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

One well-known obstacle facing most sources of renewable energy is the significantly decreased energy density with respect to fossil fuels. Energy density of wind power is further penalized by the need for wind turbines to be spaced far enough apart to not interfere with one another. Conventional horizontal axis wind turbines (HAWTs) must be spaced far apart [3–5 turbine diameters in the crosswind direction, 6–10 diameters in the downwind direction (Sørensen 2004; Hau 2005)]. This energy density imbalance requires that wind farms have a large land area to generate significant power. Vertical axis wind turbines (VAWTs) have promise in increasing the energy density of wind power beyond HAWTs. The swept area of a VAWT can be increased independently of its footprint by extending the rotor blade height, thereby increasing power extraction for the same wind farm footprint. Individual VAWT rotors can also be more closely packed than HAWTs. A recent study showed that the distance behind the turbine required to regain 95 % of the upwind velocity was 15 diameters for a traditional HAWT, but only 4 for a straight-bladed VAWT (Dabiri 2011). It has also been hypothesized that optimal layouts of VAWT arrays can benefit from constructive aerodynamic interactions of neighboring turbines (Whittlesey et al. 2010). Following these observations, it was argued that the power density of optimal VAWT arrays could be an order of magnitude higher than present HAWT farms (Dabiri 2011). Abstract 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