  This study explored the aerodynamics of a new multi-stage co-axial
vertical-axis wind turbine based on bio-inspiration from natural swimming habit
of fish. The turbine was formed from a conventional straight-bladed
vertical-axis turbine (VAWT) with a small inner rotor, also of three blades.
The azimuthal and radial locations of the inner rotor were varied. Using
numerical simulations, performance of the proposed new design was evaluated
over a range of tip-speed ratios. The preliminary results identified a 600%
increase in power output for multi-stage VAWTs at tip-speed rations TSR = 0 -
3, and a substantial drop in power coefficient at TSR > 3.0. The wake dynamics
analyses revealed that the increase was due to interactions between the blades
of one rotor and the other. This reduced the unsteady separation from the outer
rotor which produced most of the power. A detailed parametric study was also
completed, which showed the implications of geometric and kinematic details on
the performance of the proposed multistage VAWT.
