Comparison of the near‐wake between actuator‐line simulations and a simplified vortex model of a horizontal‐axis wind turbine

Sarmast, Sasan; Segalini, Antonio; Mikkelsen, Robert Flemming; Ivanell, Stefan

doi:10.1002/we.1845

Cited by 17 publications

(17 citation statements)

References 16 publications

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“…The ring vortex wake is consistent with the "Joukowsky" model of the wake, used by Sarmast et al (2016); either the bound vorticity of the blades is constant along their span or all the shed vorticity has rolled up into tip and hub vortices before reaching the far wake. This is clearly a simplification of wind turbine wakes in general, but the linearity of the Biot-Savart law allows more complex wakes to be considered as an assembly of elements such as rings.…”

Section: The Vortex Ring Model For the Wakesupporting

confidence: 71%

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The second curvature correction for the straight segment approximation of periodic vortex wakes

Wood

2018

Wind Energ. Sci.

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Abstract. The periodic, helical vortex wakes of wind turbines, propellers, and helicopters are often approximated using straight vortex segments which cannot reproduce the binormal velocity associated with the local curvature. This leads to the need for the first curvature correction, which is well known and understood. It is less well known that under some circumstances, the binormal velocity determined from straight segments needs a second correction when the periodicity returns the vortex to the proximity of the point at which the velocity is required. This paper analyzes the second correction by modelling the helical far wake of a wind turbine as an infinite row of equispaced vortex rings of constant radius and circulation. The ring spacing is proportional to the helix pitch. The second correction is required at small vortex pitch, which is typical of the operating conditions of large modern turbines. Then the velocity induced by the periodic wake can greatly exceed the local curvature contribution. The second correction is quadratic in the inverse of the number of segments per ring and linear in the inverse spacing. An approximate expression is developed for the second correction and shown to reduce the errors by an order of magnitude.

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Section: The Vortex Ring Model For the Wakesupporting

confidence: 71%

“…The labels and symbols on the figure will be defined below. O'Brien et al (2017) reviewed a range of computational models for wind turbines and Sarmast et al (2016) describe a recent application of a free-wake vortex model using straight vortex segments.…”

Section: Introductionmentioning

confidence: 99%

The second curvature correction for the straight segment approximation of periodic vortex wakes

Wood

2018

Wind Energ. Sci.

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“…Large‐eddy simulations of the flow through wind turbines have been carried out using actuator disk and actuator line models describing the rotor aerodynamics. Other models focus on single‐turbine cases, like the vortex models by Okulov and Sørensen and Segalini et al ,. which solve for the wake flow behind a stand‐alone turbine in an efficient way.…”

Section: Introductionmentioning

confidence: 99%

“…Large-eddy simulations of the flow through wind turbines have been carried out using actuator disk 4 and actuator line 5 models describing the rotor aerodynamics. Other models focus on single-turbine cases, like the vortex models by Okulov and Sørensen 6 and Segalini et al, [7][8][9] which solve for the wake flow behind a stand-alone turbine in an efficient way. Nevertheless, these fast theoretical models are not suitable for a utilization in industrial wind-farm calculations since they are based on too restricted conditions.…”

Section: Introductionmentioning

confidence: 99%

A linearized numerical model of wind-farm flows

et al. 2016

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A fast and reasonably accurate numerical three‐dimensional wake model able to predict the flow behaviour of a wind farm over a flat terrain has been developed. The model is based on the boundary‐layer approximation of the Navier–Stokes equations, linearized around the incoming atmospheric boundary layer, with the assumption that the wind turbines provide a small perturbation to the velocity field. The linearization of the actuator‐disc theory brought additional insights that could be used to understand the behaviour, as well as the limitations, of a flow model based on linear methods: for instance, it is shown that an adjustment of the turbine's thrust coefficient is necessary in order to obtain the same wake velocity field provided by the actuator disc theory within the used linear framework. The model is here validated against two independent wind‐tunnel campaigns with a small and a large wind farm aimed at the characterization of the flow above and upstream of the farms, respectively. The developed model is, in contrary to current engineering wake models, able to account for effects occurring in the upstream flow region, thereby including more physical mechanisms than other simplified approaches. The conducted simulations (in agreement with the measurement results) show that the presence of a wind farm affects the approaching flow far more upstream than generally expected and definitely beyond the current industrial standards. Despite the model assumptions, several velocity statistics above wind farms have been properly estimated providing an insight into the transfer of momentum inside the turbine rows. Copyright © 2016 John Wiley & Sons, Ltd.

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“…While in previous work such as Krogstad and Eriksen (2013) or Sarmast et al (2016) often the profiles of velocity deficits or turbulent kinetic energy are taken into consideration for evaluating the near-wake, this work uses the energy spectra to determine how far downstream the discernible effects of the distinct blades are noticable. While the analysis of the turbulent inflow in previous work (Olivares pinosa, 2017) has often been conducted using energy spectra, seldom energy spectra including the rotor effects are included in the analysis of the near-wake.…”

mentioning

confidence: 99%

Near wake analysis of actuator line method immersed in turbulent flow using large-eddy simulations

Nathan¹,

Masson²,

Dufresne³

2018

Preprint

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Abstract. The interaction between wind turbines through their wakes is an important aspect of the conception and operation of a wind farm. Wakes are characterized by an elevated turbulence level and a noticable velocity deficit which causes a decrease in energy output and fatigue on downstream turbines. In order to gain a better understanding of this phenomenon this works uses large-eddy simulations together with an actuator line model and different ambient turbulences imposed as boundary conditions. This is achieved by using the SOWFA framework from NREL (USA) which is first validated against another popular CFD 5 framework for wind energy, EllipSys3D, and then verified against the experimental results from the MEXICO and NEW MEXICO wind tunnel experiments. By using the predicted torque as a global indicator, the optimal width of the distribution kernel for the actuator line is determined for different grid resolutions. Then the rotor is immersed in homogeneous isotropic turbulence and a shear layer turbulence with different turbulence intensities, allowing to determine how far downstream the effect of the distinct blades is discernible. This can be used as an indicator for the extents of the near wake for different flow 10 conditions.

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Comparison of the near‐wake between actuator‐line simulations and a simplified vortex model of a horizontal‐axis wind turbine

Cited by 17 publications

References 16 publications

The second curvature correction for the straight segment approximation of periodic vortex wakes

The second curvature correction for the straight segment approximation of periodic vortex wakes

A linearized numerical model of wind-farm flows

Near wake analysis of actuator line method immersed in turbulent flow using large-eddy simulations

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