Abstract:This paper presents a combined experimental and computational study into the aerodynamics and performance of a small scale Vertical Axis Wind Turbine (VAWT). Wind tunnel tests were carried out to ascertain overall performance of the turbine and two and three dimensional unsteady computational fluid dynamics (CFD) models were generated to help understand the aerodynamics of this performance.Wind tunnel performance results are presented for cases of different wind velocity, tip-speed ratio and solidity as well as rotor blade surface finish. It is shown experimentally that the surface roughness on the turbine rotor blades has a significant effect on performance. Below a critical wind speed (Reynolds number of 30,000) the performance of the turbine is degraded by a smooth rotor surface finish but above it, the turbine performance is enhanced by a smooth surface finish. Both two bladed and three bladed rotors were tested and a significant increase in performance coefficient is observed for the higher solidity rotors (three bladed rotors) over most of the operating range. Dynamic stalling behaviour and the resulting large and rapid changes in force coefficients and the rotor torque are shown to be the likely cause of changes to rotor pitch angle that occurred during early testing. This small change in pitch angle caused significant decreases in performance.The performance coefficient predicted by the two dimensional computational model is significantly higher than that of the experimental and the three dimensional CFD model. The predictions show that the presence of the over tip vortices in the 3D simulations is responsible for producing the large difference in efficiency compared to the 2D predictions. The dynamic behaviour of the over tip vortex as a rotor blade rotates through each revolution is also explored in the paper.

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This paper presents a computational study of a rooftop size vertical axis wind turbine with straight blades (H-type turbine). The computational model solves for the two-dimensional and three-dimensional unsteady flow fields around the turbine based on the unsteady Reynolds averaged Navier-Stokes equations and a sliding mesh technique to connect the far-field fixed mesh to the near field rotating mesh around the rotor. Interesting flow features such as dynamic stall around the blades and the interaction of the blade wakes with the following blades are illuminated. Comparison of the 2D and 3D simulations highlight strong three dimensional effects, including the blade tip losses and the effects of the blade supporting shaft and arms. These effects significantly degrade the performance of the VAWT under investigation, pointing a way for improving VAWT designs.

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Numerical simulations have been used to analyze the equivalence of pitching and plunging motions found in a flapping NACA0012 airfoil. Two-dimensional incompressible Navier–Stokes equations are solved at Reynolds number of 103 over a range of Strouhal numbers. A novel criterion based on the Strouhal number is proposed which provides equivalence of pitching and plunging motions using the length scale traversed by the trailing edge in each case. Aerodynamic coefficients are found to match well for both the kinematics in temporal as well as spectral domains. Detailed analysis provides contribution of different mechanisms, such as vortex shedding, added mass, interaction of leading and trailing edge vortices, in the overall aerodynamic forces produced by a pitching or plunging airfoil. Wake deflection is observed for a plunging airfoil at high Strouhal numbers resulting in a bias in the lift coefficient. Further investigation reveals the dominance of second harmonic of the fundamental frequency in the lift spectrum emphasizing the role of quadratic nonlinearity in the observed phenomenon.