scholarly journals Far-wake meandering induced by atmospheric eddies in flow past a wind turbine

2018 ◽  
Vol 846 ◽  
pp. 190-209 ◽  
Author(s):  
X. Mao ◽  
J. N. Sørensen
Keyword(s):  
Wind Turbine ◽  
Strouhal Number ◽  
Large Scale ◽  
Free Stream ◽  
Stokes Equations ◽  
Wind Farms ◽  
Free Stream Velocity ◽  
Immersed Boundary ◽  
Stream Velocity ◽  
Actuator Disc

A novel algorithm is developed to calculate the nonlinear optimal boundary perturbations in three-dimensional incompressible flow. An optimal step length in the optimization loop is calculated without any additional calls to the Navier–Stokes equations. The algorithm is applied to compute the optimal inflow eddies for the flow around a wind turbine to clarify the mechanisms behind wake meandering, a phenomenon usually observed in wind farms. The turbine is modelled as an actuator disc using an immersed boundary method with the loading prescribed as a body force. At Reynolds number (based on free-stream velocity and turbine radius) $Re=1000$, the most energetic inflow perturbation has a frequency $\unicode[STIX]{x1D714}=0.8$–2, and is in the form of an azimuthal wave with wavenumber $m=1$ and the same radius as the actuator disc. The inflow perturbation is amplified by the strong shear downstream of the edge of the disc and then tilts the rolling-up vortex rings to induce wake meandering. This mechanism is verified by studying randomly perturbed flow at $Re\leqslant 8000$. At five turbine diameters downstream of the disc, the axial velocity oscillates at a magnitude of more than 60 % of the free-stream velocity when the magnitude of the inflow perturbation is 6 % of the free-stream wind speed. The dominant Strouhal number of the wake oscillation is 0.16 at $Re=3000$ and keeps approximately constant at higher $Re$. This Strouhal number agrees well with previous experimental findings. Overall the observations indicate that the well-observed stochastic wake meandering phenomenon appearing far downstream of wind turbines is induced by large-scale (the same order as the turbine rotor) and low-frequency free-stream eddies.

Numerical investigation of the flow dynamics past a three-element aerofoil

2013 ◽  
Vol 732 ◽  
pp. 401-444 ◽  
Author(s):  
Sébastien Deck ◽  
Romain Laraufie
Keyword(s):  
Numerical Investigation ◽  
Large Scale ◽  
Free Stream ◽  
Pressure Fluctuations ◽  
Phase Relationship ◽  
Feedback Mechanism ◽  
Flow Dynamics ◽  
Free Stream Velocity ◽  
Stream Velocity ◽  
Detached Eddy Simulation

AbstractA numerical investigation of the flow dynamics around a two-dimensional high-lift configuration was carried out by means of a zonal detached eddy simulation (ZDES) technique for flow conditions corresponding to aircraft approach. Both slat and flap regions have been scrutinized and compared with experimental data available in the literature. It is shown that slat and flap coves behave like shallow cavities. The distance between the upstream cusp and the downstream edge is the relevant length scale for each cove taken separately. Consistently with previous findings, this study indicates that the maximum of the broadband spectrum of slat (respectively flap) pressure fluctuations occurs for Strouhal numbers $0. 5\leq \mathit{St}\leq 4$ when based on slat chord (respectively on flap chord) and free-stream velocity. It is shown that mode $(n)$ of the slat cove and mode $(n+ 1)$ of the flap cove are very close making a coherent phase relationship possible. A large-scale coupled self-sustained oscillations mechanism between slat and flap cavities, evidenced by spectral analysis, occurs at a Strouhal number $\mathit{St}= 3{\unicode{x2013}} 6$ based on the main wing chord and free-stream velocity. This yields to an acoustic feedback mechanism characterized by a normalized frequency depending on the free stream Mach number like $\mathit{St}= (1- { M}_{0}^{2} )/ 2{M}_{0} $. The present result appears to line up with the findings by Hein et al. (J. Fluid Mech., vol. 582, 2007, pp. 179–202) who showed that two types of resonance could exist: surface waves ones, scaling with the total aerofoil length and longitudinal cavity-type resonances, scaling with the slat cove length.

