Flow topology of the wake of a rotor and its relationship with the energy efficiency

Abstract

A numerical model is used to study the flow in the wake of a two-bladed horizontal-axis rotor. The flow topology is analyzed for three different angular velocities of the rotor. For the fastest rotor there is a region downstream where the fluid loses nearly 30% of the axial inflow velocity, whereas almost 60% is lost if this region belongs to the wake of a second-in-line rotor. The second part of this study deals with the evolution of the tip helical vortices in the presence of a turbulent inflow wind. This is found to destabilize the vortices, which subsequently perform a leapfrogging motion. As the turbulence of the inflow wind strengthens, the leapfrogging starts closer to the wind rotor. If a second-in-line rotor is located in this wake, the power it extracts from the wind depends on its location with respect to the point where the first rotor’s vortices become unstable. A second rotor located downstream of this point extracts more power than one located upstream of this point.