The Role of Free-Stream Turbulence on Flow Evolution in the Wake of a VAWT Blade

Abstract

Turbulence evolution in the near wake of a VAWT blade at a constant Reynolds number of 80,000 and an angle of attack of 14 degrees has been studied experimentally under two different levels of free-stream turbulence, 0.5 and 4.6%. Mean velocity, turbulence intensity, turbulence rms velocity and turbulence strain rate profiles have been examined at several streamwise locations in the wake using hot-wire anemometry. External free-stream turbulence has been shown to notably affect the aerodynamic performance of the blade as well as turbulence characteristics of the wake flow. Aerodynamic performance of the blade has been improved under higher free-stream turbulence in the stall region, shown by increase in the lift-to-drag ratio. Free-stream turbulence also decreases the mean velocity deficit and turbulence intensity consistently in the entire wake region, while its effect on the turbulence rms velocity and turbulence strain rate is not monotonic. Substantial asymmetry has been observed in wake profiles for both Tu = 0.5 and 4.6% cases at very near wake regions. The asymmetry decreased going downstream, becoming essentially symmetric past 1.5 chord lengths downstream of the blade trailing edge. At very near wake regions, significant flow characteristic changes occurred between the wake center, wake lower, and upper parts, these changes became more subtle moving downstream.

Prediction of Wind Turbine Airfoil Performance Using Artificial Neural Network and CFD Approaches

“…According to the literature, most studies have been focused on turbulence flows and the high wind speed range. Ahmadi-Baloutaki et al [12] investigated the effect of free-stream turbulence on a NACA0015 airfoil. They presented that the free-stream turbulence in the stall region can improve the aerodynamic performance.…”