Aeroelastic research programme 2001 - 2002

The project covers the one year period from mid 2001 to mid 2002 and is the last part of a 5 years research programme on aeroelasticity. The overall objectives of the project are to improve the load and design basis for wind turbines and to ensure in collaboration with industry a continuously running process on development of new designs and solution of actual problems. Specifically the main objectives for the present period are the following: a) development of a design tool for analysis of dynamic stability. b) investigations of blade tip aerodynamics and blade tip design on basis of 3D CFD computations. c) publication of an airfoil catalogue. d) load reduction using new control strategies. e) aeroacoustic modelling of noise propagation. During the present project period the computer code HAWCModal has been finished. The code computes the modal characteristics for a turbine as function of rotational speed. It is based on the structural modelling in the aeroelastic code HAWC and uses the same input files. The computed eigen frequencies are shown in a Campbell diagram and the corresponding modal forms can be shown graphically for an operating turbine. Finally, the structural damping is also computed by the code. HAWCModal is the basis for the stability analysis tool HAWCStab which is now under develop-ment. With HAWCStab the aeroelastic stability of a turbine can be analysed. The complex aerodynamics at three different blade tip shapes have been analysed with the three-dimensional CFD code EllipSys3D. The tip vortex was visualised and the lift and drag coefficients in the tip region were analysed in order to study the influence of the tip geometry on the performance and aerodynamic damping. An airfoil cataloque containing computations on 28 different airfoils for wind turbine application in comparison with experimental data has been developed and is available via the internet. Besides the main themes of the project as mentioned above there have been research activities within different other areas. New aerodynamic routines have been implemented in the aeroelastic code HAWC which includes sub models for yaw and dynamic inflow. In order to take into account the non-linear effects from big blade deflections a new initialisation routine has been included in HAWC. During this routine the blade deflection from the average wind speed is computed at the blade nodes which afterwards are moved to these positions. In this way a time simulation can be run which the includes the main effect from non-linearity. Within CFD progress has been made on a new transition modelling. Initial results are encouraging and e.g. computations on thick airfoils seem to be improved conciderably. A new formulation of the tip correction as applied in the Blade Element Momentum (BEM) model has been derived and using this new formula a better correlation of blade loads as measured in the NREL experiment in the NASA Ames wind tunnel with model results has been achieved