scholarly journals Coherent dynamics in the rotor tip shear layer of utility-scale wind turbines

2016 ◽  
Vol 804 ◽  
pp. 90-115 ◽  
Author(s):  
Xiaolei Yang ◽  
Jiarong Hong ◽  
Matthew Barone ◽  
Fotis Sotiropoulos
Keyword(s):  
Wind Turbine ◽  
Shear Layer ◽  
Wind Turbines ◽  
Large Scale ◽  
Wind Farm ◽  
Field Experiments ◽  
Tip Vortex ◽  
Near Wake ◽  
Tip Vortices ◽  
Coherent Dynamics

Recent field experiments conducted in the near wake (up to 0.5 rotor diameters downwind of the rotor) of a Clipper Liberty C96 2.5 MW wind turbine using snow-based super-large-scale particle image velocimetry (SLPIV) (Hong et al., Nat. Commun., vol. 5, 2014, 4216) were successful in visualizing tip vortex cores as areas devoid of snowflakes. The so-visualized snow voids, however, suggested tip vortex cores of complex shape consisting of circular cores with distinct elongated comet-like tails. We employ large-eddy simulation (LES) to elucidate the structure and dynamics of the complex tip vortices identified experimentally. We show that the LES, with inflow conditions representing as closely as possible the state of the flow approaching the turbine when the SLPIV experiments were carried out, reproduce vortex cores in good qualitative agreement with the SLPIV results, essentially capturing all vortex core patterns observed in the field in the tip shear layer. The computed results show that the visualized vortex patterns are formed by the tip vortices and a second set of counter-rotating spiral vortices intertwined with the tip vortices. To probe the dependence of these newly uncovered coherent flow structures on turbine design, size and approach flow conditions, we carry out LES for three additional turbines: (i) the Scaled Wind Farm Technology (SWiFT) turbine developed by Sandia National Laboratories in Lubbock, TX, USA; (ii) the wind turbine developed for the European collaborative MEXICO (Model Experiments in Controlled Conditions) project; and (iii) the model turbine presented in the paper by Lignarolo et al. (J. Fluid Mech., vol. 781, 2015, pp. 467–493), and the Clipper turbine under varying inflow turbulence conditions. We show that similar counter-rotating vortex structures as those observed for the Clipper turbine are also observed for the SWiFT, MEXICO and model wind turbines. However, the strength of the counter-rotating vortices relative to that of the tip vortices from the model turbine is significantly weaker. We also show that incoming flows with low level turbulence attenuate the elongation of the tip and counter-rotating vortices. Sufficiently high turbulence levels in the incoming flow, on the other hand, tend to break up the coherence of spiral vortices in the near wake. To elucidate the physical mechanism that gives rise to such rich coherent dynamics we examine the stability of the turbine tip shear layer using the theory proposed by Leibovich & Stewartson (J. Fluid Mech., vol. 126, 1983, pp. 335–356). We show that for all simulated cases the theory consistently indicates the flow to be unstable exactly in the region where counter-rotating spirals emerge. We thus postulate that centrifugal instability of the rotating turbine tip shear layer is a possible mechanism for explaining the phenomena we have uncovered herein.

A Comparative Study of the Wind Loads and Wake Characteristics of a Single-Rotor Wind Turbine and Dual-Rotor Wind Turbines

2014 ◽  
Author(s):  
Ahmet Ozbay ◽  
Wei Tian ◽  
Hui Hu
Keyword(s):  
Turbulent Flow ◽  
Wind Turbine ◽  
Wind Turbines ◽  
Large Scale ◽  
Wind Farm ◽  
Wake Flow ◽  
Wind Loads ◽  
Neutral Stability ◽  
Near Wake ◽  
Wake Characteristics

An experimental study was carried out to investigate the aeromechanics and wake characteristics of dual-rotor wind turbines (DRWTs) with co- and counter-rotating configurations, in comparison to those of a conventional singlerotor wind turbine (SRWT), in order to elucidate the underlying physics to explore/optimize design of wind turbines for higher power yield and better durability. The experiments were performed in a large-scale Aerodynamic/Atmospheric Boundary Layer (AABL) wind tunnel under neutral stability conditions. In addition to measuring the power output performance of DRWT and SRWT systems, static and dynamic wind loads acting on those systems were also investigated. Furthermore, a high resolution PIV system was used for detailed near wake flow field measurements (free-run and phase-locked) so as to quantify the near wake turbulent flow structures and observe the transient behavior of the unsteady vortex structures in the wake of DRWT and SRWT systems. In the light of the promising experimental results on DRWTs, this study can be extended further to investigate the turbulent flow in the far wake of DRWTs and utilize multiple DRWTs in different wind farm operations.

