Coupled and uncoupled CFD-based design strategies for diffuser-augmented wind turbines: A comparative study

Most of the current design methodologies for ducted wind-turbines rely on an uncoupled design-procedure of the duct and the rotor. The aim of this work is to assess, for the first time, the reliability of this strategy by quantifying the differences with an advanced fully-coupled approach. Therefore, two different aero-designs are performed using these strategies. In the uncoupled approach the duct geometry is optimized to maximize the ingested mass flow rate disregarding the rotor presence. Then, a free-vortex disk is introduced, and its load is varied to maximize the power coefficient based on the device frontal-area (CP,ex). In the coupled design, the duct geometry and the rotor load are simultaneously optimized. In both cases, the CFD-actuator-disk is used as analysis method along with a gradient-based optimizer. The coupled strategy yields a higher CP,ex (0.68) adopting a compact diffuser and high stagger-angle. Contrarily, the uncoupled procedure leads to a lower CP,ex (0.59) using a large chord and a moderate stagger. A physical explanation of these differences is offered. Finally, while the presented cases refer to a given shape of the duct, the conceptual generality of the study in terms of the effective reliability of the uncoupled design remains valid in general.