Aerodynamic Loading And Performance Of Vertical Axis Wind Turbines Under Unsteady Winds

A fluctuating free-stream in unsteady wind environment presents a significant challenge on aerodynamic loading and performance of urban wind turbines. Interest in small–scale wind turbine technology as a modern energy source in unsteady wind conditions has been on increase due to the desire of consumers in urban areas to reduce carbon pollution from combustion of fossil fuels. A Vertical Axis Wind Turbine(VAWT) is known for its effectiveness in urban areas where wind has characteristics of unsteadiness with turbulence. However, a fluctuating free-stream in a turbulent inflow, unsteady wind that is typical of the built environment, presents a more significant challenge in wind turbine performance due to the existing large knowledge gaps in the VAWT rotor aerodynamics and performance. In addition, understanding of both the kinematic and aerodynamic behavior on a VAWT rotor, and the flow physics that causes the behavior is quite complex under such conditions.This dissertation employs wind tunnel experiment and Computational Fluid Dynamics(CFD) methods to analyze the aerodynamics and performance behavior of VAWTs,and the flow physics that causes the behavior. Using an urban wind resource and employing Unsteady Reynolds-Averaged Navier-Stokes(URANS) CFD, an initial study with characteristic fluctuation frequencies 0.5 Hz, 1.0 Hz, and 2.0 Hz is performed to demonstrate enhanced wind energy capture. The results show that the highest frequency of fluctuation with optimal and meaningful energy content in unsteady wind condition is approximately 1 Hz.The VAWT dynamic motion of blades for the developed CFD model was introduced by sinusoidally oscillating both VAWT blades. The validation of the numerical model with experimental data predicted the Power Coefficient(CP) performance curve for the VAWT scale. The results derived in the numerical analysis show that, within fluctuating free-stream wind conditions, thicker airfoils are desirable. Overall maximum unsteady CP of VAWT with thicker blades reveals positive deviations if the tip speed ratio λ is slightly higher than λ of the steady maximum CP, while thinner blades maximum CP marginally drops from the steady maximum CP for the same λ range. Higher frequencies of fluctuation marginally improve the unsteady wind performance of both VAWT blade profiles.High fluctuation amplitudes reveal overall performance degradation on both VAWT blade profiles more than small fluctuation amplitudes. Furthermore, the numerical method is extended to evaluate aerodynamic performance of a VAWT including assessment of wind energy potential under unsteady wind inflow, at two different rural-urban sites in Kenya.With the aid of the CFD model, unsteady wind simulations are performed and the results obtained compared with the empirical methods.The experimental investigations were performed on a small-scale Savonius turbine rotor in a closed-loop circuit wind tunnel to investigate unsteady rotor aerodynamics under turbulent inflow. The aerodynamic power performance of the VAWT model is measured using a rotary torque sensor. The use of turbulence-generating mechanism that generates unsteady turbulent inflow in the wind tunnel is detailed, and simulations using a CFD code performed to compare the experimental data with numerical data. A systematic analysis of torque and power coefficients, including their variations at uniform inflow,was also presented to predict the power performance. Results revealed that turbulence of the inflow impacted fluctuating loads on the turbine blade and, ultimately, its aerodynamic performance.The findings lend substantially to our understanding of the aerodynamics and performance behavior on VAWTs operating in unsteady wind condition that is typical of a built environment, and the flow physics that causes the behavior. The results from the study will, hopefully, be of importance to the wind energy technology stakeholders, and industrial aerodynamics applications that require designs for urban wind turbines reflecting realistic unsteady wind operating conditions.