Drone-Based Experimental Investigation of Three-Dimensional Flow Structure of a Multi-Megawatt Wind Turbine in Complex Terrain

2015 ◽  
Vol 137 (5) ◽  
Author(s):  
B. Subramanian ◽  
N. Chokani ◽  
R. S. Abhari
Keyword(s):  
Wind Speed ◽  
Wind Turbine ◽  
Wind Turbines ◽  
Wind Direction ◽  
Complex Terrain ◽  
Wind Farm ◽  
Three Dimensional ◽  
Energy Yield ◽  
Near Wake ◽  
Tip Vortices

The aerodynamic characteristics of wakes in complex terrain have a profound impact on the energy yield of wind farms and on the fatigue loads on wind turbines in the wind farm. In order to detail the spatial variations of the wind speed, wind direction, and turbulent kinetic energy (TKE) in the near-wake, comprehensive drone-based measurements at a multi-megawatt (MW) wind turbine that is located in complex terrain have been conducted. A short-time Fourier transform (STFT)-based analysis method is used to derive time-localized TKE along the drone's trajectory. In upstream and in the near-wake, the vertical profiles of wind speed, wind direction, and TKE are detailed. There is an increase in the TKE from upstream to downstream of the wind turbine, and whereas, the characteristic microscale length scales increase with increasing height above the ground upstream of the turbine, in the near-wake the microscale lengths are of constant, smaller magnitude. The first-ever measurements of the pressure field across a multi-MW wind turbines rotor plane and of the tip vortices in the near-wake are also reported. It is shown that the pitch between subsequent tip vortices, which are shed from the wind turbines blades, increases in the near-wake as the wake evolves. These details of the near-wake can have an important effect on the subsequent evolution of the wake and must be incorporated into the three-dimensional (3D) field wake models that are currently under intensive development.

Wake Analysis of a Finite Width Gurney Flap

2015 ◽  
Author(s):  
D. Holst ◽  
A. B. Bach ◽  
C. N. Nayeri ◽  
C. O. Paschereit ◽  
G. Pechlivanoglou
Keyword(s):  
Wind Turbine ◽  
Wind Turbines ◽  
Large Scale ◽  
Reynolds Numbers ◽  
Tip Vortex ◽  
Finite Width ◽  
Low Reynolds Numbers ◽  
Gurney Flaps ◽  
Gurney Flap ◽  
Pitch System

The results of stereo Particle-Image-Velocimetry measurements are presented in this paper to gain further insight into the wake of a finite width Gurney flap. It is attached to an FX 63-137 airfoil which is known for a very good performance at low Reynolds numbers and is therefore used for small wind turbines and is most appropriate for tests in the low speed wind tunnel presented in this study. The Gurney flaps are a promising concept for load control on wind turbines but can have adverse side effects, e.g. shedding of additional vortices. The investigation focuses on frequencies and velocity distributions in the wake as well as on the structure of the induced tip vortices. Phase averaged velocity fields are derived of a Proper-Orthogonal-Decomposition based on the stereo PIV measurements. Additional hot-wire measurements were conducted to analyze the fluctuations downstream of the finite width Gurney flaps. Experiments indicate a general tip vortex structure that is independent from flap length but altered by the periodic shedding downstream of the flap. The influence of Gurney flaps on a small wind turbine is investigated by simulating a small 40 kW turbine in Q-Blade. They can serve as power control without the need of an active pitch system and the starting performance is additionally improved. The application of Gurney flaps imply tonal frequencies in the wake of the blade. Simulation results are used to estimate the resulting frequencies. However, the solution of Gurney flaps is a good candidate for large scale wind turbine implementation as well. A FAST simulation of the NREL 5MW turbine is used to generate realistic time series of the lift. The estimations of control capabilities predict a reduction in the standard deviation of the lift of up to 65%. Therefore finite width Gurney flaps are promising to extend the lifetime of future wind turbines.

scholarly journals Impact of Economic Indicators on the Integrated Design of Wind Turbine Systems

2018 ◽  
Vol 8 (9) ◽  
pp. 1668 ◽  
Author(s):  
Jianghai Wu ◽  
Tongguang Wang ◽  
Long Wang ◽  
Ning Zhao
Keyword(s):  
Wind Turbine ◽  
Wind Turbines ◽  
Energy Production ◽  
Large Scale ◽  
Wind Farm ◽  
Net Present Value ◽  
Integrated Design ◽  
Aerodynamic Performance ◽  
System Level ◽  
Blade Length

