlapping of wake region is analysed.