Three dimensional cross flows as well as quasi steady

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Unformatted text preview: ion. Three-dimensional cross flows as well as quasi-steady and unsteady separation events arise from both the stochastic nature of the inflow and the components of the turbine architecture. Rapid changes in wind speed or direction dynamically alter the local angle of attack along the span. Both angle of attack and dynamic pressure change radially along the span, and are further effected by variable speed or pitch operation. Because of these many influences, accurate and reliable prediction of wind turbine aerodynamics remains a unique challenge for today’s computational modelers. The overall contribution of dynamic stall to wind turbine structural loads continues to undergo opinion shifts in the research community. Several years ago, the effects of dynamic stall were deemed inconsequential as interest centered around predicting power performance. This conclusion was not surprising since the rapid transient loads produced by a single dynamic stall event occur over a small percentage of the total turbine rotation cycle. Opinions changed when it was shown that some type of dynamic stall model was necessary to adequately predict both peak and fatigue loads. Currently, the impulsive loading introduced via dynamic stall aerodynamic models can be shown to amplify, damp, or have little effect on the resulting structural loads depending on a particular time series, turbulent inflow condition, or turbine architecture. As such, the ability of current aerodynamic models to adequately predict quasi-steady, three-dimensional, post-stall performance remains in question, let alone three-dimensional dynamic stall performance in an extremely stochastic inflow environment. Most wind turbine structural design codes rely on Blade Element Momentum Theory (BEMT) to simulate blade aerodynamic performance. This algorithm allows the aerodynamics model to run quickly, enabling practical design trade-off analyses. The use of empirical two-dimensional wind 1 tunnel test data to obtain quasi-static aerodynamic loads and necessary assumptions associated with BEMT prevents these models from capturing full three-dimensional effects. These aerodynamics models include dynamic stall models, again empirically derived using two-dimensional wind tunnel test data collected from pitching and plunging type motions. Further refinements can be made for three-dimensional tip and separation effects. It is not clear, however, if the resulting integrated aerodynamics models sufficiently capture the actual fluid physics of highly three-dimensional, dynamically separated flows in the presence of stochastic inflows. Nor is it clear to what degree these models are able to produce adequate predictions experienced in the full operating envelope of a field environment. Data from the Unsteady Aerodynamics Experiment are often used for validation of aerodynamic design models. In fact, some 400 authors have cited use of the Unsteady Aerodynamics Experiment data set including 26 papers this year alone. Much of the data are readily available over the internet through collaborative IEA interactions [1]. The most widely used data sets represent “baseline” conditions. Under these conditions, wind speed and yaw error do not undergo extreme variations during multiple successive rotation cycles, and approximate steady, deterministic inflow conditions as closely as possible. These “baseline” data comprise less than 3% of all cycles collected, providing a clear indication of the highly stochastic nature of actual field operation. Unfortunately, only successful model validation comparisons are likely to be published in the open literature. However, model predictions that agree closely with field test data correspond to relatively benign operating conditions. In general, model predictions fail to agree well with Unsteady Aerodynamics Experiment data, where separation and strong three-dimensional effects dominate the blade/inflow interaction. Most analysts and designers agree that aerodynamic model predictions in such flow regimes are suspect, and should be used only with appropriate safety factors based on design and f...
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This note was uploaded on 11/13/2011 for the course AEE 495 taught by Professor O.uzol during the Spring '11 term at Middle East Technical University.

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