This article presents a framework to integrate and optimize the design of large-scale wind turbines. Annual energy production, load analysis, the structural design of components and the wind farm operation model are coupled to perform a system-level nonlinear optimization. As well as the commonly used design objective levelized cost of energy (LCoE), key metrics of engineering economics such as net present value (NPV), internal rate of return (IRR) and the discounted payback time (DPT) are calculated and used as design objectives, respectively. The results show that IRR and DPT have the same effect as LCoE since they all lead to minimization of the ratio of the capital expenditure to the energy production. Meanwhile, the optimization for NPV tends to maximize the margin between incomes and costs. These two types of economic metrics provide the minimal blade length and maximal blade length of an optimal blade for a target wind turbine at a given wind farm. The turbine properties with respect to the blade length and tower height are also examined. The blade obtained with economic optimization objectives has a much larger relative thickness and smaller chord distributions than that obtained for high aerodynamic performance design. Furthermore, the use of cost control objectives in optimization is crucial in improving the economic efficiency of wind turbines and sacrificing some aerodynamic performance can bring significant reductions in design loads and turbine costs.

Near-wake behaviour of a utility-scale wind turbine

2018 ◽  
Vol 859 ◽  
pp. 204-246 ◽  
Author(s):  
Teja Dasari ◽  
Yue Wu ◽  
Yun Liu ◽  
Jiarong Hong
Keyword(s):  
Wind Turbine ◽  
Large Scale ◽  
Velocity Ratio ◽  
Rate Of Change ◽  
Tip Vortex ◽  
Time Duration ◽  
Near Wake ◽  
Field Station ◽  
Operation Conditions ◽  
Utility Scale

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) $\times$ 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 are in clear contrast with the expansion states typically associated with wind turbine wakes. These contraction states feature a pronounced upsurge of velocity in the central portion of the wake. The wake velocity ratio $R_{w}$, defined as the ratio of the spatially averaged velocity of the inner wake to that of the outer wake, is introduced to categorize the instantaneous near wake into expansion ($R_{w}<1$) and contraction states ($R_{w}>1$). Based on the $R_{w}$ criterion, the wake contraction occurs 25 % of the time during a 30 min time duration of SLPIV measurements. The contraction states are found to be correlated with the rate of change of blade pitch by examining the distribution and samples of time sequences of wake states with different turbine operation parameters. Moreover, blade pitch change is shown to be strongly correlated to the tower and blade strains measured on the turbine, and the result suggests that the flexing of the turbine tower and the blades could indeed lead to the interaction of the rotor with the turbine wake, causing wake contraction. The visualization of tip vortex behaviour demonstrates the presence of a state of consistent vortex formation as well as various types of disturbed vortex states. The histograms corresponding to the consistent and disturbed states are examined over a number of turbine operation/response parameters, including turbine power and tower strain as well as the fluctuation of these quantities, with different conditional sampling restrictions. This analysis establishes a clear statistical correspondence between these turbine parameters and tip vortex behaviours under different turbine operation conditions, which is further substantiated by examining samples of time series of these turbine parameters and tip vortex patterns. This study not only offers benchmark datasets for comparison with the-state-of-the-art numerical simulation, laboratory and field measurements, but also sheds light on understanding wake characteristics and the downstream development of the wake, turbine performance and regulation, as well as developing novel turbine or wind farm control strategies.

Modeling of the atmospheric boundary layer under stability stratification for wind turbine wake production

2019 ◽  
pp. 0309524X1988092
Author(s):  
Mohamed Marouan Ichenial ◽  
Abdellah El-Hajjaji ◽  
Abdellatif Khamlichi
Keyword(s):  
Boundary Layer ◽  
Atmospheric Boundary Layer ◽  
Wind Turbine ◽  
Wind Turbines ◽  
Large Scale ◽  
Wind Farm ◽  
Layer Structure ◽  
Atmospheric Stability ◽  
Weather Conditions ◽  
Boundary Layer Structure

The assessment of climatological site conditions, airflow characteristics, and the turbulence affecting wind turbines is an important phase in developing wake engineering models. A method of modeling atmospheric boundary layer structure under atmospheric stability effects is crucial for accurate evaluation of the spatial scale of modern wind turbines, but by themselves, they are incapable to account for the varying large-scale weather conditions. As a result, combining lower atmospheric models with mesoscale models is required. In order to realize a reasonable approximation of initial atmospheric inflow condition used for wake identification behind an NREL 5-MW wind turbine, different vertical wind profile models on equilibrium conditions are tested and evaluated in this article. Wind farm simulator solvers require massive computing resources and forcing mechanisms tendencies inputs from weather forecast models. A three-dimensional Flow Redirection and Induction in Steady-state engineering model was developed for simulating and optimizing the wake losses of different rows of wind turbines under different stability stratifications. The obtained results were compared to high-fidelity simulation data generated by the famous Simulator for Wind Farm Applications. This work showed that a significant improvement related to atmospheric boundary layer structure can be made to develop accurate engineering wake models in order to reduce wake losses.

