Investigation of the near-wake behaviour of a utility-scale wind turbine (original) (raw)
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Near-wake behaviour of a utility-scale wind turbine
Journal of Fluid Mechanics, 2018
Super-large-scale particle image velocimetry (SLPIV) and the associated flow visualization technique using natural snowfall have been shown to be effective tools to probe the turbulent velocity field and coherent structures around utility-scale wind turbines (Hong et al.Nat. Commun., vol. 5, 2014, article 4216). Here, we present a follow-up study using the data collected during multiple deployments from 2014 to 2016 around the 2.5 MW turbine at the EOLOS field station. These data include SLPIV measurements in the near wake of the turbine in a field of view of 115 m (vertical) times\timestimes 66 m (streamwise), and the visualization of tip vortex behaviour near the elevation corresponding to the bottom blade tip over a broad range of turbine operational conditions. The SLPIV measurements provide velocity deficit and turbulent kinetic energy assessments over the entire rotor span. The instantaneous velocity fields from SLPIV indicate the presence of intermittent wake contraction states which...
Effect of turbine nacelle and tower on the near wake of a utility-scale wind turbine
Journal of Wind Engineering and Industrial Aerodynamics, 2019
Super-large-scale particle image velocimetry (SLPIV) using natural snowfall is used to investigate the influence of nacelle and tower generated flow structures on the near-wake of a 2.5 MW wind turbine at the EOLOS field station. The analysis is based on the data collected in a field campaign on March 12 th , 2017, with a sample area of 125 m (vertical) × 70 m (streamwise) centred on the plane behind the turbine support tower. The SLPIV measurement provides the velocity field over the entire rotor span, revealing a region of accelerated flow around the hub caused by the reduction in axial induction at the blade roots. The in-plane turbulent kinetic energy field shows an increase in turbulence in the regions of large shear behind the blade tips and the hub, and a reduction in turbulence behind the tower where the large-scale turbulent structures in the boundary layer are broken up. Snow voids reveal coherent structures shed from the blades, nacelle, and tower. The hub wake meandering frequency is quantified and found to correspond to the vortex shedding frequency of an Ahmed body (0.06). Persistent hub wake deflection is observed and shown to be connected with the turbine yaw error. In the region below the hub, strong interaction between the tower-and blade-generated structures is observed. The temporal characteristics of this interaction are quantified by the co-presence of two dominant frequencies, one corresponding to the blade vortex shedding at the blade pass frequency and the other corresponding to tower vortex shedding at 0.2. This study highlights the influence of the tower and nacelle on the behaviour of the near-wake, informing model development and elucidating the mechanisms that influence wake evolution.
Natural snowfall reveals large-scale flow structures in the wake of a 2.5-MW wind turbine
Nature Communications, 2014
To improve power production and structural reliability of wind turbines, there is a pressing need to understand how turbines interact with the atmospheric boundary layer. However, experimental techniques capable of quantifying or even qualitatively visualizing the large-scale turbulent flow structures around full-scale turbines do not exist today. Here we use snowflakes from a winter snowstorm as flow tracers to obtain velocity fields downwind of a 2.5-MW wind turbine in a sampling area of B36 Â 36 m 2. The spatial and temporal resolutions of the measurements are sufficiently high to quantify the evolution of bladegenerated coherent motions, such as the tip and trailing sheet vortices, identify their instability mechanisms and correlate them with turbine operation, control and performance. Our experiment provides an unprecedented in situ characterization of flow structures around utility-scale turbines, and yields significant insights into the Reynolds number similarity issues presented in wind energy applications.
Energies
Wind turbines are usually clustered in wind farms which causes the downstream turbines to operate in the turbulent wakes of upstream turbines. As turbulence is directly related to increased fatigue loads, knowledge of the turbulence in the wake and its evolution are important. Therefore, the main objective of this study is a comprehensive exploration of the turbulence evolution in the wind turbine’s wake to identify characteristic turbulence regions. For this, we present an experimental study of three model wind turbine wake scenarios that were scanned with hot-wire anemometry with a very high downstream resolution. The model wind turbine was exposed to three inflows: laminar inflow as a reference case, a central wind turbine wake, and half of the wake of an upstream turbine. A detailed turbulence analysis reveals four downstream turbulence regions by means of the mean velocity, variance, turbulence intensity, energy spectra, integral and Taylor length scales, and the Castaing param...
An Experimental Study of the Near Wake of Horizontal Axis Wind Turbines
The present work considers an experimental investigation of wind turbine near wake by using Particle Image Velocimetry (PIV) visualization technique. The PIV technique gives a complete picture of all points at the domain under consideration. The study is focused on the effect of tip speed ratio () and Reynolds number (Re c ) on the near wake characteristics. A threeblade model of wind turbine with airfoil SG 6040 16% is tested in water channel at Re c range between 1.28 ×10 . Various tip speed ratios are tested between =2 and =12. Experiments are also performed at constant =8 and variable Re c 4 and 7.68 ×10 4 1 in the range between 2.56 ×10 and 5.12 ×10 4 . The results show that as increases both wake width and length increase up to =9. For higher values of l, the wake width remains constant. The turbulence intensity measurements show that an increase of causes an increase of turbulence intensity in the wake. Experiments at constant tip speed ratio λ = 8 and variable Reyno...
