A comparison of wake measurements in motor‐driven and flow‐driven turbine experiments

When studying flow phenomena in a scaled laboratory experiment or in a computational simulation, it is often not possible to achieve dynamic similarity with the fullscale flow of interest. In the case of wind and water turbine experiments, a scaled model turbine may not perform as well as in the field, or perhaps not at all, due to a mismatch of Reynolds number and other scaling difficulties such as increased bearing friction. Grant and Parkin (2000) note that in very small-scale model turbine experiments, the blades may operate below their design Reynolds number, causing extensive flow separation that can limit the extrapolation of these model tests to full-scale rotors. Despite this, both wind and water tunnel experiments are used as a practical means to study the flow characteristics of turbines even when dynamic similarity cannot be achieved. Recent progress has been made in understanding the aerodynamics of cross-flow turbines, also known as vertical-axis wind/water turbines (VAWTs). Bachant and Wosnik (2014) examined the effect of Reynolds number on the near-wake characteristics of VAWTs. They found that turbine performance becomes nearly independent of Reynolds number above turbine diameter Reynolds number of ReD = 8× 10 . They also observed that near-wake statistics, such as mean velocity and turbulence intensity, showed only slight differences across the full range of Reynolds number that was examined, i.e., ReD = 3–13 × 10. Abstract We present experimental data to compare and contrast the wake characteristics of a turbine whose rotation is either driven by the oncoming flow or prescribed by a motor. Velocity measurements are collected using two-dimensional particle image velocimetry in the nearwake region of a lift-based, vertical-axis turbine. The wake of this turbine is characterized by a spanwise asymmetric velocity profile which is found to be strongly dependent on the turbine tip speed ratio (TSR), while only weakly dependent on Reynolds number (Re). For a given Re, the TSR is controlled either passively by a mechanical brake or actively by a DC motor. We find that there exists a finite region in TSR versus Re space where the wakes of the motor-driven turbine and flow-driven turbine are indistinguishable to within experimental precision. Outside of this region, the sign of the net circulation in the wake changes as TSR is increased by the motor. Shaft torque measurements show a corresponding sign change above this TSR threshold set by circulation, indicating a transition from net torque due to lift to net torque due to drag produced by the turbine blades, the latter of which can give wake measurements that are inconsistent with a flow-driven turbine. The results support the claim that the turbine kinematics and aerodynamic properties are the sole factors that govern the