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Research Papers

Guidelines for Volume Force Distributions Within Actuator Line Modeling of Wind Turbines on Large-Eddy Simulation-Type Grids

[+] Author and Article Information
Pankaj K. Jha

Department of Aerospace Engineering,
The Pennsylvania State University,
University Park, PA 16802

Matthew J. Churchfield, Patrick J. Moriarty

Senior Engineer
National Wind Technology Center,
National Renewable Energy Laboratory,
Golden, CO 80401

Sven Schmitz

Assistant Professor
Department of Aerospace Engineering,
The Pennsylvania State University,
University Park, PA 16802

Contributed by the Solar Energy Division of ASME for publication in the JOURNAL OF SOLAR ENERGY ENGINEERING. Manuscript received April 25, 2013; final manuscript received December 5, 2013; published online January 10, 2014. Assoc. Editor: Yves Gagnon.

J. Sol. Energy Eng 136(3), 031003 (Jan 10, 2014) (11 pages) Paper No: SOL-13-1123; doi: 10.1115/1.4026252 History: Received April 25, 2013; Revised December 05, 2013

The objective of this work is to develop and test a set of general guidelines for choosing parameters to be used in the state-of-the-art actuator line method (ALM) for modeling wind turbine blades in computational fluid dynamics (CFD). The actuator line method is being increasingly used for the computation of wake interactions in large wind farms in which fully blade-resolving simulations are expensive and require complicated rotating meshes. The focus is on actuator line behavior using fairly isotropic grids of low aspect ratio typically used for large-eddy simulation (LES). Forces predicted along the actuator lines need to be projected onto the flow field as body forces, and this is commonly accomplished using a volumetric projection. In this study, particular attention is given to the spanwise distribution of the radius of this projection. A new method is proposed where the projection radius varies along the blade span following an elliptic distribution. The proposed guidelines for actuator line parameters are applied to the National Renewable Energy Laboratory's (NREL's) Phase VI rotor and the NREL 5-MW turbine. Results obtained are compared with available data and the blade-element code XTurb-PSU. It is found that the new criterion for the projection radius leads to improved prediction of blade tip loads for both blade designs.

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Figures

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Fig. 2

Vorticity magnitude in an axial wake plane (NREL 5 MW wind turbine, VWind = 8 m/s). In this example, the wind turbine rotor is subject to uniform inflow conditions. Root and tip vortices as well as the expansion of the streamtube are clearly visible in the near wake. The wake flow becomes unstable approximately 1.5 rotor diameters downstream of the turbine, where the laminar shear layer transitions to turbulent flow. The breakdown of large turbulent eddies into smaller ones is visible further downstream.

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Fig. 1

A contour of the streamwise velocity normalized by freestream velocity taken in a plane perpendicular to the actuator line and at midspan using purely linear interpolation (a) or a spatially varying blend of midpoint and upwind interpolation (b). The point where the actuator line intersects these contours is shown with a black dot.

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Fig. 9

Spanwise variation of AOA for the NREL phase VI rotor (Vwind = 7 m/s). ALM parameters: ε/c* = 0.67, Δgrid/R = 1/37, Δbgrid = constant.

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Fig. 10

Spanwise variation of force for the NREL phase VI rotor (Vwind = 7 m/s). ALM parameters: ε/c* = 0.67, Δgrid/R = constant, Δbgrid = 1.5.

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Fig. 3

Equivalent elliptic distribution of Gaussian radius, ε. Here, the equivalent blade ellipse that defines the Gaussian radius ε(r) is shown on −R/2 ≤ r ≤ +R/2 for demonstration purposes only. For the actual computations of rotating blades and reporting the blade loads, the equivalent ellipse was shifted such that 0 ≤ r ≤ + R.

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Fig. 4

Examples of the “equivalent” elliptic planform to define the Gaussian radius ε

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Fig. 13

Spanwise variation of force for the NREL 5-MW turbine (Vwind = 8 m/s). ALM parameters: Δgrid/R = 1/64.

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Fig. 14

Wake structure and strength for the NREL 5-MW turbine (Vwind = 8 m/s) showing isosurface of vorticity magnitude 0.5 s− 1

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Fig. 5

Spanwise variation of AOA for the NREL phase VI rotor (Vwind = 7 m/s). ALM parameters: εgrid = constant, Δgrid/R = 1/32, Δbgrid = 1.

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Fig. 6

Spanwise variation of force for the NREL phase VI rotor (Vwind = 7 m/s). ALM parameters: εgrid = 2, Δgrid/R = 1/32, Δbgrid = 1.

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Fig. 11

Spanwise variation of AOA and velocity magnitude for the NREL phase VI rotor (Vwind = 7 m/s). ALM parameters: ε/c* = 0.67, Δgrid/R = constant, Δbgrid = 1.5.

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Fig. 12

Spanwise variation of force for the NREL 5-MW turbine (Vwind = 8 m/s). ALM parameters: ε/c* = 1.33, 0.67, Δgrid/R = constant, Δbgrid = 1.5.

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Fig. 7

Spanwise variation of AOA for the NREL phase VI rotor (Vwind = 7 m/s). ALM parameters: ε/c = constant, Δgrid/R = 1/37, Δbgrid = 1.

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Fig. 8

Spanwise variation of force for the NREL phase VI rotor (Vwind = 7 m/s). ALM parameters: ε/c = 0.57, Δgrid/R = 1/37, Δbgrid = constant.

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