Aerodynamic Rotor Performance of a 3300-kW Modern Commercial Large-Scale Wind Turbine Installed in a Wind Farm

2020 ◽  
Vol 143 (3) ◽  
Author(s):  
Mehmet Bilgili ◽  
Mehmet Tontu ◽  
Besir Sahin
Keyword(s):  
Wind Speed ◽  
Wind Turbine ◽  
Wind Turbines ◽  
Large Scale ◽  
Wind Farm ◽  
Thrust Force ◽  
Speed Ratio ◽  
Flow Angle ◽  
Onshore Wind ◽  
Turbine Performance

Abstract Wind turbine technology in the world has been developed by continuously improving turbine performance, design, and efficiency. Over the last 40 years, the rated capacity and dimension of the commercial wind turbines have increased dramatically, so the energy cost has declined significantly, and the industry has moved from an idealistic position to an acknowledged component of the power generation industry. For this reason, a thorough examination of the aerodynamic rotor performance of a modern large-scale wind turbine working on existing onshore wind farms is critically important to monitor and control the turbine performance and also for forecasting turbine power. This study focuses on the aerodynamic rotor performance of a 3300-kW modern commercial large-scale wind turbine operating on an existing onshore wind farm based on the measurement data. First, frequency distributions of wind speeds and directions were obtained using measurements over one year. Then, wind turbine parameters such as free-stream wind speed (U∞), far wake wind speed (UW), axial flow induction factor (a), wind turbine power coefficient (CP), tangential flow induction factor (a′), thrust force coefficient (CT), thrust force (T), tip-speed ratio (λ), and flow angle (ϕ) were calculated using the measured rotor disc wind speed (UD), atmospheric air temperature (Tatm), turbine rotational speed (Ω), and turbine power output (P) parameters. According to the results obtained, the maximum P, CP, CT, T, and Ω were calculated as approximately 3.3 MW, 0.45, 0.6, 330 kN, and 12.9 rpm, respectively, while the optimum λ, ϕ, U∞, and Ω for the maximum CP were determined as 7.5–8.5, 6–6.3°, 5–10 m/s, and 6–10 rpm, respectively. These calculated results can contribute to assessing the economic and technical feasibility of modern commercial large-scale wind turbines and supporting future developments in wind energy and turbine technology.

Wake Analysis of a Finite Width Gurney Flap

2015 ◽  
Vol 138 (6) ◽  
Author(s):  
D. Holst ◽  
A. B. Bach ◽  
C. N. Nayeri ◽  
C. O. Paschereit ◽  
G. Pechlivanoglou
Keyword(s):  
Wind Turbine ◽  
Wind Turbines ◽  
Large Scale ◽  
Reynolds Numbers ◽  
Tip Vortex ◽  
Finite Width ◽  
Piv Measurements ◽  
Gurney Flaps ◽  
Gurney Flap ◽  
Pitch System

The results of stereo particle-image-velocimetry (PIV) measurements are presented in this paper to gain further insight into the wake of a finite width Gurney flap. It is attached to an FX 63-137 airfoil which is known for a very good performance at low Reynolds numbers and is therefore used for small wind turbines and is most appropriate for tests in the low speed wind tunnel presented in this study. The Gurney flaps are a promising concept for load control on wind turbines but can have adverse side effects, e.g., shedding of additional vortices. The investigation focuses on frequencies and velocity distributions in the wake as well as on the structure of the induced tip vortices. Phase-averaged velocity fields are derived of a proper-orthogonal-decomposition (POD) based on the stereo PIV measurements. Additional hot-wire measurements were conducted to analyze the fluctuations downstream of the finite width Gurney flaps. Experiments indicate a general tip vortex structure that is independent from flap length but altered by the periodic shedding downstream of the flap. The influence of Gurney flaps on a small wind turbine is investigated by simulating a small 40 kW turbine in QBlade. They can serve as power control without the need of an active pitch system and the starting performance is additionally improved. The application of Gurney flaps implies tonal frequencies in the wake of the blade. Simulation results are used to estimate the resulting frequencies. However, the solution of Gurney flaps is a good candidate for large-scale wind turbine implementation as well. A FAST simulation of the NREL 5 MW turbine is used to generate realistic time series of the lift. The estimations of control capabilities predict a reduction in the standard deviation of the lift of up to 65%. Therefore, finite width Gurney flaps are promising to extend the lifetime of future wind turbines.

