30 Aug 2022
30 Aug 2022
Status: this preprint is currently under review for the journal WES.

Turbulence structures and entrainment length scales in large offshore wind farms

Abdul Haseeb Syed1, Jakob Mann1, Andreas Platis2, and Jens Bange2 Abdul Haseeb Syed et al.
  • 1Department of Wind and Energy Systems, Technical University of Denmark, 4000 Roskilde, Denmark
  • 2Environmental Physics, Geo- and Environmental Center, Eberhard Karls University of Tübingen, 72076 Tübingen, Germany

Abstract. The flow inside and around large offshore wind farms can range from smaller structures associated with the mechanical turbulence generated by wind turbines to larger structures indicative of the mesoscale flow. In this study, we explore the variation in turbulence structures and dominant scales of vertical entrainment above large offshore wind farms located in the North Sea, using data obtained from a research aircraft. The aircraft was flown upstream, downstream, and above wind farm clusters. Under neutrally stratified conditions, there is high ambient turbulence in the atmosphere and an elevated energy dissipation rate compared to stable conditions. The intensity of small-scale turbulence structures is increased above and downstream of the wind farm, and it prevails the mesoscale fluctuations. But in stable stratification, mesoscale flow structures are not only dominant upstream of the wind farm but also downstream. We observed that the vertical flux of horizontal momentum is the main source of energy recovery in large offshore wind farms, and it strongly depends on the magnitude of the length scales of the vertical wind velocity component. The dominant length scales of entrainment range from 20 to ~60 m above the wind farm in all stratification strengths, and in the wake flow these scales range from 10 m to ~100 m only under near-neutral stratification. For strongly stable conditions, negligible vertical entrainment of momentum was observed even just 2 km downstream of large wind farms. We also observed that there is a significant lateral momentum flux above the offshore wind farms, especially under strongly stable conditions, which suggests that these wind farms do not satisfy the conditions of an "infinite wind farm".

Abdul Haseeb Syed et al.

Status: open (until 27 Oct 2022)

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Abdul Haseeb Syed et al.

Abdul Haseeb Syed et al.


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Short summary
Wind turbines extract energy from the incoming wind flow which needs to be recovered. In very large offshore wind farms, the energy is recovered mostly from above the wind farm in a process called 'entrainment'. In this study, we analyzed the effect of atmospheric stability on the entrainment process in large offshore wind farms using measurements recorded by a research aircraft. This is the first time that in-situ measurements are used to study the energy recovery process above wind farms.