Combining unsteady blade pressure measurements and a free-wake vortex model to investigate the cycle-to-cycle variations in wind turbine aerodynamic blade loads in yaw

Prediction of the unsteady aerodynamic flow phenomenon on wind turbines is challenging and still subject to considerable uncertainty. Under yawed rotor conditions, the wind turbine blades are subjected to unsteady flow conditions as a result of the blade advancing and retreating effect and the development of a skewed vortical wake created downstream of the rotor plane. Blade surface pressure measurements conducted on the NREL Phase VI rotor in yawed conditions have shown that dynamic stall causes the wind turbine blades to experience significant cycle-to-cycle variations in aerodynamic loading. These effects were observed even though the rotor was subjected to a fixed speed and a uniform and steady wind flow. This phenomenon is not normally predicted by existing dynamic stall models integrated in wind turbine design codes. This paper couples blade pressure measurements from the NREL Phase VI rotor to a free-wake vortex model to derive the angle of attack time series at the different blade sections over multiple rotor rotations and three different yaw angles. Through the adopted approach it was possible to investigate how the rotor self-induced aerodynamic load fluctuations influence the unsteady variations in the blade angles of attack and induced velocities. The hysteresis loops for the normal and tangential load coefficients plotted against the angle of attack were plotted over multiple rotor revolutions. Although cycle-to-cycle variations in the angles of attack at the different blade radial locations and azimuth positions are found to be relatively small, the corresponding variations in the normal and tangential load coefficients may be significant. Following a statistical analysis, it was concluded that the load coefficients follow a normal distribution at the majority of blade azimuth angles and radial locations. The results of this study provide further insight on how existing engineering models for dynamic stall may be improved through the integration of stochastic models to be able to account for the cycle-to-cycle variability in the unsteady wind turbine blade loads under yawed conditions.peer-reviewe

Predictions of the cycle-to-cycle aerodynamic loads on a yawed wind turbine blade under stalled conditions using a 3D empirical stochastic model

This paper investigates a new approach to model the stochastic variations in the aerodynamic loads on yawed wind turbines experienced at high angles of attack. The method applies the one-dimensional Langevin equation in conjunction with known mean and standard deviation values for the lift and drag data. The method is validated using the experimental data from the NREL Phase VI rotor in which the mean and standard deviation values for the lift and drag are derived through the combined use of blade pressure measurements and a free-wake vortex model. Given that direct blade pressure measurements are used, 3D flow effects arising from the co-existence of dynamic stall and stall delay are taken into account. The model is an important step towards verification of several assumptions characterized as the estimated standard deviation, Gaussian white noise of the data and the estimated drift and diffusion coefficients of the Langevin equation. The results using the proposed assumptions lead to a good agreement with measurements over a wide range of operating conditions. This provides motivation to implement a general fully independent theoretical stochastic model within a rotor aerodynamics model, such as the free-wake vortex or blade-element momentum code, whereby the mean lift and drag coefficients can be estimated using 2D aerofoil data with correction models for 3D dynamic stall and stall delay phenomena, while the corresponding standard derivations are estimated through CFD.peer-reviewe

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

New vortex-lift and tangential-force models for HAWT aerodynamic load prediction

Horizontal axis wind turbines (HAWTs) experience three-dimensional rotational and unsteady aerodynamic phenomena at the rotor blades sections. These highly unsteady three-dimensional effects have a dramatic impact on the aerodynamic load distributions on the blades, in particular, when they occur at high angles of attack due to stall delay and dynamic stall. Unfortunately, there is no complete understanding of the flow physics yet at these unsteady 3D flow conditions, and hence, the existing published theoretical models are often incapable of modelling the impact on the turbine response realistically. The purpose of this paper is to provide an insight on the combined influence of the stall delay and dynamic stall on the blade load history of wind turbines in controlled and uncontrolled conditions. New dynamic stall vortex and nonlinear tangential force coefficient modules, which integrally take into account the three dimensional rotational effect, are also proposed in this paper. This module along with the unsteady influence of turbulent wind speed and tower shadow is implemented in a blade element momentum (BEM) model to estimate the aerodynamic loads on a rotating blade more accurately. This work presents an important step to help modelling the combined influence of the stall delay and dynamic stall on the load history of the rotating wind turbine blades which is vital to have lighter turbine blades and improved wind turbine design systems