Further Developments in Numerical Simulations of Wind Turbine Flows Using the Actuator Line Method

Abstract

Current large-scale wind turbine installations are sited using layouts based on site topology, real estate costs and restrictions, and turbine power output. Existing optimization programs attempt to site multiple turbines based on simple geometric turbine wake models, which typically overestimate individual turbine output. In addition, advanced Computational Fluid Dynamics (CFD) modeling of individual turbine wake fields have revealed complex flow patterns and “wake meandering” which have not been taken into account in current optimization and flow field models. CFD models of entire turbine fields have had limited application because of the enormous compute resources required; limitations of the simplified turbine models used which do not provide high resolution results in the wake field; and the lack of efforts to adapt the results of complex CFD output to analytical models which can be incorporated into wind turbine siting optimization routines. In this paper, we report on our efforts to simulate flow past wind turbines using a new adaptation of the Actuator Line (AL) method for turbine blade modeling. This method creates a geometric representation of each rotating turbine blade. Grid points in the CFD flow field are selected within the outline of the blades and near downstream planes, and the aerodynamic forces are calculated using traditional blade element equations. The forces are distributed using an automated routine which dynamically determines the application area based on the number of applied grid points at each time step. Turbine blades are rotated in time with progressing CFD field calculations. This method distributes blade forces without using a geometric distribution function used in other recent research. Blade forces are then input as body forces into the Navier Stokes equations in the host CFD program. A Smagorisnky LES turbulence model is employed to model turbulent effects. To improve accuracy and reduce computing power requirements, the advanced parallel CFD code, NEK5000, is used in this study. FORTRAN subroutines are written to generate the actuator line and blade geometry, and to calculate the blade lift and drag forces. These subroutines are then linked to the solver source code and compiled. Details of the actuator line setup and calculations, LES turbulence model, CFD flow simulation setup, and results from current turbine runs will be presented. Current results are consistent with published research. A roadmap to ongoing development will also be discussed.