Meeting the goal of 20 percent US energy by wind by 2030 will require a diverse fleet of wind energy harvesters. Wind turbines of various styles and of increasing size will need to be installed in a variety of environments including off shore. Maintenance and repair of very large turbines can be costly and involve significant down time, thereby challenging the development of affordable wind energy. Currently, horizontal-axis wind turbines (HAWTs) are the most popular type, with vertical-axis wind turbines (VAWTs) being the next most common. One thing that has hindered the development of VAWTs is their reputation for having vibration and fatigue problems. This reputation was established well before the development of modern blade materials. But VAWTs have some important advantages. These include independence of wind direction, reduced tower size, lower center of mass, and lower generator placement. Furthermore, HAWT size may be limited by gravity. Also, VAWT efficiency improves when placed in wind-farm arrays. Thus, VAWTs may turn out to be better for some applications, including large off-shore installations, and are therefore getting renewed interest. To date, VAWTs have been modeled in much less depth than HAWTs. These circumstances all converge to the need for better models for VAWTs, including VAWT blade vibration. The purpose of this work is to develop a model for VAWT blade vibration, and to use the model to understand the role of VAWT blade parameters on vibration, to enable the design of reliable VAWTs in the future.
In this work, vibration models of H-rotor VAWT blades will be formulated. The simple geometry of H-rotor blades allows the work to focus on the complexity of the mechanics of VAWT function. The model will include nonlinear beam theory and a semi-empirical model of the nonlinear aerodynamic forces. Preliminary insight into the model suggests the existence of parametric and direct excitation, and nonlinearity, all of which together can interact to produce a variety of resonances and instabilities. Reduced-order modeling and asymptotic analysis will lead to identification of resonances and instabilities. Critical cases will be simulated numerically in more depth. The result will be an understanding of the mechanisms of resonances and instabilities of VAWT blades, as well as the role of parameters, leading to design recommendations for increased reliability, reduced maintenance costs, and less down time.
