Aeroelastic research programme

The project constitutes a continuously running 5 years research program in aeroelasticity and the main part of the project has been concentrated to obtain results on six specific milestones. A few important results will briefly be described below. Furhter details can be found in the summary report where also project activities outside the six milestones are described and where a thorough list of published material from the project can be found. Double stall: Within the previous EFP-97 project in aeroelasticity it was shown that the major cause of double stall probably is the presence of an unstable laminar separation bubble at the leading edge of the airfoil. Furhter it was shown that the specific development of the bubble strongly depends on the geometry of the leading edge of the airfoil. It should therefor be possible to avoid double stall for new airfoil designs and possibly also on existing blades by redesign of the leading edge of the airfoil. This has been the main subject on double stall within the present project. Two basically different methods to avoid double stall has been investigated. The first is derived from the results of the numerical CFD flow calculations of the bubble which show that forcing turbulent flow before the bubble arise will stabilise the flow and reduce the risk of double stall. Based on wind tunnel experiments the method was shown to work in principal. However, when the turbulent flow was forced to occur from the leading edge e.g. by a trip wire or by roughness the lift was reduced substantially and leading to a low performance for the blade. The other method to avoid double stall is to stabilise the flow with a minor modification of the geometry of the leading edge but with the constraint that the new contour is outside the old so that an existing airfoil can be modified by adding a list. Using a previously developed optimisation tool for airfoil design a modified leading edge design was carried out for the NACA 63-415 and 416 airfoil, which is used onthe LM 19.1 blades, where the double stall problem often is seen. The theoretical method developed to analyse the risk of double stall for an airfoil shows much improved performance for the modified designs and they will now be tested in full scale experiments on a rotor. Limits of dynamic stability: The importance of this subject is expected to be more and more important with the increase in size and flexibility of the turbines. To improve the prediction capabilities of dynamic stability two new numerical tools, the ANSYS and the ADAMS model have been implemented. This is a general purpose finite element program and a general purpose dynamic mechanism programme, respectively, which can be considered as a supplement to the existing aeroealstic programmes. The new tools will be used to evaluate the importance of different degrees of freedom which are not presently been taken into account by the areroelastic models. Extreme loads on a parked rotor: The loads at extreme high wind conditions on a parked rotor can for more wind turbine components be the ultimate loads. However, so far the calculation of these loads has been uncertain because the aeroelastic models are not designed for calculations on a parked rotor. A full 3D CFD calculation has therefor been carried out on a fixed blade. The results show a strong 3D flow around the blade and its influence on the variation of the airfoil coefficients along the blade, which have been quite uncertain so far