Secondary flows in the wake of a vertical axis wind turbine of solidity 0.5 working at a tip speed ratio of 2.2

The influence of the end effects in the wake development of a vertical axis wind turbine is studied using a LargeEddy Simulation methodology, coupled with an Immersed-Boundary technique, for a value of solidity sigma = Nc/D = 0.5 (N: number of blades; c: chord length of the blades; D: diameter of the turbine), a tip speed ratio TSR = R omega/ U infinity = 2.206 (R: radius of the turbine; omega: angular speed; U infinity: free-stream velocity) and a Reynolds number Re = DU infinity/nu = 180, 000 (nu: kinematic viscosity). The end effects at the tip of the blades originate significant secondary flows, especially on the windward side. They promote additional lateral displacement of the momentum deficit from the leeward side towards the windward side, increase the levels of turbulence and reinforce the asymmetry of the wake system. Turbulence is also intensified by the shear produced at the top and bottom boundaries of the wake between the free-stream and the decelerated flow downstream of the turbine. Significant vertical flows are produced, whose intensity and orientation were found dependent on the horizontal coordinate between leeward and windward sides. Their overall effect consists in contributing to momentum recovery downstream of the turbine, which is indeed faster at the locations closer to its spanwise boundaries, especially on the leeward side.

Automated summary

The study found that the end effects at the tip of the blades of a vertical axis wind turbine cause significant secondary flows, especially on the windward side. They lead to additional lateral displacement of the momentum deficit from the leeward side towards the windward side, increasing turbulence levels and reinforcing the asymmetry of the wake system.