5 research outputs found
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 Modified Beddoes–Leishman Model for Unsteady Aerodynamic Blade Load Computations on Wind Turbine Blades
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