Comparison of a coupled near- and far-wake model with a free-wake vortex code

Pirrung, Georg Raimund; Riziotis, Vasilis A.; Madsen, Helge Aagaard; Hansen, Morten Hartvig; Kim, Taeseong

doi:10.5194/wes-2-15-2017

Cited by 39 publications

(40 citation statements)

References 13 publications

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“…Therefore, it will not accurately model the dynamics of the tip vortex due to dynamic changes in loading. This is also the case for any common BEM tip loss correction; the vortex dynamics can be accounted for by the near wake model . Nevertheless, the present modification is expected to improve simulation results in dynamic situations, for example, due to turbulent inflow, because it takes the rotation of the resulting force into account.…”

Section: A Modified Tip Correctionsupporting

confidence: 76%

“…This is also the case for any common BEM tip loss correction; the vortex dynamics can be accounted for by the near wake model. 11 Nevertheless, the present modification is expected to improve simulation results in dynamic situations, for example, due to turbulent inflow, because it takes the rotation of the resulting force into account. FIGURE 1 Sketches of the aerodynamic forces at an airfoil.…”

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A simple improvement of a tip loss model for actuator disc simulations

et al. 2020

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The loading of a wind turbine decreases towards the blade tip because of the velocities induced by the tip vortex. This tip loss effect has to be taken into account when performing actuator disc simulations, where the single blades of the turbine are not modeled. A widely used method applies a factor on the axial and tangential loading of the turbine. This factor decreases when approaching the blade tip. It has been shown that the factor should be different for the axial and tangential loading of the turbine to model the rotation of the resulting force vector at the airfoil sections caused by the induced velocity. The present article contains the derivation of a simple correction for the tangential load factor that takes this rotation into account. The correction does not need any additional curve fitting but just depends on the local airfoil characteristics and angle of attack. Actuator disc computations with the modified tip loss correction show improved agreement with results from actuator line, free wake lifting line, and blade element momentum simulations.

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Section: A Modified Tip Correctionsupporting

confidence: 76%

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A simple improvement of a tip loss model for actuator disc simulations

et al. 2020

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“…Hence determining the correction 10 velocity is an iterative procedure. We use the technique by Pirrung et al (2017a) established for the NWM to accelerate and ensure its convergence. Furthermore the activation of the correction is delayed until the starting vorticity of the rotor has been transported at least one blade length away from the rotor plane.…”

Section: Tip/smearing Correction For the Actuator Linementioning

confidence: 99%

A vortex-based tip/smearing correction for the actuator line

Forsting¹,

Pirrung²,

Ramos‐García³

2019

Preprint

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The actuator line was intended as a lifting line technique for CFD applications. In this paper we proof -theoretically and practically -that smearing the forces of the actuator line in the flow domain necessarily leads to smeared velocity fields. By combining a near-wake representation of the trailed vorticity with a viscous vortex core model, the missing induction from the smeared velocity is recovered. This novel dynamic smearing correction is verified for basic wing test cases and rotor simulations of a multi-MW turbine. The latter cover the entire operational wind speed range as well as yaw, strong turbulence and pitch 5 step cases. The correction is validated with lifting line simulations with and without viscous core, that are representative of an actuator line without and with smearing correction, respectively. The dynamic smearing correction makes the actuator line effectively act as a lifitng line, as it was originally intended. 10 The actuator line (AL) technique developed by Sørensen and Shen (2002) is a lifting-line (LL) representation of the wind turbine rotor suitable for computational fluid dynamics (CFD) simulations. It captures transient physical features like shed and trailed vorticity (including root/tip vortices) , without the computational cost associated with resolving the full rotor geometry. The AL model thus enables Large-eddy simulations (LES) of large wind farms in realistic, turbulent atmospheric boundary layers (Vollmer et al., 2017).15However, different to LL vortex formulations the blade forces are dispersed in the flow domain -most commonly in form of a Gaussian projection -to avoid numerical instability. A length scale -also referred to as smearing factor -controls this force redistribution, whose lower limit is linked to the grid size through numerical stability requirements (Troldborg et al., 2009). Mikkelsen (2003 observed a large sensitivity of the blade velocities to this length scale, which consequently also propagated to the blade forces. Especially in regions along the blade exhibiting stark load changes, as around the root and tip, forces 20 are substantially over-predicted. Meaning this effect is exacerbated by non-tapered and low aspect ratio blades. As actuator disc formulations suffer from similar issues towards the blade tip, their Glauert (1935) type tip corrections are also frequently applied to ALs (Shen et al., 2005). Yet, these correct discs for missing discrete blades and thus should be unnecessary -strictly even invalid -for ALs. Shives and Crawford (2013) and Jha et al. (2014) achieved a reduction in the force over-prediction Wind Energ. Sci. Discuss., https://doi.

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“…Therefore differences are expected close to the root and the tip of the blade, where the induction from a helical wake deviates most from the induction due to a cylindrical wake. An option to address these limitations is to couple a vortex-based near wake model to the BEM code (Pirrung et al, 2016(Pirrung et al, , 2017. However, the work in IEA Task 29 has shown 685 that care has to be taken when coupling the induction dynamics (Schepers et al, 2018b).…”

Section: Dynamic Inflowmentioning

confidence: 99%

Implementation of the Blade Element Momentum Model on a Polar Grid and its Aeroelastic Load Impact

Madsen¹,

Larsen²,

Pirrung³

et al. 2019

Preprint

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Abstract. We show that the up-scaling of wind turbines from rotor diameters of 15–20 m to presently large rotors of 150–200 m has changed the requirements for the aerodynamic Blade Element Momentum (BEM) models in the aeroelastic codes. This is because the typical scales in the inflow turbulence are now comparable with the rotor diameter of the large turbines. Therefore the spectrum of the incoming turbulence relative to the rotating blade has increased energy content on 1P, 2P, ..., nP and the annular mean induction approach in a classical BEM implementation might no longer be a good approximation for large rotors. We present a complete BEM implementation on a polar grid that models the induction response to the considerable 1P, 2P, ..., nP inflow variations, including models for yawed inflow, dynamic inflow and radial induction. At each time step in an aeroelastic simulation the induction derived from a local BEM approach is updated at all the stationary grid points covering the swept area so the model can be characterized as an engineering actuator disc (AD) solution. The induction at each grid point varies slowly in time due to the dynamic inflow filter but the rotating blade now samples the induction field; as a result the induction seen from the blade is highly unsteady and has a spectrum with distinct 1P, 2P, ..., nP peaks. The load impact mechanism from this unsteady induction is analyzed and it is found that the load impact strongly depends on the turbine design and operating conditions. For operation at low to medium thrust coefficients (conventional turbines at above rated wind speed or low induction turbines in the whole operating range) it is found that the grid BEM gives typically 8–10 % lower 1 Hz fatigue loads than the classical annular mean BEM approach. At high thrust coefficients the grid BEM can give slightly increased fatigue loads. In the paper the implementation of the grid based BEM is described in detail and finally several validation cases are presented.

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Comparison of a coupled near- and far-wake model with a free-wake vortex code

Cited by 39 publications

References 13 publications

A simple improvement of a tip loss model for actuator disc simulations

A simple improvement of a tip loss model for actuator disc simulations

A vortex-based tip/smearing correction for the actuator line

Implementation of the Blade Element Momentum Model on a Polar Grid and its Aeroelastic Load Impact

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