運用有限單元梁元素分析複合材料風力葉片

Abstract

目前在分析風力葉片結構特性上，大部分研究仍採用有限單元中的薄殼元素進行模擬，此套方法的發展也十分地完整，但對於應用至較為複雜的葉片分析如結構最佳化等運算時，過多的網格便可能導致整體運算量驟增而效率降低，因此，為了減少分析的耗時，本研究提出以有限單元之梁元素模型結合數值方法來計算2.5米複合材料風力葉片，並以其受力變形及自然頻率值作為與殼元素模型進行比較的標準。在此之前，將以三種截面較葉片更為簡易的梁結構先進行理論驗證，以梁元素模型和使用分析軟體的殼元素模型之受力變形作判斷，最終三種截面結構之變形量差異均落在3.5%之內，證明此梁元素模型的計算結果具有相當的可靠性，可進一步套用至2.5米葉片的結構計算上。 本文所使用之葉片翼型為NACA4418，實際製作時採樹脂轉注成形製造法，葉片完成後將進行靜態負載實驗，以此實驗結果驗證分析軟體之殼元素模型的正確性，接著再以本文之梁元素方法和上述之殼元素模型進行葉片受力變形與自然頻率的結果比較，本研究最終的結果差異均保持在5%之內，顯示本文所提出的計算方法已達到設定的目標，在模擬葉片結構之特性上有相當的準確性，且相對於殼元素而言，梁元素不僅在達到模型收斂時的元素個數需求較低，造就其能有效縮短後續運算的時間，在修改葉片參數上亦十分方便。

Currently, most of the researches on structural characteristics of wind turbine blades are still carried out by using shell elements of the finite element method. However, for more complex analysis of blade such as structural optimization and so on, the use of a large amount of elements may be accompanied with increasing processing time and thus makes the analysis inefficient. In this study, a beam element is proposed to analyze the deformation and natural frequency of a 2.5m composite wind turbine blade. Prior to this, three kinds of simple sections different from blade section will be used to validate the feasibility of the beam element. In the validation of the proposed element, it has been shown that the errors of structural deformations between the present method and the Abaqus solution using shell elements are within 3.5%, which prove that the present procedure is quiet reliable and can be applied to the analysis of 2.5m composite wind turbine blade. The blade comprising airfoil of NACA4418 is fabricated using the VARTM (Vacuum Assistant Resin Transfer Modeling) method. The blade was subjected to static testing. The experimental deflection of the wind blade is used to validate the suitability of the Abaqus finite element model constructed using shell elements. The comparison between the results of blade deformation and natural frequency obtained using the present method and the Abaqus shell element model has shown that the errors are within 5%. In this study, it has been shown that the blade analysis procedure proposed in this study has reached the goal with acceptable accuracy in simulating the structural characteristics of wind blade. The advantage of using beam elements in analyzing the mechanical behavior of wind blades includes not only the reduction of the element number but also the shortening of computational time. Besides, the proposed method provides easy means for modifying the parameters during simulation.