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

The dynamic stall phenomenon in Vertical Axis Wind Turbines (VAWTs) appears under some operating conditions that have not been very well established. 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 x 10^5. This paper has elucidated how the several parameters governing VAWTs operation affect the stall-onset angle and therefore has contributed to a much better understanding of the causes that induce the stall in these devices

Unsteady Aerodynamics of Vertical Axis Wind Turbines

This thesis aims to substantially contribute to the understanding of the unsteady aerodynamics that remains unclear in the VAWTs. The analysis has been carried out by using computational fluid dynamics techniques very carefully verified with experimental data and using a modified Leishman-Beddoes dynamic stall algorithm. The results of this investigation are based on the analysis of oscillating aerofoils at constant and time-varying Reynolds and VAWTs with one, two and three blades evaluated at the tip speed ratio range of 1.5-5 using four symmetrical aerofoils and two-cambered aerofoils. The results have shown that: (i) the stall-onset angle in the VAWTs is defined by the combined effect of the tip speed ratio, pitch angle, angular frequency and relative velocity in the final value of the non-dimensional pitch rate when the angle of attack approaches the static stall angle; (ii) Under the fully attached regime, the symmetrical aerofoils performed better than the cambered aerofoils especially at the downstream zone of the rotor. This zone has shown to play the most significant role in the reduction of the overall torque coefficient and this reduction increases with the tip speed ratio, thus, a poor lift force coefficient at negative angles of attack can mitigate the advantages of a high lift/drag aerofoil observed at positive angles of attack. (iii) The curvature effect becomes more relevant with the increase of the tip speed ratio but also appears to be affected by the number of blades, therefore, a fast tool to design VAWTs needs to take into account this phenomenon in order to give reliable results. In the methods proposed in this thesis, despite being limited to the cases investigated, they demonstrate to be a potential tool to predict the unsteady loads in the VAWTs. The present investigation gives sufficient aerodynamic information to design strategies that improve VAWT performance

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

In this paper, the 10 kW WindQuest Vertical Axis Wind Turbine (VAWT) has been instrumented by strain gauges during its trials in the Ifremer in situ test site of Brest to study the effects of the structural dynamic response of the blades under operating conditions. Static and dynamic effects have been investigated as a function of the rotational speed when the rotor operates under stable wind conditions. The analysis segregates the influence of the gravitational, inertial, and aerodynamic loading components on the flapwise bending stress of the blades. The study of the cyclic variations on the blade strain at different Tip-Speed Ratios leads to the identification of the dynamic stall effect on the unsteady loads, while the spectral analysis describes the system eigenfrequencies excited by the interaction of the wind and the structure's motion. The results provide useful data to validate numerical models for VAWT blades with similar design and evaluate the structural fatigue