Dynamic wind loads and wake characteristics of a wind turbine model in an atmospheric boundary layer wind (original) (raw)
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This paper is dedicated to the investigation and analysis of wind turbine wake. An experimental work is undertaken in wind tunnel on a horizontal axis wind turbine model. The velocity field in the wake is measured using PIV with phase synchronization in order to relate velocity and vortices to the rotating blades. The tip vortices are investigated in successive azimuthal positions of the rotor. A specially developed algorithm based on the circulation maximum detects the positions of the vortex cores and permits to use conditional averaging technique. The analysis of obtained velocity fields enables to determine the vortex core diameter, the swirl velocity distribution and the vortex diffusion as functions of the vortex age. The quality of obtained results permits to use them as reference for the validation of numerical computations.
1180 Investigation of Wind Turbine Flow and Wake
The Proceedings of the International Conference on Jets, Wakes and Separated Flows (ICJWSF), 2013
This paper is dedicated to the investigation and analysis of wind turbine wake. An experimental work is undertaken in wind tunnel on a horizontal axis wind turbine model. The velocity field in the wake is measured using PIV with synchronization in order to relate velocity and vortices to the rotating blades. The tip vortices are investigated in successive azimuthal planes and the analysis of vortex characteristics permits to express the vortex core diameter, the swirl velocity distribution and the vortex diffusion as functions of the vortex age. The positions of the vortex cores are obtained by a specially developed algorithm based on circulation calculation. The quality of the obtained results permits their use as a reference for validation of numerical computations.
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.
Field measurements in the wake of a model wind turbine
Journal of Physics: Conference Series, 2014
As a first step to study the dynamics of a wind farm, we experimentally explored the flow field behind a single wind turbine of diameter 1.17 m at a hub height of 6.25 m. A 10 m tower upstream of the wind farm characterizes the atmospheric conditions and its influence on the wake evolution. A vertical rake of sonic anemometers is clustered around the hub height on a second tower, 6D downstream of the turbine. We present preliminary observations from a 1hour block of data recorded in near-neutral atmospheric conditions. The ratio of the standard deviation of power to the inflow velocity is greater than three, revealing adverse effects of inflow turbulence on the power and load fluctuations. Furthermore, the wake defect and Reynolds stress and its gradient are pronounced at 6D. The flux of energy due to Reynolds stresses is similar to that reported in wind tunnel studies. The swirl and mixing produces a constant temperature wake which results in a density jump across the wake interface. Further field measurements will explore the dynamics of a model wind farm, including the effects of atmospheric variability.
Wind Tunnel Investigation of the Near-wake Flow Dynamics of a Horizontal Axis Wind Turbine
Journal of Physics: Conference Series, 2014
Experiments conducted in a large wind tunnel setup investigate the 3D flow dynamics within the near-wake region of a horizontal axis wind turbine. Particle Image Velocimetry (PIV) measurements quantify the mean and turbulent components of the flow field. Measurements are performed in multiple adjacent horizontal planes in order to cover the area behind the rotor in a large radial interval, at several locations downstream of the rotor. The measurements were phase-locked in order to facilitate the reconstruction of the threedimensional flow field. The mean velocity and turbulence characteristics clearly correlate with the near-wake vortex dynamics and in particular with the helical structure of the flow, formed immediately behind the turbine rotor. Due to the tip and root vortices, the mean and turbulent characteristics of the flow are highly dependent on the azimuth angle in regions close to the rotor and close to the blade tip and root. Further from the rotor, the characteristics of the flow become phase independent. This can be attributed to the breakdown of the vortical structure of the flow, resulting from the turbulent diffusion. In general, the highest levels of turbulence are observed in shear layer around the tip of the blades, which decrease rapidly downstream. The shear zone grows in the radial direction as the wake moves axially, resulting in velocity recovery toward the centre of the rotor due to momentum transport.
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
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.
ICOWES2013 Conference 17-19 June 2013, Lyngby, 2013
"The mixing properties of the self-induced flow in a wind turbine wake are studied. The wake of a model horizontal axis wind turbine is analysed with the Particle Image Velocimetry technique and a triple decomposition of the flow. The process of wake re-energising is studied and its dependency on the wake flow structures and stability is shown. The streamwise development of the wake velocity is presented, as well as its clear dependency on the onset of the pairwise instability of the tip-vortices. The mean flow kinetic energy transport and turbulence production is calculated for different regions of the wake. The main conclusion is that the stability of the tip-vortex helix has a strong influence on the mixing of the wake with the outer flow and its re-energising. A thorough estimation of the energy transport at wake scale and the modelling of its dependency on the turbine characteristics would be a first step towards a rotor design process which does not only take into account the aerodynamic and power optimisation of the rotor itself, but also the re-energising properties of the wake, namely the “design of the wake”."
Turbulence Transport Phenomena in the Wakes of Wind Turbines
A true physical understanding of the subtleties involved in the recovery process of the wake momentum deficit downstream of utility-scale wind turbines in the atmosphere has not been obtained to date. While the wind energy community has now a better understanding of some of the effect of the atmospheric stability state on wind turbine power production and wake recovery within an array of wind turbines, available field data are, in general, not acquired at sufficient spatial and temporal resolution that would allow to dissecting some of the mysteries of wake turbulence. It is here that the Actuator Line Method (ALM) has evolved to become the technology standard in the wind energy community for modeling the wakes of single wind turbines as well as arrays of wind turbines and wind farms immersed in an atmospheric boundary-layer flow. This work presents the ALM embedded into an OpenFOAM-LES solver (ALM/LES) and applies it to two small wind farms, the first one consisting of an array of two NREL 5-MW turbines separated by seven rotor diameters in neutral and unstable atmospheric boundary-layer (ABL) flow and the second one consisting of five NREL 5-MW wind turbines arranged in two staggered arrays of two and three turbines, respectively, in unstable ABL flow. Detailed statistics involving power spectral density (PSD) of turbine power along with standard deviations reveal the effects of atmospheric turbulence and its space and time scales. Furthermore, the effect of turbulence generated directly by upstream wind turbines on the power response of downstream wind turbines is quantified. High-resolution surface data extracts in addition to selected Reynolds-stress statistics provide new insight into the complex recovery process of the wake momentum deficit governed by turbulence transport phenomena. A typical wind turbine wake can be divided into three main regions, see Fig. 2.: i) Near Wake, ii) Intermediate Wake, and iii) Far Wake. The near wake is characterized by
Investigation of the Wind Turbine Vortex Structure
Proc. of 14th Int Symp …, 2008
This paper describes the investigation of vortex wake flow downstream of a three blades rotor of a horizontal-axis wind turbine (HAWT) with 0.5 m diameter, placed in a semi-closed return wind tunnel. The phase-locked measurements are carried out by means of ...