Experimental Study on Performance of Vertical Axis Wind Turbine with NACA 4-Digital Series of Blades

2012 ◽  
Vol 488-489 ◽  
pp. 1055-1061 ◽  
Author(s):  
W.C. Hsieh ◽  
J.M. Miao ◽  
C.C. Lai ◽  
C.S. Tai
Keyword(s):  
Wind Turbine ◽  
Output Power ◽  
Free Stream ◽  
Rotation Speed ◽  
Vertical Axis ◽  
Free Stream Velocity ◽  
Stream Velocity ◽  
Test Model ◽  
Vertical Axis Wind Turbine ◽  
Blade Section

The experimental studies of output power performances of a vertical-axis-wind-turbine (VAWT) had been conducted in suction-type low speed wind tunnel with various free stream velocity. Torque and rotation speed of blades were measured by using torque meter and optical detector to analyze the effect of blade-section shape on the performance of wind turbine. The test model of experiments in the research was H-rotor VAWT. Three shapes of the NACA 4-digital series blade-section, NACA0022, NACA6404, and NACA6422 were taken in this work. Effects of thickness and camber of blade-section, blade numbers, and blade setting angles on the performance of VAWT have been analyzed in detail. The results show that NACA6422 blade-section has rotation speed of 42% higher than that of NACA0022 when the free stream velocity is below 12 m/s and the blade numbers are 4-blade type. Wind turbines with NACA6422 blades also showed that about 10% higher output power than that of NACA0022 blades among the tested range of free stream velocity. Results indicated that wind turbine with blades of anti-symmetric and thick blade-section was generally more suitable for applying to VAWT. All results of this study can be used the optimization design of VAWT blades in further.

scholarly journals Development of a Computational System to Improve Wind Farm Layout, Part I: Model Validation and Near Wake Analysis

Energies ◽  
2019 ◽  
Vol 12 (5) ◽  
pp. 940 ◽  
Author(s):  
Rafael Rodrigues ◽  
Corinne Lengsfeld
Keyword(s):  
Wind Turbine ◽  
Pitch Angle ◽  
Wind Farm ◽  
Free Stream ◽  
Free Stream Velocity ◽  
Speed Ratio ◽  
Stream Velocity ◽  
Near Wake ◽  
Tip Speed Ratio ◽  
Wind Farm Layout

The first part of this work describes the validation of a wind turbine farm Computational Fluid Dynamics (CFD) simulation using literature velocity wake data from the MEXICO (Model Experiments in Controlled Conditions) experiment. The work is intended to establish a computational framework from which to investigate wind farm layout, seeking to validate the simulation and identify parameters influencing the wake. A CFD model was designed to mimic the MEXICO rotor experimental conditions and simulate new operating conditions with regards to tip speed ratio and pitch angle. The validation showed that the computational results qualitatively agree with the experimental data. Considering the designed tip speed ratio (TSR) of 6.6, the deficit of velocity in the wake remains at rate of approximately 15% of the free-stream velocity per rotor diameter regardless of the free-stream velocity applied. Moreover, analysis of a radial traverse right behind the rotor showed an increase of 20% in the velocity deficit as the TSR varied from TSR = 6 to TSR = 10, corresponding to an increase ratio of approximately 5% m·s−1 per dimensionless unit of TSR. We conclude that the near wake characteristics of a wind turbine are strongly influenced by the TSR and the pitch angle.

A study of non-parallel and nonlinear effects on the localized receptivity of boundary layers

1995 ◽  
Vol 290 ◽  
pp. 29-37 ◽  
Author(s):  
J. D. Crouch ◽  
P. R. Spalart
Keyword(s):  
Free Stream ◽  
Stokes Equations ◽  
Nonlinear Effects ◽  
Free Stream Velocity ◽  
Navier Stokes ◽  
Stream Velocity ◽  
Navier Stokes Equations ◽  
Dimensional Boundary ◽  
Instability Growth ◽  
The Difference

The acoustic receptivity due to localized surface suction in a two-dimensional boundary layer is studied using a finite-Reynolds-number theory and direct numerical simulation of the Navier-Stokes equations. Detailed comparisons between the two methods are used to determine the bounds for application of the theory. Results show a 4% difference between the methods for receptivity in the neighbourhood of branch I with low suction levels, low acoustic levels, and a moderate frequency; we attribute this difference to non-parallel effects, not included in the theory. The difference is larger for receptivity upstream of branch I, and smaller for receptivity downstream of branch I. As the peak suction level is increased to 1% of the free-stream velocity, the simulations show a nonlinear deviation from the theory. Suction levels as small as 0.1% are shown to have a significant effect on the instability growth between branch I and branch II. Increasing the acoustic amplitude to 1% of the steady free-stream velocity produces no significant nonlinear effect.

scholarly journals Dynamic Stall Analysis of S809 Pitching Airfoil in Unsteady Free Stream Velocity

