Abstract

Previous attempts to describe the structure of wind turbine wakes and their mutual interaction were mostly limited to large-eddy and Reynolds-averaged Navier–Stokes simulations using finite-volume solvers. We employ the higher-order spectral-element code Nek5000 to study the influence of numerical aspects on the prediction of the wind turbine wake structure and the wake interaction between two turbines. The spectral-element method enables an accurate representation of the vortical structures, with lower numerical dissipation than the more commonly used finite-volume codes. The wind-turbine blades are modeled as body forces using the actuator-line method (ACL) in the incompressible Navier–Stokes equations. Both tower and nacelle are represented with appropriate body forces. An inflow boundary condition is used which emulates homogeneous isotropic turbulence of wind-tunnel flows. We validate the implementation with results from experimental campaigns undertaken at the Norwegian University of Science and Technology (NTNU Blind Tests), investigate parametric influences and compare computational aspects with existing numerical simulations. In general the results show good agreement between the experiments and the numerical simulations both for a single-turbine setup as well as a two-turbine setup where the turbines are offset in the spanwise direction. A shift in the wake center caused by the tower wake is detected similar to experiments. The additional velocity deficit caused by the tower agrees well with the experimental data. The wake is captured well by Nek5000 in comparison with experiments both for the single wind turbine and in the two-turbine setup. The blade loading however shows large discrepancies for the high-turbulence, two-turbine case. While the experiments predicted higher thrust for the downstream turbine than for the upstream turbine, the opposite case was observed in Nek5000.

Highlights

  • The wind-energy sector has been subject to continuous progress throughout the last decades due to the attempt of many countries to emancipate themselves from fossil fuels

  • This paper studies the wake of a single wind-turbine setup and the interaction between two consecutive wind turbines by means of numerical simulation and compares them to experimental results (”Blind Tests”) by Krogstad and Eriksen [11] and Krogstad et al [12] and numerical simulations using the finite-volume solver EllipSys3D [7]

  • The turbulent stress peaks generated by the tip vortices and the wake shear layer in Nek5000 is five times smaller than in the experiment due to the larger vortex core size of the actuator-line method (ACL) method which is defined by the width of the Gaussian and the finite resolution of the simulation

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Summary

Introduction

The wind-energy sector has been subject to continuous progress throughout the last decades due to the attempt of many countries to emancipate themselves from fossil fuels. Apart from being subjected to the changing characteristics of the atmospheric boundary layer, individual turbines are exposed to the wakes of other turbines. The wake from an upstream turbine usually leads to an increase in turbulent intensity and fatigue loading of downstream turbines. Due to the decreased energy content of the flow in the wake of a turbine the downstream turbines extract less energy from the atmosphere than.

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