Gusty winds producing steady power

Wind turbines are growing in more ways than one. While they follow a simple concept that has proven its value for centuries, their development faces many complex challenges.

Growth - no end in sight

Over the last decade, the wind energy industry has been a success with average growth rates of more than 20% per year. Wind turbines have enjoyed political support in Europe for many years, a trend that caught on lately in North America, China and India which helps to ensure similar industry growth rates for the coming decade. This momentum has several reasons: wind (energy) is readily available, which helps the cause of energy independence; it is cleaner than burning fossil fuels; and its cost competitiveness has increased. The way to improved competitiveness requires higher power output per wind turbine, which means growing dimensions, larger forces impacting wind turbines and ever bigger engineering challenges - challenges that are often difficult to meet and require new methods to reach better system reliability.

Unique engineering challenges

Wind turbines should unfailingly operate for at least 20 years or 120,000 operating hours; to understand what this means remember that the operational lifetime of cars is around 4,000 to 6,000 hours with significantly smaller loads. During their lifetime wind turbines must reliably withstand highly dynamic, asymmetric and unsteady wind forces as well as other environmental influences like precipitation, icing, and in offshore installations, wave loads and salt water.  

Addressing all these requirements is not easy for developers who can rely only in a limited way on empirical values and test results. Physical tests, frequently indispensable parts of the development process in conventional development scenarios, are not only time-consuming and expensive due to their "trial and error" approach, but are possible only with limitations due to size, expected life time and site dependent wind loads of turbines. As a result, more emphasis is placed on numerical simulation processes. On the computer, controlled test conditions (weather and wind) can be created, and application of new materials and different design variants can easily and quickly be checked for all critical operational conditions. Thus engineers get an early overview of the behavior of the entire system, its subsystems and components, and gain an improved understanding of various loads and system responses. The spectrum of simulation solutions employ ranges from simple structural calculations to complex multi-disciplinary system simulations.

Demanding simulation challenges

Current and future dimensions of wind turbines entail that the structures must withstand extreme forces. System sizes combined with these forces result in flexible structures whose responses to loads are determined by the interaction of all components comprising the system. When simulating wind turbines one often weighs accuracy versus speed and too often, speed requirements lead to accuracy compromises that prevent truly dependable, reliable predictions. Overcoming these compromises is an area that MSC, in cooperation with its customers, is successfully working on.

System simulation - Loads calculation wind gust

Simulation supporting wind turbine design

Simulation serves many aspects of wind turbine design. First, it is necessary to calculate dynamic loads caused by various standard or critical wind conditions impacting turbine components like blades, gear boxes, bearings, tower and other structures. Using these loads designers perform detailed analysis of components to ensure structural strength and durability. When development reaches the final stages, full system simulations are done to optimize overall performance, including performance under low wind conditions. 

Virtual tests via simulation help not only with the design of wind turbines, but also create important results needed for type certification. This process involves thousands of virtual tests for all crucial wind conditions and supplies data that would be difficult to obtain in a timely manner through physical tests.

MSC's simulation offering for the wind energy industry

For the applications described, MSC offers highly efficient simulation solutions. They are successfully used by most of the leading wind turbine OEMs and are based on superior technology for multi-body dynamics (MD Adams) and FEA (MD Nastran). Based upon this proven solver foundation, MSC has developed WE industry specific solutions for efficient e.g. wind turbine system or rotor blade modelling and is offering the means to manage thousands of simulation results through SimManager.

At the start of a design cycle, it is important to rapidly perform many simulations to understand the most appropriate design options; later, it is more important to obtain accurate results to ensure strength and durability for all components and their interaction. Our solutions allow representation of components in different degrees of fidelity, enabling users to get the results they need as quickly as possible.

Advanced simulation capabilities

Multi-body dynamics is the standard tool for system simulation and obtaining coupled responses of a virtual wind turbine. Traditionally, multi-body dynamics has been done with rigid bodies for speed reasons, but in the case of large wind turbines structural elasticity and flexibility is a fact that can no longer be ignored. For this reason advanced flexible body modeling is a prerequisite for high fidelity predictions. The best implementation is a combined application of multi-body simulation and FEM analysis. Here, the modal reduction method can be used to account for the behavior of flexible bodies in multi-body simulation. The appropriate components are then first calculated with the FEM method using MD Nastran and subsequently the so-called Craig-Bampton modes are exported to Adams. Comparisons have shown that multi-body simulation systems with flexible bodies require shorter calculation times than multi-disciplinary models of comparable accuracy.

Detail analysis - Stresses in 3D gear train

An advantage of this procedure is that the simulation models are highly scalable. Critical areas, such as the drive train, can be modeled in great detail in 3D representation with flexible components. For many tasks, however, a more or less simplified representation is sufficient, which keeps the complexity of the simulation model reasonable.

Growing size and reliability

Virtual product development plays a critical role in wind turbine manufacturers' success. To achieve the engineering goals coming from the need for larger and more competitive systems, modeling has to be as realistic and accurate as possible. To accomplish this, MSC cooperates with leading vendors and research institutions to improve virtual test capabilities to make new systems more efficient and reliable.

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