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A wind-tunnel study of the wake development behind wind turbines over sinusoidal hills
KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. University of Southampton, Faculty of Engineering and the Environment.ORCID iD: 0000-0002-8038-1127
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.ORCID iD: 0000-0001-8667-0520
2018 (English)In: Wind Energy, ISSN 1095-4244, E-ISSN 1099-1824, Vol. 21, no 8, p. 605-617Article in journal (Refereed) Published
Abstract [en]

In the present work, the wake development behind small-scale wind turbines is studied when introducing local topography variations consisting of a series of sinusoidal hills. Additionally, wind-tunnel tests with homogeneous and sheared turbulent inflows were performed to understand how shear and ambient turbulence influence the results. The scale of the wind-turbine models was about 1000times smaller than full-size turbines, suggesting that the present results should only be qualitatively extrapolated to real-field scenarios. Wind-tunnel measurements were made by means of stereoscopic particle image velocimetry to characterize the flow velocity in planes perpendicular to the flow direction. Over flat terrain, the wind-turbine wake was seen to slowly approach the ground while it propagated downstream. When introducing hilly terrain, the downward wake deflection was enhanced in response to flow variations induced by the hills, and the turbulent kinetic energy content in the wake increased because of the speed-up seen over the hills. The combined wake observed behind 2 streamwise aligned turbines was more diffused and when introducing hills, it was more prone to deflect towards the ground compared to the wake behind an isolated turbine. Since wake interactions are common at sites with multiple turbines, this suggested that it is important to consider the local hill-induced velocity variations when onshore wind farms are analysed. Differences in the flow fields were seen when introducing either homogeneous or sheared turbulent inflow conditions, emphasizing the importance of accounting for the prevailing turbulence conditions at a given wind-farm site to accurately capture the downstream wake development.

Place, publisher, year, edition, pages
John Wiley & Sons, 2018. Vol. 21, no 8, p. 605-617
Keywords [en]
Wind turbines, Complex terrain, Wind-tunnel measurements
National Category
Fluid Mechanics and Acoustics
Research subject
Engineering Mechanics
Identifiers
URN: urn:nbn:se:kth:diva-221704DOI: 10.1002/we.2181ISI: 000438224900001000438224900001Scopus ID: 2-s2.0-85044577908OAI: oai:DiVA.org:kth-221704DiVA, id: diva2:1176461
Funder
Swedish Research Council
Note

QCR 20180122

Available from: 2018-01-22 Created: 2018-01-22 Last updated: 2018-08-01Bibliographically approved
In thesis
1. Wind turbines over a hilly terrain: performance and wake evolution
Open this publication in new window or tab >>Wind turbines over a hilly terrain: performance and wake evolution
2018 (English)Licentiate thesis, comprehensive summary (Other academic)
Alternative title[sv]
Vindturbiner över en kuperad terräng: prestanda och vakutbredning
Abstract [en]

The aim of this licentiate thesis is to investigate wind-turbines placed in a complex-terrain environment. This is done by studying the flow around small-scale wind-turbine models placed over a landscape model with hills, and by comparing the results with corresponding data obtained over a flat terrain model. The studied flow features include the wind-turbine wake development and the turbine performance under different conditions, the effects from wake interactions, the influence of the ambient turbulence levels and the influence from a complex topography. Wind-tunnel measurements have been performed using particle image velocimetry and hot-wire anemometry to measure the velocity field. Additionally, numerical simulations, based on RANS modelling and actuator-disc techniques, have been made to support the experimental data and to gain further knowledge about the investigated flow cases.

The results reveal that the hills promote a downward wake deflection behind the turbines and enhance the wind-turbine wake diffusion. As a consequence of this, and with the flow acceleration introduced by the hills, an improved power performance is seen for turbines exposed to wake-interference effects. A correlation is observed between the turbulence levels present in the flow, and the magnitude to which the hill-induced flow gradients influence the wake: Stronger wake deflections due to the hills are seen when the wind-turbine wake is more diffused. This is for instance the case when the wake of two tandem turbines is studied, or when higher ambient turbulence levels are present in the wind tunnel.

A good qualitative agreement is seen when comparing the experimental and numerical results. The simulation results further indicate that the hills give rise to modulations of the wind-turbine wake. It is shown that these modulations can be reasonably captured by means of wake-superposition techniques, given that a wake model with sufficient accuracy is chosen. 

Abstract [sv]

Syftet med denna licentiatavhandling är att öka förståelsen om hur vindturbiner påverkas av en omgivande komplex terräng. Huvudsakligen betraktas luftströmningen kring småskaliga vindturbinsmodeller som placerats över en landskapsmodell med kullar. I tillägg görs jämförelser med resultat som erhållits då vindtubinerna placerats över en platt landskapsmodell. De studerade strömningsaspekterna inkluderar vindturbinernas vakutveckling och prestanda under olika förhållanden, inverkan från vakinteraktioner, inflytande från omgivande turbulensnivåer och inverkan från en komplex topografi. Vindtunnelmätningar har utf ̈orts där PIV och varmtrådsanemometri användes för att uppmäta hastighetsfält. I tillägg har numeriska simuleringar utförts baserade på RANS-modellering, där turbinens rotor beskrevs av en porös skiva. Simuleringarna gjordes som komplement till de experimentella mätresultaten för att få en ökad förståelse om de undersökta strömningsfallen.

Resultaten från mätningarna och simuleringarna med kullar visar att terrängvariationerna främjar en nedåtgående vakförskjutning bakom turbinerna och ökar vindturbinernas vakdiffusion. Detta, i kombination med luftens acceleration över kullarna, resulterar i att en högre effektprestanda utvinns från en vindturbin vars inströmmande luftflöde störs av vaken från en framförliggande turbin. Vidare observeras kraftigare nedågående vakförskjutningar på grund av det kullriga landskapet då vindturbinsvakarna är mer diffunderade. Detta är exempelvis fallet då vaken bakom två turbiner placerade i en tandemkonfiguration studeras, eller när höga omgivande turbulensnivåer uppmäts i vindtunneln.

En bra kvalitativ överensstämmelse kan ses mellan de experimentella och numeriska resultat som uppnås. Resultaten från simuleringarna indikerar dessutom att landskapet med kullar ger upphov till moduleringar av vindturbinens vak. Det visas att dessa moduleringar kan beskrivas någorlunda väl med hjälp av vaksuperpositionsmetoder, givet att en vakmodell med tillräckligt hög noggrannhet väljs. 

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2018. p. 56
Keywords
Fluid mechanics, wind turbines, complex terrain, wind-tunnel measurements, Strömningsmekanik, vindturbiner, komplex terräng, vindtunnelmätningar
National Category
Applied Mechanics
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-221675 (URN)978-91-7729-674-4 (ISBN)
Presentation
2018-02-23, D3, Lindstedtsvägen 5, D-huset, KTH Campus, Stockholm, 10:15 (English)
Opponent
Supervisors
Funder
Swedish Research Council, 2014-5406
Note

QC 20180122

Available from: 2018-01-22 Created: 2018-01-19 Last updated: 2018-01-22Bibliographically approved

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