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Many of the aerodynamic phenomena contributing to the observed effects on wind turbines are now known, but the details of the flow are still poorly understood and are challenging to predict accurately. This article emphasizes two key areas that need continued serious consideration for wind turbine predictive analyses: the modelling of the rotor wake and the modelling of the unsteady aerodynamics of the blade sections. Issues discussed herein include the modelling of the induced velocity field produced by the vortical wake behind the turbine, the various unsteady aerodynamic issues associated with the blade sections, and the non-linear intricacies of dynamic stall. Fundamental limits exist in the capabilities of all models, and ambiguities can also arise in how these models should be properly applied. A challenge for analysts of the future is to use complementary experimental measurements and modelling techniques to better understand the aerodynamic problems found on wind turbines, and to develop more rigorous models with wider ranges of application.The aerodynamic modelling of wind turbines has spanned the gamut from parsimonious models based on simple momentum and blade element momentum (BEM) theories, through 'engineering models' based on blade element and dynamic inflow or vortex wake methods, to computational fluid dynamics (CFD) methods that solve numerically the Euler or Navier-Stokes equations. In this article, the focus will be on the use of engineering methods, which represent a majority of the methods currently being used to design wind turbines. While the other approaches using BEM theory, and perhaps even CFD, can play a useful role in the design * Only for k < O 0Ð01 can the flow be assumed steady or quasi-steady.

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