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

Abstract

The objective of this work is to develop and test a set of general guidelines for choosing parameters to be used in the stateoftheart 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 bladeresolving 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 largeeddy 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 5MW turbine. Results obtained are compared with available data and the bladeelement code XTurbPSU. It is found that the new criterion for the projection radius leads to improved prediction of blade tip loads for both blade designs.