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Wind turbines over a hilly terrain: performance and wake evolution
KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
2018 (English)Licentiate thesis, comprehensive summary (Other academic)Alternative title
Vindturbiner över en kuperad terräng: prestanda och vakutbredning (Swedish)
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
Keyword [en]
Fluid mechanics, wind turbines, complex terrain, wind-tunnel measurements
Keyword [sv]
Strömningsmekanik, vindturbiner, komplex terräng, vindtunnelmätningar
National Category
Applied Mechanics
Research subject
Engineering Mechanics
Identifiers
URN: urn:nbn:se:kth:diva-221675ISBN: 978-91-7729-674-4 (print)OAI: oai:DiVA.org:kth-221675DiVA: diva2:1176088
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
List of papers
1. Effects From Complex Terrain on Wind-Turbine Performance
Open this publication in new window or tab >>Effects From Complex Terrain on Wind-Turbine Performance
2017 (English)In: Journal of energy resources technology, ISSN 0195-0738, E-ISSN 1528-8994, Vol. 139, no 5, article id 051205Article in journal (Refereed) Published
Abstract [en]

In this work, experimental measurements are made to study wind turbines over complex terrains and in presence of the atmospheric boundary layer. Thrust and power coefficients for single and multiple turbines are measured when introducing sinusoidal hills and spires inducing an artificial atmospheric boundary layer. Additionally, wake interaction effects are studied, and inflow velocity profiles are characterized using hot-wire anemometry. The results indicate that the introduced hills have a positive impact on the wind-turbine performance and that wake-interaction effects are significantly reduced during turbulent inflow conditions.

Place, publisher, year, edition, pages
ASME Press, 2017
Keyword
Marine Current Turbines, Boundary-Layer, Wake, Flow, Simulation
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-215373 (URN)10.1115/1.4036048 (DOI)000411139800007 ()2-s2.0-85018483540 (Scopus ID)
Conference
2nd International Conference on Next Generation of Wind Energy (ICNGWE), AUG 24-26, 2016, Lund Univ, Lund, Sweden
Funder
Swedish Research Council
Note

QC 20171009

Available from: 2017-10-09 Created: 2017-10-09 Last updated: 2018-01-22Bibliographically approved
2. A wind-tunnel study of the wake development behind wind turbines over sinusoidal hills
Open this publication in new window or tab >>A wind-tunnel study of the wake development behind wind turbines over sinusoidal hills
(English)In: Wind Energy, ISSN 1095-4244, E-ISSN 1099-1824Article in journal (Refereed) Accepted
Keyword
Wind turbines, Complex terrain, Wind-tunnel measurements
National Category
Fluid Mechanics and Acoustics
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-221704 (URN)
Funder
Swedish Research Council
Note

QCR 20180122

Available from: 2018-01-22 Created: 2018-01-22 Last updated: 2018-02-07Bibliographically approved
3. Qualitative analysis of wind-turbine wakes over hilly terrain
Open this publication in new window or tab >>Qualitative analysis of wind-turbine wakes over hilly terrain
2017 (English)In: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 854, no 1, article id 012023Article in journal (Refereed) Published
Abstract [en]

In this work, wind-turbine wakes are studied over flat and hilly terrains. Measurements made by using stereoscopic PIV are compared to data obtained from numerical simulations using RANS equations and an actuator-disc method. The numerical and experimental data show similar qualitative trends, indicating that the wind-turbine wake is perturbed by the presence of the hills. Additionally, a faster flow recovery at hub height is seen with the hilly terrain, indicating that the hills presence is beneficial for downstream turbines exposed to wake-interaction effects. The Jensen wake model is implemented over the hilly terrain and it is shown that this model cannot accurately capture the wake modulations induced by the hills. However, by superimposing a wind-turbine wake simulated over flat terrain on the hilly-terrain flow field, it is illustrated that the commonly-used wake-superposition technique can yield reasonable results if the used wake model has sufficient accuracy.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2017
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-213258 (URN)10.1088/1742-6596/854/1/012023 (DOI)2-s2.0-85023634607 (Scopus ID)
Conference
Wake Conference 2017, Uppsala University's Gotland CampusVisby, Sweden, 30 May 2017 through 1 June 2017
Note

QC 20170828

Available from: 2017-08-29 Created: 2017-08-29 Last updated: 2018-01-22Bibliographically approved

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