Wake Alleviating Devices for Offshore Wind Turbines

2014 ◽  
Author(s):  
Vera Klimchenko
Keyword(s):  
Wind Turbine ◽  
Wind Turbines ◽  
Tip Vortex ◽  
Offshore Wind ◽  
Offshore Wind Turbine ◽  
Test Case ◽  
Horizontal Axis Wind Turbine ◽  
Tip Vortices ◽  
Fixed Area ◽  
Blade Tips

The wake behind an offshore wind turbine can persist for several turbine diameters, so decreasing the space between wind turbines in an array leads to strong wake-turbine interactions and a decrease in efficiency of the wind turbines downstream. The dominant structures in the wake of a horizontal axis wind turbine are large helical tip vortices. Implementing devices on the blade tips of a wind turbine can induce mixing into the tip vortex core, encouraging breakup of the tip vortices and wake dissipation. A wake that dissipated more quickly can maximize the farm-level efficiency by allowing more turbines to be installed in a fixed area. This study focuses on quantifying the effectiveness of three different blade-mounted devices in speeding up the dissipation of the wake of an offshore horizontal axis wind turbine. Experiments were conducted in a low speed, low turbulence wind tunnel. A small scale wind turbine model was designed using optimum rotor theory to match the tip speed ratio of an offshore wind turbine. The baseline case consisted of a wind turbine rotor without blade-mounted devices. It was tested in the wind tunnel under a range of free stream conditions, and the rotational speed was measured to determine the operational tip speed ratios. A second test case was the same rotor, but with winglets at the blade tips designed to weaken the tip vortices. A third test case was the baseline case rotor with serrated blade tips, designed to introduce turbulence into the core of the tip vortex. Smoke flow visualization and particle image velocimetry (PIV) were used to observe the dissipation of the turbines’ wake. The effectiveness of the blade-mounted devices on wake dissipation was evaluated with a special interest in optimizing the overall energy harvested by an offshore wind farm of a fixed area. It was shown that both tip treatments tested have the capacity to reenergize the flow and decrease the momentum deficit in the wake of a wind turbine.

Comparison of Experimental and Numerically Predicted Three-Dimensional Wake Behavior of Vertical Axis Wind Turbines

2018 ◽  
Vol 140 (12) ◽  
Author(s):  
Joseph Saverin ◽  
Giacomo Persico ◽  
David Marten ◽  
David Holst ◽  
George Pechlivanoglou ◽  
...  
Keyword(s):  
Wind Turbine ◽  
Wind Turbines ◽  
Large Scale ◽  
Computational Cost ◽  
Three Dimensional ◽  
Vertical Axis ◽  
Tip Vortex ◽  
Design And Optimization ◽  
Aerodynamic Model ◽  
Vertical Axis Wind Turbines

The evolution of the wake of a wind turbine contributes significantly to its operation and performance, as well as to those of machines installed in the vicinity. The inherent unsteady and three-dimensional (3D) aerodynamics of vertical axis wind turbines (VAWT) have hitherto limited the research on wake evolution. In this paper, the wakes of both a troposkien and a H-type VAWT rotor are investigated by comparing experiments and calculations. Experiments were carried out in the large-scale wind tunnel of the Politecnico di Milano, where unsteady velocity measurements in the wake were performed by means of hot wire anemometry. The geometry of the rotors was reconstructed in the open-source wind-turbine software QBlade, developed at the TU Berlin. The aerodynamic model makes use of a lifting line free-vortex wake (LLFVW) formulation, including an adapted Beddoes-Leishman unsteady aerodynamic model; airfoil polars are introduced to assign sectional lift and drag coefficients. A wake sensitivity analysis was carried out to maximize the reliability of wake predictions. The calculations are shown to reproduce several wake features observed in the experiments, including blade-tip vortex, dominant and minor vortical structures, and periodic unsteadiness caused by sectional dynamic stall. The experimental assessment of the simulations illustrates that the LLFVW model is capable of predicting the unsteady wake development with very limited computational cost, thus making the model ideal for the design and optimization of VAWTs.

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