2015 ◽  
Vol 32 (2) ◽  
pp. 227-235 ◽  
Author(s):  
H. R. Karbasian ◽  
S. A. Moshizi ◽  
M. J. Maghrebi
Keyword(s):  
Free Stream ◽  
Stokes Equations ◽  
Simple Algorithm ◽  
Numerical Results ◽  
Free Stream Velocity ◽  
Dynamic Stall ◽  
Stream Velocity ◽  
Horizontal Axis Wind Turbine ◽  
Dimensional Parameter ◽  
Reduced Frequency

AbstractIn this paper, the dynamic stall of S809 airfoil that widely used in horizontal axis wind turbine, in different reduced frequencies is investigated. The simulation was carried out numerically handling Navier-Stokes equations. For this purpose, the segregated solver with SIMPLE algorithm was chosen to solve the momentum equations. The effect of turbulence on the flow field is taken into account using Shear Stress Transport (SST) K-ω turbulence model. The obtained numerical results are compared with experimental and a few numerical results. The results indicate that the K-ω SST model is in good agreement with experimental results for both steady and unsteady conditions. Furthermore, a non-dimensional parameter, relating the acceleration of unsteady free stream velocity and acceleration of pitch motion (known as reduced frequency), is also investigated. In addition, the results show that any increase in the reduced frequency increases the instantaneous aerodynamic characteristics of oscillating airfoil.

Overview of Subsynchronous Oscillation in Grid-connected Wind Farm

2020 ◽  
Vol 13 (7) ◽  
pp. 969-979
Author(s):  
Xu Pei-Zhen ◽  
Lu Yong-Geng ◽  
Cao Xi-Min
Keyword(s):  
Power Systems ◽  
Wind Turbine ◽  
Large Scale ◽  
Wind Farm ◽  
Wind Farms ◽  
Induction Generator ◽  
Future Research ◽  
Stable Operation ◽  
Subsynchronous Oscillation ◽  
Different Types

Background: Over the past few years, the subsynchronous oscillation (SSO) caused by the grid-connected wind farm had a bad influence on the stable operation of the system and has now become a bottleneck factor restricting the efficient utilization of wind power. How to mitigate and suppress the phenomenon of SSO of wind farms has become the focus of power system research. Methods: This paper first analyzes the SSO of different types of wind turbines, including squirrelcage induction generator based wind turbine (SCIG-WT), permanent magnet synchronous generator- based wind turbine (PMSG-WT), and doubly-fed induction generator based wind turbine (DFIG-WT). Then, the mechanisms of different types of SSO are proposed with the aim to better understand SSO in large-scale wind integrated power systems, and the main analytical methods suitable for studying the SSO of wind farms are summarized. Results: On the basis of results, using additional damping control suppression methods to solve SSO caused by the flexible power transmission devices and the wind turbine converter is recommended. Conclusion: The current development direction of the SSO of large-scale wind farm grid-connected systems is summarized and the current challenges and recommendations for future research and development are discussed.

scholarly journals Analysis of Wind Turbine Aging through Operation Data Calibrated by LiDAR Measurement

Energies ◽  
2021 ◽  
Vol 14 (8) ◽  
pp. 2319
Author(s):  
Hyun-Goo Kim ◽  
Jin-Young Kim
Keyword(s):  
Power Generation ◽  
Wind Speed ◽  
Wind Turbine ◽  
Wind Farm ◽  
Free Stream ◽  
Turbine Blades ◽  
Wind Farms ◽  
Lidar Measurement ◽  
Wind Turbine Blades ◽  
Wake Effect

This study analyzed the performance decline of wind turbine with age using the SCADA (Supervisory Control And Data Acquisition) data and the short-term in situ LiDAR (Light Detection and Ranging) measurements taken at the Shinan wind farm located on the coast of Bigeumdo Island in the southwestern sea of South Korea. Existing methods have generally attempted to estimate performance aging through long-term trend analysis of a normalized capacity factor in which wind speed variability is calibrated. However, this study proposes a new method using SCADA data for wind farms whose total operation period is short (less than a decade). That is, the trend of power output deficit between predicted and actual power generation was analyzed in order to estimate performance aging, wherein a theoretically predicted level of power generation was calculated by substituting a free stream wind speed projecting to a wind turbine into its power curve. To calibrate a distorted wind speed measurement in a nacelle anemometer caused by the wake effect resulting from the rotation of wind-turbine blades and the shape of the nacelle, the free stream wind speed was measured using LiDAR remote sensing as the reference data; and the nacelle transfer function, which converts nacelle wind speed into free stream wind speed, was derived. A four-year analysis of the Shinan wind farm showed that the rate of performance aging of the wind turbines was estimated to be −0.52%p/year.

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