Brian C. Owens

Effect of Finite Thickness and Free Edges on Stresses in Plain Weave Composites

Many years ago, free edge effects were studied extensively, perhaps excessively, for ordinary tape laminate composites. Various techniques were used to show that the free edge effects extended only a short distance from the free edge and that stacking sequence had a significant impact on interlaminar stresses, which could cause premature failure. In contrast, the analytical study of woven composites has focused on periodic analysis of unit cells. Since such analyses assume an infinite array of unit cells, free edge effects were not considered. This article investigates the significance of traction free surfaces on stresses in woven composites. Effects due to both free edge and free lateral surfaces (due to finite thickness) were investigated. The significance of the free edge effect was found to be sensitive to the waviness of the plain weave mat. Also, the stress distribution is quite different for finite and infinitely thick stacks of woven mats.

Aeroelastic Modeling of Large Offshore Vertical-axis Wind Turbines: Development of the Offshore Wind Energy Simulation Toolkit.

The availability of offshore wind resources in coastal regions makes offshore wind energy an attractive opportunity. There are, however, significant challenges in realizing offshore wind energy with an acceptable cost of energy due to increased infrastructure, logistics, and operations and maintenance costs. Vertical-axis wind turbines (VAWTs) are potentially ideal candidates for offshore applications, with many apparent advantages over the horizontal-axis wind turbine configuration in the offshore arena. VAWTs, however, will need to undergo much development in the coming years. Thus, the Offshore Wind ENergy Simulation (OWENS) toolkit is being developed as a design tool for assessing innovative floating VAWT configurations. This paper presents an overview of the OWENS toolkit and provides an update on the development of the tool. Verification and validation exercises are discussed, and comparisons to experimental data for the Sandia National Laboratories 34meter VAWT test bed are presented. A discussion and demonstration of a “loose” coupling approach to external loading modules, which allows a greater degree of modularity, is given. Results for a realistic VAWT structure on a floating platform under aerodynamic loads are shown and coupling between platform and turbine motions is demonstrated. Finally, future plans for development and use of the OWENS toolkit are discussed.

Simulation of oxidation in textile composites

Modeling strategies were developed in order to reduce the computational effort required to simulate oxidation in textile composites. The oxidation behavior in heterogeneous configurations was found to be complex and dependent on a number of factors. A decoupled subdomain strategy for analyzing textile composites was evaluated and used to simulate the oxidation behavior in a graphite/PMR-15 plain-weave composite.

Evaluation of Global/Local Strategies for Detailed Stress Analysis of Textile Composites

Gl obal/local strategies have long been used in conjunction with finite element anal ysis to analyze regions of interest in configurations that are too l arge for a complete detailed analysis. Material properties and/or architecture are somehow approxi mated in the global section, and detailed properties and architecture are used in the l ocal section. Textile composites represent a configuration suitable to global/local analysis because the macrolevel response of the composites is so dependent upon the complex microstructure of the weave. In this work, several global/local strategies for detailed stress analysis of textile composites were i nvestigated. Different approxi mati ons of the textile material properties and architecture were eval uated for use in the global region. It was found that using homogenized properties for the global region yields accurate local solutions only in limited cases. Addi tionally, it was found that using multi-fiel d macroelements to model the global textile architecture yiel ds very accurate soluti ons in the local region.

Modal Dynamics and Stability of Large Multi-megawatt Deepwater Offshore Vertical-axis Wind Turbines: Initial Support Structure and Rotor Design Impact Studies

The availability of offshore wind resources in coastal regions, along with a high concentration of load centers in these areas, makes offshore wind energy an attractive opportunity for clean renewable electricity production. High infrastructure costs such as the offshore support structure and operation and maintenance costs for offshore wind technology, however, are significant obstacles that need to be overcome to make offshore wind a more cost-effective option. A vertical-axis wind turbine (VAWT) rotor configuration offers a potential transformative technology solution that significantly lowers cost of energy for offshore wind due to its inherent advantages for the offshore market. This paper presents an initial design impact study for assessing the dynamic stability of large multi-megawatt deepwater offshore VAWTs. The analysis and understanding of very large, highly flexible VAWT structures is further complicated by the rigid body modes of a floating support structure. A newly developed design tool for offshore VAWTs is employed to assess the stability of very large multi-megawatt VAWT configurations in a deepwater environment. To gain a fundamental understanding of tower resonance in VAWTs, an analytical expression for characterizing critical per-rev excitations for VAWT configurations is developed and presented. The influence of various support condition on structural modes of a VAWT is also investigated. For offshore deployment, a monopile support condition may exacerbate resonance concerns while floating platform supports may provide a means to alleviate resonance concerns. The effect of the large rotating structure on the rigid body modes of the turbine/platform system is also examined.

Design Studies for Deep-Water Floating Offshore Vertical Axis Wind Turbines.

Deep-water offshore sites are an untapped opportunity to bring large-scale offshore wind energy to coastal population centers. The primary challenge has been the projected high costs for floating offshore wind systems. This work presents a comprehensive investigation of a new opportunity for deep-water offshore wind using large-scale vertical axis wind turbines. Owing to inherent features of this technology, there is a potential transformational opportunity to address the major cost drivers for floating wind using vertical axis wind turbines. The focus of this report is to evaluate the technical potential for this new technology. The approach to evaluating this potential was to perform system design studies focused on improving the understanding of technical performance parameters while looking for cost reduction opportunities. VAWT design codes were developed in order to perform these design studies. To gain a better understanding of the design space for floating VAWT systems, a comprehensive design study of multiple rotor configuration options was carried out. Floating platforms and moorings were then sized and evaluated for each of the candidate rotor configurations. Preliminary LCOE estimates and LCOE ranges were produced based on the design study results for each of the major turbine and system components. The major outcomes of this study are a comprehensive technology assessment of VAWT performance and preliminary LCOE estimates that demonstrate that floating VAWTs may have favorable performance and costs in comparison to conventional HAWTs in the deep-water offshore environment where floating systems are required, indicating that this new technology warrants further study.

Efiect of Mesh Reflnement on Various Measures of Convergence for Plain Weave Composites

The increasing popularity of textile composite materials in advanced structures presents challenges that require reliable predictive tools. Finite element analysis has been employed to study the behavior of textile composites and gain greater insight on these materials. However, there has been virtually no study on the convergence of flnite element models of textile composite materials. Herein, a variety of metrics will be studied including volume averaged quantities such as efiective engineering properties and local measures such as point stresses. A plain weave architecture was chosen due to symmetries allowing for a smaller analysis region that could be analyzed using very reflned meshes. The efiect of a glass or graphite material system was investigated as well as the efiect of the waviness of the architecture. Results showed the convergence of some metrics are sensitive to material system while others are sensitive to the waviness of architecture.