Three-dimensional flow field around and downstream of a subscale model rotating vertical axis wind turbine

Three-dimensional, three-component mean velocity fields have been measured around and downstream of a scale model vertical axis wind turbine (VAWT) operated at tip speed ratios (TSRs) of 1.25 and 2.5, in addition to a non-rotating case. The five-bladed turbine model has an aspect ratio (height/diameter) of 1 and is operated in a water tunnel at a Reynolds number based on turbine diameter of 11,600. Velocity fields are acquired using magnetic resonance velocimetry (MRV) at an isotropic resolution of 1/50 of the turbine diameter. Mean flow reversal is observed immediately behind the turbine for cases with rotation. The turbine wake is highly three-dimensional and asymmetric throughout the investigated region, which extends up to 7 diameters downstream. A vortex pair, generated at the upwind-turning side of the turbine, plays a dominant role in wake dynamics by entraining faster fluid from the freestream and aiding in wake recovery. The higher TSR case shows a larger region of reverse flow and greater asymmetry in the near wake of the turbine, but faster wake recovery due to the increase in vortex pair strength with increasing TSR. The present measurement technique also provides detailed information about flow in the vicinity of the turbine blades and within the turbine rotor. The details of the flow field around VAWTs and in their wakes can inform the design of high-density VAWT wind farms, where wake interaction between turbines is a principal consideration.