Performance analysis of diffuser-augmented wind turbines through a CFD-based actuator disk method coupled with a Blade-Element approach

Due to their potential to enhance power output, potentially even exceeding the Betz–Joukowsky limit, diffuser-augmented wind turbines are gaining interest, especially for small-scale applications. However, the lack of fast and reliable analysis methods tailored to these devices still hinders their widespread adoption. This paper applies, for the first time, a coupled actuator-disk/Blade-Element-Theory model, embedded in a Computational Fluid Dynamics code, to the analysis of diffuser-augmented wind turbines, and includes a thorough validation of the proposed approach. The flow field is obtained through a Reynolds-Averaged Navier–Stokes solver, while turbine effects are modelled via body forces iteratively evaluated using a Blade-Element scheme. An additional original aspect is that porting this methodology from open to ducted rotors requires a tip-correction-factor model specifically adapted to account for rotor–duct interactions near the blade tips. The results are validated in terms of global performance coefficients using available experimental data. For two turbines, local flow quantities, including wake velocities, are successfully compared with data from a more advanced blade-resolved Reynolds-Averaged Navier–Stokes solver. A relevant new finding is that, contrary to expectations, the actuator disk approach accurately predicts the pressure recovery in the duct divergent part, where strong interactions between the duct boundary layer and tip vortices take place. A further new result is that, for small tip gaps, the blade-forces near the tip exhibit a decreasing–increasing–decreasing trend, associated with tip-vortex/duct interaction. This behaviour cannot be reproduced by simply adapting conventional models for open rotors, thus highlighting the need for a dedicated correction strategy.