Application of the actuator surface concept to wind turbine rotor aerodynamics

Watters, Christophe Sibuet; Breton, Simon-Philippe; Masson, Christian

doi:10.1002/we.365

Cited by 22 publications

(9 citation statements)

References 22 publications

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“…Full solutions including viscous effects using computational fluid dynamics (CFD) are presented by many authors (e.g. Madsen et al 2010;Sibuet Watters & Masson 2010;Troldborg, Sørensen & Mikkelsen 2010). Since the calculation time required for a complete solution of blade and wake flow is still too long, the blade is often represented by an actuator line as first proposed by Sørensen & Shen (2002).…”

Section: Brief History Of Rotor Aerodynamicsmentioning

confidence: 99%

The role of conservative forces in rotor aerodynamics

et al. 2014

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The theory to predict the performance and loads on rotors (propellers, screws,\ud windmills) has a history of more than a century. Apart from modern computational\ud fluid dynamics and vortex panel models taking the true blade geometry into account,\ud most other models proceed from an infinitely thin actuator disc or line. These models\ud assume an externally defined force field distributed at the disc or line, representing\ud the loads on the real rotor. Given this force field, the flow is solved by momentum\ud balances or by the equations of motion. The use of external force fields was discussed\ud in textbooks of the first decades of the 20th century, but has received little attention\ud since then. This paper investigates the higher-order effect of adding thickness to the\ud actuator disc or changing the actuator line to a blade with cross-sectional dimensions.\ud For the generation of a Rankine vortex by a force field acting on an actuator disc with\ud thickness, an exact solution has been found in which not only the thrust and torque\ud determine the flow, but also a radial force. This force is conservative, in contrast to\ud the other force components. For rotor blades, a conservative normal and radial force\ud acting on the chordwise bound vorticity is present. This explains the experimentally\ud observed inboard motion of the tip vortex of model wind turbine rotors before the\ud wake induction field drives it outboard. Simulations by computational fluid mechanics\ud and a vortex panel code reproduce the inboard motion, but an actuator line analysis,\ud in which the chordwise vorticity is absent, does not. The conservative load is only\ud 1–2% of the thrust on the entire blade but =10% of the thrust at the tip (r/R>0.9).\ud Conservative forces at the disc and rotor blade vanish for vanishing disc thickness or\ud blade cross-section, so play no role in any of the infinitely thin actuator disc or line\ud methods. However, if higher-order effects of non-zero dimensions are to be modelled,\ud the conservative force field has to be included.peer-reviewe

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Section: Brief History Of Rotor Aerodynamicsmentioning

confidence: 99%

The role of conservative forces in rotor aerodynamics

et al. 2014

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“…This approach inherently assumed that the induction due to the wake is the same at the monitor point as it is at the blade, which is not true because shed and trailed vortices will have a much greater influence at the blade than on a location one chord length upstream of it. Sibuet Watters et al also studied the actuator surface method and calculated α and V rel by using a sectional average along the blade chord. This inherently assumes that the induction due to the bound circulation is symmetric about the center of the chord.…”

Section: Introductionmentioning

confidence: 97%

Mesh and load distribution requirements for actuator line CFD simulations

Shives

Crawford

2012

Wind Energy

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Fundamental numerical testing has been carried out to determine mesh density and force distribution guidelines for an actuator line-based computational fluid dynamics method for simulating kinetic turbines. The method computes forces from lifting surfaces (i.e. wings or blades) by using the evolving flowfield and tabulated airfoil data. The forces are applied to the flow as momentum source terms distributed with a Gaussian smoothing function about the physical locations of the blade/wing quarter-chord line. The chosen length scale of the Gaussian distribution affects the magnitude and distribution of the resulting induction and necessitates a minimum grid resolution for accurate results. Tests have been conducted to determine appropriate distribution length scales and mesh spacing by using an infinite span wing and finite span wings with constant and elliptical spanwise circulation distributions. These test cases were chosen because they have simple analytical solutions derived from lifting line theory. The eventual goal is to simulate turbine rotors; however, these fundamental test cases provide a means to evaluate the required mesh spacing and the appropriate distribution length scale without the complexity of modeling a turbine rotor wake. It was found that the source distribution length scale should be proportional to the local airfoil chord length c with a ratio =c of approximately 1/4 and that the mesh spacing at the actuator line should satisfy = grid 4. This limit is likely somewhat code specific and should be evaluated for all solvers used for actuator line simulations. ' Note that with this configuration, the simulated circulation could vary slightly along the wingspan because of variation in the simulated relative velocity; however, this varied only very slightly (< 2%) in the observed results.

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“…At any blade cross section, the local lift and drag forces, which are evaluated locally using tabulated lift and drag coefficients as function of the local angle of attack, are imposed at points moving with the blades within the simulation domain. The actuator surface model (ASM) is another method to represent lifting surfaces . In the ASM, the blades are represented as an infinitesimal sheet with a vorticity source.…”

Section: Introductionmentioning

confidence: 99%

Optimal smoothing length scale for actuator line models of wind turbine blades based on Gaussian body force distribution

2017

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The actuator line model (ALM) is a commonly used method to represent lifting surfaces such as wind turbine blades within large-eddy simulations (LES). In the ALM, the lift and drag forces are replaced by an imposed body force that is typically smoothed over several grid points using a Gaussian kernel with some prescribed smoothing width . To date, the choice of has most often been based on numerical considerations related to the grid spacing used in LES. However, especially for finely resolved LES with grid spacings on the order of or smaller than the chord length of the blade, the best choice of is not known. In this work, a theoretical approach is followed to determine the most suitable value of , based on an analytical solution to the linearized inviscid flow response to a Gaussian force. We find that the optimal smoothing width opt is on the order of 14%-25% of the chord length of the blade, and the center of force is located at about 13%-26% downstream of the leading edge of the blade for the cases considered. These optimal values do not depend on angle of attack and depend only weakly on the type of lifting surface. It is then shown that an even more realistic velocity field can be induced by a 2-D elliptical Gaussian lift-force kernel. Some results are also provided regarding drag force representation.Numerical simulations of flow over wind turbines often cannot afford to resolve the entire rotating blade geometry and the associated flow details. 1 Actuator disk models (ADMs) greatly reduce resolution requirements by replacing the rotor by a distributed body force at the rotor disk location. 2, 3 However, the ADM misses important information about the instantaneous blade location, its detailed aerodynamic properties and so on. An intermediate approach is provided by the actuator line model (ALM), 4 a technique for representing lifting surfaces within flow simulations with characteristics that fall between full blade resolution and the ADM. The ALM has been particularly popular in large-eddy simulations (LES) of wind turbine blades. 4-12 At any blade cross section, the local lift and drag forces, which are evaluated locally using tabulated lift and drag coefficients as function of the local angle of attack, are imposed at points moving with the blades within the simulation domain. The actuator surface model (ASM) is another method to represent lifting surfaces. 13,14 In the ASM, the blades are represented as an infinitesimal sheet with a vorticity source. Previous work has shown improvements in the flow fields produced by the ASM compared with the ALM. 13 The present work focuses on the ALM because of its simplicity and its wide and common usage in representing wind turbine blades in large-eddy simulations concerning rotor and wake studies. 4

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Application of the actuator surface concept to wind turbine rotor aerodynamics

Cited by 22 publications

References 22 publications

The role of conservative forces in rotor aerodynamics

The role of conservative forces in rotor aerodynamics

Mesh and load distribution requirements for actuator line CFD simulations

Optimal smoothing length scale for actuator line models of wind turbine blades based on Gaussian body force distribution

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