The Bolund Experiment, Part I: Flow Over a Steep, Three-Dimensional Hill

Abstract

We present an analysis of data from a measurement campaign performed at the Bolund peninsula in Denmark in the winter of [2007][2008]. Bolund is a small isolated hill exhibiting a significantly steep escarpment in the main wind direction. The physical shape of Bolund represents, in a scaled-down form, a typical wind turbine site in complex terrain. Because of its small size the effect of atmospheric stratification can be neglected, which makes the Bolund experiment ideal for the validation of neutral flow models and hence model scenarios most relevant to wind energy. We have carefully investigated the upstream conditions. With a 7-km fetch over water, the incoming flow is characterized as flow over flat terrain with a local roughness height based on the surface momentum flux. The nearly perfect upstream conditions are important in forming a meaningful quantitative description of the flow over the Bolund hill. Depending on the wind direction, we find a maximum speed-up of 30% at the hill top accompanied by a maximum 300% enhancement of turbulence intensity. A closer inspection reveals transient behaviour with recirculation zones. From the wind energy context, this implies that the best site for erecting a turbine based on resource constraints unfortunately also imposes a penalty of high dynamic loads. On the lee side of Bolund, recirculation occurs with the turbulence intensity remaining significantly enhanced even at one hill length downstream. Its transient behaviour and many recirculation zones place Bolund in a category in which the linear flow theory is not applicable.

Validation of the simpleFoam (RANS) solver for the atmospheric boundary layer in complex terrain

“…For the normalized TKE (Fig. 6b), the snappyHexMesh simulation follows more closely the experimental data, but both meshes underestimate the jump in the TKE in the 01002-p. 6 First Symposium on OpenFOAM in Wind Energy upwind side of the hill, with the underestimation being less pronounced for the terrainBlockMesher simulation.…”

“…The Bolund hill, located north of the RISØ National Laboratory (Denmark), is a steep escarped island of about 130 m lenth, 12 m height and 75 m width [6]. The island is connected to the coast by a narrow strip of land.…”

“…We compare our simulations with the classical benchmark cases of Askervein [5] and Bolund [6] hills. Additionally, we test a new OpenFOAM-based tool for generating structured meshes in complex terrain [3].…”

“…Additionally, the mesh can be refined in regions with complex orography. We study both the classical benchmark case of Askervein hill [5] and the more recent Bolund island data set [6]. Our purpose is two-folded: to validate the performance of OpenFOAM steady state solvers, and the suitability of the new meshing tool to generate high quality structured meshes, which will be used in the future for performing more computationally intensive LES simulations in complex terrain.…”

Wind Turbine Control Based on Computational Fluid Dynamics (CFD) and Its Economic Effects

“…One technical issue which needs to be resolved in the near future in the field of wind power generation is to establish a numerical wind synopsis prediction technique which allows 1) accurate assessment of the local wind synopsis for wind turbines and 2) identification of local wind risks (terrain-induced turbulence) to wind turbines [1][2][3][4][5][6][7][8] .…”