A field study of the wake behind a 2 MW wind turbine
Atmospheric Environment (1967), 1988
The wake behind the 2 MW wind turbine situated on flat country at Ntisudden, Sweden has been mapped in a field experiment where several measuring techniques were employed. A high resolution sodar appeared to be a very powerful tool which enabled detailed studies of both mean and turbulence wake profiles to be carried out. Measurements with this system were made with three antennae in three different modes of operation and at several distances in the range 20-3.60 from the turbine (D being the rotor diameter, 75 m). Additional detailed data were obtained with turbulence instruments at three levels at a 145 m tower, situated 30 downwind, during conditions with the wind blowing from the turbine towards the tower. Finally good quality data of the centre line velocity deficit and of the longitudinal turbulence intensity were obtained with Tala kites at distances of up to 10.5D.
Near-wake flow structure downwind of a wind turbine in a turbulent boundary layer
Experiments in Fluids, 2011
Wind turbines operate in the surface layer of the atmospheric boundary layer, where they are subjected to strong wind shear and relatively high turbulence levels. These incoming boundary layer flow characteristics are expected to affect the structure of wind turbine wakes. The near-wake region is characterized by a complex coupled vortex system (including helicoidal tip vortices), unsteadiness and strong turbulence heterogeneity. Limited information about the spatial distribution of turbulence in the near wake, the vortex behavior and their influence on the downwind development of the far wake hinders our capability to predict wind turbine power production and fatigue loads in wind farms. This calls for a better understanding of the spatial distribution of the 3D flow and coherent turbulence structures in the near wake. Systematic wind-tunnel experiments were designed and carried out to characterize the structure of the near-wake flow downwind of a model wind turbine placed in a neutral boundary layer flow. A horizontal-axis, three-blade wind turbine model, with a rotor diameter of 13 cm and the hub height at 10.5 cm, occupied the lowest one-third of the boundary layer. High-resolution particle image velocimetry (PIV) was used to measure velocities in multiple vertical stream-wise planes (x–z) and vertical span-wise planes (y–z). In particular, we identified localized regions of strong vorticity and swirling strength, which are the signature of helicoidal tip vortices. These vortices are most pronounced at the top-tip level and persist up to a distance of two to three rotor diameters downwind. The measurements also reveal strong flow rotation and a highly non-axisymmetric distribution of the mean flow and turbulence structure in the near wake. The results provide new insight into the physical mechanisms that govern the development of the near wake of a wind turbine immersed in a neutral boundary layer. They also serve as important data for the development and validation of numerical models.
Near-Wake Flow Dynamics of a Horizontal Axis Wind Turbine
2012
Experiments have been conducted in a large wind tunnel setup in order to study the flow structures within the near-wake region of a horizontal axis wind turbine. Particle Image Velocimetry (PIV) has been employed to quantify the mean and turbulent components of the flow field. The measurements have been performed in multiple adjacent horizontal planes in order to cover the area behind the rotor in a large radial interval, at several locations downstream Moreover, the PIV results have also been compared with the results obtained from the velocity measurements performed by previous investigators in small wind tunnel setups , in order to assess the scaling effects, and in particular the effect of local chord Reynolds number. The tip speed ratio is considered to be similar for all measurement to satisfy the kinematic similarity requirement. The comparison shows that the axial velocity profiles are highly dependent on Reynolds number. This is an important finding in terms of simulating scaled models of wind turbines and wind farms in wind tunnel settings. v Keywords: horizontal axis wind turbine, wake, wind tunnel experiments, Particle Image Velocimetry, large scale experimental setup, turbulent flow field, phase-locked measurement vi ACKNOLEDGEMENTS I wish to express my profound appreciation and sincere gratitude to my supervisor, Professor Horia M. Hangan, for his thoughtful insights, invaluable advices, stimulating suggestions, as well as his continuous encouragement, patience support and guidance throughout this research. I gratefully acknowledge my co-supervisor, Professor Kamran Siddiqui, for his support, expert help, thoughtful advices and useful guidelines throughout this research in particular in PIV measurements and data analysis. I would also like to thank the staff of the Boundary Layer Wind Tunnel Laboratory and the
Visualization of the tip vortices in a wind turbine wake
Journal of Visualization, 2012
In the present study, an experimental study was conducted to characterize the formation and the evolution of the helical tip vortices and turbulent flow structures in the wake of a horizontal axis wind turbine model placed in an atmospheric boundary layer wind. A high-resolution particle image velocimetry system was used to make detailed flow field measurements to quantify the time evolution of the helical tip vortices in relation to the position of the rotating turbine blades in order to elucidate the underlying physics associated with turbine power generation and fatigue loads acting on the wind turbines.
Effects of Inflow Conditions on Wind Turbine Performance and near Wake Structure
Open Journal of Fluid Dynamics
Knowledge about the structure and development of wakes behind wind turbines is important for power optimization of wind power farms. The high turbulence levels in the wakes give rise to undesired unsteady loadings on the downstream turbines, which in the long run might cause fatigue damages. In the present study, the near wake behind a small-scale model wind turbine was investigated experimentally in a wind tunnel. The study consists of measurements with particle image velocimetry using two different inlet conditions: a freely developing boundary layer, causing an almost uniform inflow across the rotor disc, and an inflow with strong shear across the rotor disc, in order to model the atmospheric boundary layer. The results show a faster recovery of the wake in the case with shear inflow, caused by the higher turbulence levels and enhanced mixing of momentum. The increased inlet turbulence levels in this case also resulted in a faster breakdown of the tip vortices as well as different distributions of the streamwise and vertical components of the turbulence intensity in the wake. An analysis comparing vortex statistics for the two cases also showed the presence of strong tip vortices in the case with lower inlet turbulence, while the case with higher inlet turbulence developed a different distribution of vortices in the wake.