A 3-D stall-delay model for horizontal axis wind turbine performance prediction

Most design and analysis methods widely used for horizontal axis wind turbine performance prediction, such as the PROP code, are based on the traditional 2-D blade element/momentum theory (BEMT) methods, which are inadequate and underpredict the wind turbine rotor power output in the high-wind/peak-power condition, owing to effects of rotation on the wind turbine blade boundary layer. Although the deficiencies of the methods have been known for some time, this area has been neglected. The continued development of viable and well-established stall-regulated wind-turbine technology makes this research topic timely and particularly relevant to reducing the cost of wind energy. The main aim of the present paper is to describe and analyze the fundamental flow phenomena that characterize the boundary layer on rotating blades, and to develop a preliminary stall-delay model that modifies the 2-D airfoil data so as to simulate the 3-D stall-delay effects. The following steps were taken in the development of the model: 1) analysis of the 3-D integral boundary- layer equations for a reference system rotating with the blade, 2) description of the effects of rotor rotation on the separation point and its causes, and 3) determination of a simple correction formula to obtain rotating rotor lift coefficient C_i(a) and drag coefficient C_d(a) data from measured 2-D airfoil data. The preliminary 3-D stall-delay model consists of two key parameters (the ratio of local chord to local radius c/r the ratio of rotation speed to freestream velocity A) and three empirical correction factors(a, b, d). The stall-delay model is consistent with the blade element/momentum theory method and the Viterna/Tangler model, and the 3-D stall-delay model can be incorporated into the state of the art performance prediction codes, such as PROP. Through comparison with the field test data, the new model for 3-D stall-delay shows good agreement between predictions and experiments. The new model should be of great use in existing codes for horizontal axis wind turbine design and analysis.