Towards performance improvements in vertical axis wind turbines: Improving wind tunnel simulations of the atmospheric boundary layer and wake interaction studies

Abstract

The performance and wake structure of a model vertical axis wind turbine (VAWT) is studied, subject to various blade geometries, modes of operation, and inflow conditions.

To facilitate this investigation, the neutral atmospheric boundary layer (ABL) must be simulated in a short recirculating wind tunnel. This is achieved using a modified form of Counihan's method, composed of passive devices including a barrier wall, elliptical vortex generators, and wall roughness elements. Improvements to these elements resulted in a $1:1000$ scale representation of the neutral ABL, with turbulence characteristics that match well with both rough wall boundary layer measurements and full-scale atmospheric measurements alike. Good spanwise and streamwise uniformity of this simulated ABL was achieved in only five boundary layer depths, $\delta$, allowing approximately half of the wind tunnel test section for turbine measurements.

The effect of several blade geometries were tested, including straight blades and blades of varying sweep magnitude and direction, and measurements of the flow both upstream of the turbine and in its wake were performed using particle image velocimetry (PIV). The measurements were performed at two tip speed ratios, $\lambda=1$ and 3, and in both uniform and ABL inflow conditions. The wake is highly three-dimensional, and both proper orthogonal decomposition (POD) and phase-locked measurements reveal a wake structure dominated by the effects of dynamic stall. Though the straight blades resulted in the highest performance of any of the tested blade geometries, apparent distortion of the swept blades due to sweep angle is suspected. Despite these undesired effects, the swept blades do result in higher planform energy flux into the turbine wake, confirming previous observations and correlating with a more rapid recovery of the turbine wake. Finally, ABL inflow is shown to have a positive effect on turbine performance, increasing turbine efficiency and decreasing the effects of dynamic stall.