Development and scaling of a vertical axis wind turbine wake

Abstract

A wind tunnel experiment is conducted to observe the downstream development of the wake past a model vertical axis wind turbine (VAWT). The flow domain is composed of a streamwise-spanwise plane, obtained via particle image velocimetry, at mid-height of the VAWT rotor. The flow field is assessed by analyzing contours of mean velocities and components in the Reynolds stress tensor. Profiles of the aforementioned quantities and flow parameters are discussed in the context of downstream flow development. The wake is skewed in the direction of the rotor rotation causing an imbalance in the displacement thickness mirrored about the centerline. The wake is divided into two portions, prior and after x/D=2.5, as the effects due to the rotation of the turbine diminish after x/D=2.5. The latter region experiences a reduced wake expansion and decreased velocity deficit compared to the region x/D≤2.5. The velocity and Reynolds stress profiles are then scaled by utilizing various scalings. In search of an improved scaling, a shift to the spanwise coordinate is employed. Downstream dependencies are removed from the Reynolds stress profiles when normalized by the turbulence kinetic energy, k, thus collapsing the profiles. Effects absorbed include the variations in momentum as well as the wake skewness since an imbalance in momentum is present due to the rotation of the VAWT. Findings have implications in modeling due to an improved scaling as well as a basic understanding of the near-wake behavior of a VAWT.