Abstract

Unstable atmospheric conditions are often observed during the daytime over land and for significant periods offshore, and are hence relevant for wake studies. A simple k-ε RANS turbulence model for simulation of wind turbine wakes in the unstable surface layer is presented, which is based on Monin-Obukhov similarity theory (MOST). The turbulence model parametrizes buoyant production of turbulent kinetic energy (TKE) without the use of an active temperature equation, and flow balance is ensured throughout the domain by modifications of the turbulence transport equations. Large eddy simulations and experimental data from the literature are used for validation of the model.

Highlights

  • Wind turbine wakes have been studied for decades using many different methodologies, including wind tunnels, field experiments, analytical engineering models, and numerical simulations

  • Another peculiarity is that higher turbulence intensity (TI) leads to larger shear, because the velocity gradient scales with u∗, which scales with Iref (Eq 10); this is a consequence of specifying both TI and hub height velocity

  • We have proposed a simple k-ε Reynolds-Averaged Navier Stokes (RANS) model, the "cstB" model, for simulation of wind turbine wakes in the unstable surface layer

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Summary

Introduction

Wind turbine wakes have been studied for decades using many different methodologies, including wind tunnels, field experiments, analytical engineering models, and numerical simulations. A sub-category of "numerical simulations" is the Reynolds-Averaged Navier Stokes (RANS) approach, which is a Computational Fluid Dynamics (CFD) method that solves for the mean fields This means that no time history of the flow is obtained, 15 the computational resources required for RANS are very small compared to higher-fidelity CFD methods, making RANS an attractive option for parametric studies or for isolating various physical effects (c.f. van der Laan et al, 2021). Examples include Churchfield et al (2012), Abkar and Porté-Agel (2015), Ghaisas et al (2017) and Xie and Archer (2017), which 45 simulate wakes in both unstable and neutral conditions for a wide variety of cases All these studies agree with the general consensus and explain it with the increased TI encountered in unstable conditions due to buoyant production of turbulence. Modifications to the MOST k-ε-fP equations in the unstable regime are suggested in this paper and validated against various field experiments and LES’s

Simulation setup
Inflow profile for unstable ASL
Wind Turbine representation
Modification of the k-ε-fP model in the unstable ASL
Buoyant production term
Turbulence closure with fP in non-neutral conditions
Validation with experiments and LES
NTK41 case
V80-Abkar case
V80-Keck case
Findings
Conclusions
Full Text
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