Locations of vortices and their impacts on the aerodynamic performances of a diffuser and a DAWT

Geometric features of an unloaded Diffuser Augmented Wind Turbine (DAWT), are the main parameters controlling the aerodynamic performances of this wind-energy device. Results obtained from wind-tunnel experiments, Particle Image Velocimetry (PIV) measurements and numerical simulations, show that the wind velocity of the airflow increases while approaching the diffuser. It attains a maximum value at the diffuser's throat, where a wind turbine should be mounted, and then it decreases. The wind velocity increase, depends mostly on the positions of a two contra-rotating vortices generated behind the diffuser's flange due to the Kelvin-Helmholtz instabilities. The best aerodynamic performances of the diffuser are obtained when these vortices are located too near from the flange without being introduced into the area situated immediately downstream the diffuser's outlet section. At the same time, the flange's height, the open angle and the length of the diffuser reach their optimal values. Beyond these values, one of the two vortices tends to submerge in the area delimited by the diffuser's outlet section. As a result, a boundary-layer separation takes place at the diffuser's inner wall, and the geometric characteristics become without significant effect on increasing the wind velocity. In practice, this means that the diffuser could be more efficient if equipped with a control system, able to hold the vortices too near from the flange.