Turbulence and entrainment length scales in large wind farms

Abstract

A number of large wind farms are modelled using large eddy simulations to elucidate the entrainment process. A reference simulation without turbines and three farm simulations with different degrees of imposed atmospheric turbulence are presented. The entrainment process is assessed using proper orthogonal decomposition, which is employed to detect the largest and most energetic coherent turbulent structures. The dominant length scales responsible for the entrainment process are shown to grow further into the wind farm, but to be limited in extent by the streamwise turbine spacing, which could be taken into account when developing farm layouts. The self-organized motion or large coherent structures also yield high correlations between the power productions of consecutive turbines, which can be exploited through dynamic farm control.This article is part of the themed issue 'Wind energy in complex terrains'.

Vortex simulations of wind turbines operating in atmospheric conditions using a prescribed velocity‐vorticity boundary layer model

“…In finite‐volume solvers, a simplified boundary layer model using a distribution of prescribed body forces in combination with an actuator line model have previously been used in the wind energy community, see, eg, Troldborg et al The prescribed approach, both in vortex methods and finite‐volume solvers, can be seen as a simplified engineering approach to modeling the atmospheric boundary layer. However, the prescribed approach is extremely useful in order to perform parametric studies like those carried out by Andersen et al, as it allows full control of the different inflow characteristics. Such control can be key in engineering studies where a set of predefined or measured atmospheric conditions need to be reproduced as accurately as possible, cf, eg, Hasager et al…”

Instantaneous Response and Mutual Interaction between Wind Turbine and Flow

“…The influence of the incoming turbulence is deemed minor in the present case as the analysis will focus on turbines operating deep inside the wind farm, where the turbine generated turbulence is dominant. Deep inside the wind farm, the free stream turbulence intensity tends to govern the amount energy entrainment and hence the mean wind speed, while the large atmospheric turbulence length scales are broken down to scales related to the wind turbine spacing as shown by Andersen et al [22]. …”

Introduction Wind farms in complex terrains: an introduction

“…A number of LESs have been carried out that provide good insight into the power-extraction process and the momentum transfer taking place in the wind-farm internal boundary layer [9,10]. The description obtained from studies on single turbines must be extended since several wakes may simultaneously interact, making the flow highly complex.…”

“…This theme issue of the Philosophical Transactions of the Royal Society A mainly builds on presentations [2][3][4][5][6][7][8][9][10][11] made at the EUROMECH Colloquium 576, Wind Farms in Complex Terrain, which took place at KTH Royal Institute of Technology in Stockholm, Sweden, over 3 days in June 2016. The main focus of the Colloquium was the various issues related to complex terrains, mainly from the aerodynamic, meteorological and noise-propagation points of view.…”

“…The topics include wind statistics on both the micro-and macro-scale level [2], the effect of surface roughness on the description of boundary-layer flow physics [3,4], the effect of complex terrain on sound propagation [5], as well as various strategies for modelling the flow field around wind turbines [6][7][8] and wind farms [9][10][11].…”