A FREE WAKE METHOD FOR VERTICAL-AXIS WIND TURBINE PERFORMANCE PREDICTION

摘要

The main objective of this study was to develop a new free wake method to analyze the global performance and unsteady blade loads of vertical-axis wind turbines. This has been accomplished by means of a three-dimensional unsteady aerodynamic solution based on a basic vortex flow coupled to an unsteady airfoil performance model. The numerical procedure implies a time marching scheme; according to that at each time step the strength of the bound circulation and latest wake are computed by using Kutta-Joukowski law and the previously shed wake vortices with unchanged strengths. Dynamic stall effects were simulated by an indicial model (as dynamic stall model) which tackles this problem at a more physical level of approximation but still in a sufficiently sample manner. Analysis of the computed results presented in the previous section has demonstrated certain success of the vortex model coupled with the indicial model regarding its ability to accurately predict the instantaneous aerodynamic characteristics of Darrieus wind turbine. The results also tend to support the fact that among other semi-empirical models, the indicial model allows the best representation of dynamic stall for the prediction of wind turbine performance. The vortex model yields results with regard to rotor wake velocities, which are at least in good qualitative agreement with flow visualization experiments. A more detailed experimental study of the turbine wake it is necessary too more rigorously validate the aerodynamic model.