Tip-vortex instability and turbulent mixing in wind-turbine wakes

Lignarolo, L. E. M.; Ragni, Daniele; Scarano, Fulvio; Ferreira, Carlos; Bussel, G.J.W. Van

doi:10.1017/jfm.2015.470

Cited by 125 publications

(124 citation statements)

References 58 publications

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“…The stronger fluctuations in the WT wake are due to the presence of concentrated tip vortices. These are normally accounted for in the calculation of the added turbulence (Lignarolo et al, 2015a). The latter one represents the flow turbulence caused by the presence of the turbine, which is added to the ambient turbulence as explained by Crespo et al (1999) and includes both coherent periodic structures, as the tip-vortices, and random velocity fluctuations, which are not separated in the classical double Reynolds decomposition applied in this work.…”

Section: Discussionmentioning

confidence: 99%

“…When the equation is applied to a control volume including the wake shear layer but not encompassing the AD/WT, as shown by Lignarolo et al (2015a) for i ¼ 1 and j ¼ 2, the term U ¼ Àuðu 0 v 0 Þ represents the flux in the radial direction of the streamwise mean-flow kineticenergy. In other words, the term represents the entrainment of free-stream kinetic energy across the wake shear layer.…”

Section: F Theorymentioning

confidence: 99%

“…This behaviour is attributed to the convected tip-vortex in the WT wake. (Lignarolo et al, 2015a), which dissipates and diffuses after the instability and is not present in the AD wake. In the inner region of the wake, the presence of the WT blades turbulence is prominent.…”

Section: J Renewable Sustainable Energy 8 023301 (2016)mentioning

confidence: 99%

“…In the WT wake both random fluctuations and periodic vortices are accounted for as turbulence, as mentioned in Sec. III E. Lignarolo et al (2015a) have shown that the periodic vortical fluctuations do not lead to a net positive transport of mean-flow kinetic-energy (and therefore entrainment of energy in the WT wake). The entrainment in the AD wake shear layer is relatively stable, preserving the same intensity from the very near wake until the end of the measured field.…”

Section: F Wake Mixingmentioning

confidence: 99%

“…Following the experimental studies of Dobrev et al (2008), Felli et al (2011), andSherry et al (2010), Lignarolo et al (2014a) have shown how the instability of the tip-vortex helix has a major effect on the wake mixing and re-energising mechanism. Lignarolo et al (2015a) have conducted a detailed analysis of the effect on the turbulence field and wake mixing due to the presence of the leapfrogging instability. Bolnot et al (2011) simulated numerically the helical wake using an array of axisymmetric vortex-rings for studying the onset of pair-wise instability in helicopter and windunknown_hyphen;turbine rotor wakes.…”

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confidence: 99%

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Experimental comparison of a wind-turbine and of an actuator-disc near wake

Lignarolo

Ragni

Ferreira

et al. 2016

Journal of Renewable and Sustainable Energy

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The actuator disc (AD) model is commonly used to simplify the simulation of horizontal-axis wind-turbine aerodynamics. The limitations of this approach in reproducing the wake losses in wind farm simulations have been proven by a previous research. The present study is aimed at providing an experimental analysis of the near-wake turbulent flow of a wind turbine (WT) and a porous disc, emulating the actuator disc numerical model. The general purpose is to highlight the similarities and to quantify the differences of the two models in the near-wake region, characterised by the largest discrepancies. The velocity fields in the wake of a wind turbine model and a porous disc (emulation of the actuator disc numerical model) have been measured in a wind tunnel using stereo particle image velocimetry. The study has been conducted at low turbulence intensity in order to separate the problems of the flow mixing caused by the external turbulence and the one caused by the turbulence induced directly by the AD or the WT presence. The analysis, as such, showed the intrinsic differences and similarities between the flows in the two wakes, solely due to the wake-induced flow, with no influence of external flow fluctuations. The data analysis provided the time-average three-component velocity and turbulence intensity fields, pressure fields, rotor and disc loading, vorticity fields, stagnation enthalpy distribution, and mean-flow kinetic-energy fluxes in the shear layer at the border of the wake. The properties have been compared in the wakes of the two models. Even in the absence of turbulence, the results show a good match in the thrust and energy coefficient, velocity, pressure, and enthalpy fields between wind turbine and actuator disc. However, the results show a different turbulence intensity and turbulent mixing. The results suggest the possibility to extend the use of the actuator disc model in numerical simulation until the very near wake, provided that the turbulent mixing is correctly represented.

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Section: Discussionmentioning

confidence: 99%

Section: F Theorymentioning

confidence: 99%

Section: J Renewable Sustainable Energy 8 023301 (2016)mentioning

confidence: 99%

Section: F Wake Mixingmentioning

confidence: 99%

mentioning

confidence: 99%

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Experimental comparison of a wind-turbine and of an actuator-disc near wake

Lignarolo

Ragni

Ferreira

et al. 2016

Journal of Renewable and Sustainable Energy

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POD‐based analysis of a wind turbine wake under the influence of tower and nacelle

et al. 2020

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The wake produced by a model wind turbine is investigated using proper orthogonal decomposition (POD) of numerical data obtained by large eddy simulations at a diameter‐based Reynolds number Re=6.3×105. The blades are modeled employing the actuator line method and an immersed boundary method is used to simulate tower and nacelle. Two simulations are performed: one accounts only for the blades effect; the other includes also tower and nacelle. The two simulations are analyzed and compared in terms of mean flow fields and POD modes that mainly characterize the wake dynamics. In the rotor‐only case, the most energetic modes in the near wake are composed of high‐frequency tip and root vortices, whereas in the far wake, low‐frequency modes accounting for mutual inductance instability of tip vortices are found. When tower and nacelle are included, low‐frequency POD modes are present already in the near wake, linked to the von Karman vortices shed by the tower. These modes interact nonlinearly with the tip vortices in the far wake, generating new low‐frequency POD modes, some of them lying in the frequency range of wake meandering. An analysis of the mean kinetic energy (MKE) entrainment of each POD mode shows that tip vortices sustain the wake mean shear, whereas low‐frequency modes contribute to wake recovery. This explains why tower and nacelle induce a faster wake recovery.

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Review of wake management techniques for wind turbines

Houck

2021

Wind Energy

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Summary The progression of wind turbine technology has led to wind turbines being incredibly optimized machines often approaching their theoretical maximum production capabilities. When placed together in arrays to make wind farms, however, they are subject to wake interference that greatly reduces downstream turbines' power production, increases structural loading and maintenance, reduces their lifetimes, and ultimately increases the levelized cost of energy. Development of techniques to manage wakes and operate larger and larger arrays of turbines more efficiently is now a crucial field of research. Herein, four wake management techniques in various states of development are reviewed. These include axial induction control, wake steering, the latter two combined, and active wake control. Each of these is reviewed in terms of its control strategies and use for power maximization, load reduction, and ancillary services. By evaluating existing research, several directions for future research are suggested.

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Tip-vortex instability and turbulent mixing in wind-turbine wakes

Cited by 125 publications

References 58 publications

Experimental comparison of a wind-turbine and of an actuator-disc near wake

Experimental comparison of a wind-turbine and of an actuator-disc near wake

POD‐based analysis of a wind turbine wake under the influence of tower and nacelle

Review of wake management techniques for wind turbines

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