A critical analysis of the stall onset in vertical axis wind turbines

Abstract

The dynamic stall phenomenon in Vertical Axis Wind Turbines (VAWTs) appears, under some operating conditions, to be not very well defined, such as at a low tip speed ratio. Some studies have focused on describing the topology of the dynamic stall but little attention has been paid to understand how all the operating VAWT parameters influence the moment of stall inception. This paper focuses on analysing the influence of the tip speed ratio, pitch angle, reduced frequency, relative velocity and Reynolds number on the stall-onset angle of VAWTs. CFD simulations with an oscillating NACA0015 describing the angle of attack and relative velocity in VAWTs were employed. The results have revealed that an increase in the stall-onset occurs anytime the operating parameters increase the value of the non-dimensional pitch rate and the Reynolds number at the moment the angle of attack approaches to the static stall angle. The stall-onset angle showed a linear increase with the non-dimensional pitch rate in the range of Reynolds number tested, namely 0.8 − 3.3 × 10 5 . This paper has elucidated how the several parameters governing VAWTs operation effect the stall-onset angle and therefore has contributed to a much better understanding of the causes that induce the stall in these devices.

Blade strain analysis from field measurements on a vertical axis wind turbine

“…In VAWT devices, while rotating around the vertical axis, the blades experience aerodynamic loads cyclic variations in both the relative wind velocity and the angle of attack impacting the blades [5]. Thus, the estimations of the unsteady loads affecting the blades become a very challenging task in both numerical and experimental approaches [6,7]. The average power coefficient as a function of the tip speed ratio is the parameter most frequently reported [8].…”