Velocity Field Measurements in Wake of a Straight-Bladed Vertical Axis Wind Turbine

Small vertical axis wind turbine (VAWT) is promising for the development of wind energy in the built environment due to its insensitivity to yaw. In general, computer codes based on the blade element and momentum (BEM) theory, which have much less calculation time than CFD codes, are used for design and performance prediction of wind turbines. However, the results on small VAWTs obtained by the BEM theory often do not accord with the experimental results due to the low Reynolds number, the dynamic stall effects, and so forth. The three-dimension nature of the flow field around the VAWT rotor may be one of the reasons for the discord. This study aims to elucidate the actual flow field around small VAWT. In this study, velocity field measurements were carried out in the wake of a small straight-bladed VAWT by using an ultrasonic anemometer. The measurements of the V and W velocity components showed the downwash and the blow-up, which proceeded toward the equator level and merged into the horizontal flow. Counter flow of the U component was also observed in the mainstream direction. The wake velocity profile simulated by using the Double-Multiple Streamtube (DMS) model based on the BEM theory showed a similar behavior to that of the measured wake. However, the high thrust coefficient estimated from the measured deficit in the U velocity component almost doubles the estimation by the BEM theory.