Comparison study between wind
turbine and power kite wakes
Thomas Haas
Ph.D. candidate and AWESCO fellow
Supervision: Prof. Dr. Ir. Johan Meyers
Wake Conference 2017
May 30th – June 1st, 2017, Visby
Airborne Wind Energy Principle
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• Emerging wind energy technology
• Power harvesting tethered airborne devices
• High-altitude operation
• On-board and ground-based generation
[Airborne Wind Energy, Springer]
Airborne Wind Energy Generation modes
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Ground-based generation
• Fast flying tethered wing
• High tension unrolls tether from drum
• Drum drives electric generator on ground
• Cyclic power generation (reel in/out phases)
On-board generation
• Tethered plane with on-board turbines
• High relative airspeed of crosswind
• Tether conducts electricity
• Vertical take-off and landing
→ “Pumping mode”
→ “Drag mode”
Airborne Wind Energy Technology
Advantages
• Stronger and more consistent winds
• Require less material as wind turbines
• Eased ground-based maintenance
• Rapid and flexible deployment
Challenges
• Operation in variable wind conditions
• Autonomous launch and landing
• No prevalent design or concept
• Large scale commercial operation
Implementation
• Several companies in the field
• Prototypes of 50kW and 600kW
• Upscaling to multiple MW in future
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[https://x.company/makani]
Research question:
How do kite systems interact
with their wind environment?
Outline
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1. Introduction
2. Motivation
3. Methodology
4. Results
5. Outlook
Motivation Background
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Objective
• Difference between wind turbine-like wake and power kite wake
• Investigate the influence of torque on the wake development
Turbine mode
- Inspired from wind turbine operation
- Torque virtually captured by the generator
- Net addition of torque onto the flow
Drag mode
- Additional forces for on-board turbines
- Aero. torque balances turbine torque
- Total torque added to flow is zero
Outline
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1. Introduction
2. Motivation
3. Methodology
4. Results
5. Outlook
Methodology CFD code
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Large Eddy Simulation
• Filtered incompressible Navier-Stokes equations
• Subgrid scale modelling using Smagorinsky model
Inhouse code from KULeuven
• Pseudo-spectral flow solver SPWind [1,2]
• Fringe region technique to circumvent BC periodicity
• Synthetic turbulence generation using Mann model (tugen library)
Actuator Line Technique
• No implicit force computation from airfoil characteristics
• Optimal force distribution based on Betz-Joukowski limit
• Smearing out of local forces onto LES grid using Gaussian filter
[1] Calaf et al. 2010, [2] Munters, Meneveau and Meyers 2016
Methodology Force distribution
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Optimality condition from Betz-Joukowski limit
• Aerodynamic drag is neglected
• Optimal induction factors: 𝑎 = 1 3 and 𝑎′ = 𝑎(1 − 𝑎) 𝜆2𝜇2
• Derive flow angle and lift distribution
𝐿 = 𝜌4𝜋𝑅
𝑈∞2
𝐵𝜆𝑎(1 − 𝑎) (1 − 𝑎)2+(𝜆𝜇(1 − 𝑎′))2 tan(𝜙) =
1 − 𝑎
𝜆𝜇(1 − 𝑎′)
Methodology Force distribution
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Optimality condition from Betz-Joukowski limit
• Aerodynamic drag is neglected
• Optimal induction factors: 𝑎 = 1 3 and 𝑎′ = 𝑎(1 − 𝑎) 𝜆2𝜇2
• Derive flow angle and lift distribution
Additional turbine forces D
• 2 turbines (33% and 66% of span)
• Additional force in tangential direction
• Distributed over 10% of the actuator line segments
• Total torque on flow is zero
𝑟 × 𝐿 sin 𝜙 𝑑𝑟 +
𝑟𝑜
𝑟𝑖
𝑟 × 𝐷 𝑑𝑟 =
𝑟𝑜
𝑟𝑖
0
Methodology Simulation setup
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Grid resolution 𝑁𝑥 × 𝑁𝑦 × 𝑁𝑧 = 640 × 160 × 320 ~ 30 ∙ 106 grid points
Kite span 𝑏 = 0.43𝑅
Cell size ∆𝑥 × ∆𝑦 × ∆𝑧≈ 0.1𝑏 × 0.1𝑏 × 0.05𝑏
Operation conditions 𝜆 = 7, 𝑈∞ = 10𝑚/𝑠
Inflow turbulence Turbulence over sea 𝐿𝑇 , 𝛼𝜖2/3, Γ = [60𝑚, 0.022𝑚
4
3𝑠−2, 2.85]
Parallel computation 320 CPUs
Outline
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1. Introduction
2. Motivation
3. Methodology
4. Results
5. Outlook
Results Wake spreading in uniform inflow
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Contours of time-averaged axial induction 𝒂 = 𝟏 − 𝒖𝒙 𝑼∞
• 3 locations x/R = 0, 3, 6
• Inward and outward wake spreading
• “Jet effect” inside annulus core region
• No difference between turbine and drag mode
a = -0,05
a = 0
a = +0,2
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Results Flow structure in uniform inflow
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Contours of instantaneous axial induction at annulus plane 𝒂 = 𝟏 −𝒖𝒙
𝑼∞
a = -0.05
a = 0
a = +0.2
Tip vortices
Results Flow structure in uniform inflow
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Contours of instantaneous tangential induction at annulus plane 𝒂′ =−𝒖𝒕
𝜴𝒓
a’ = -0.12
a’ = -0.06
a’ = -0.02
a’ ~ 0
Additional structures
Results Instantaneous fields of vorticity magnitude in uniform inflow
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Pair of counter-
rotating tip vortices
Pair of tip vortices
+ turbine vortices
Turbine mode
Drag mode
New research questions:
• Formation mechanism
• Setup dependency
Results Vortex interaction in drag mode
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• Slower downstream advection of turbine vortices
Results Vortex interaction in drag mode
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• Slower downstream advection of turbine vortices
• Outward advection
Results Vortex interaction in drag mode
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• Slower downstream advection of turbine vortices
• Outward advection
• Rapid vortex breakdown
Results Instantaneous fields of vorticity magnitude in turbulent inflow
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Turbine mode
Drag mode
• Same behavior at
annulus plane
• Additional interaction
with ambient turbulence
• Rapid loss of coherence
Results Far wake development: velocity deficit
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• Faster wake recovery with turbulent inflow 𝑥 𝑅
Outline
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1. Introduction
2. Motivation
3. Methodology
4. Results
5. Outlook
Conclusions and future work
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Investigation of wake characteristics in context of airborne wind energy
• Two different operation modes: turbine-like and torque-free operation
• Two different inflow conditions: uniform and turbulent inflow
Principal outcome
• Wake spreading in both inward and outward direction
• Shedding from additional vortices at location of on-board turbines
• No substantial influence on wake development
• Faster wake recovery with turbulent inflow
Future work
• Further investigation of formation mechanism of turbine vortices
• Computation with aerodynamic model based on 2D airfoil data
• Simulation in atmospheric boundary layer
Acknowledgments
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Support from European Commission through Horizon2020 programme AWESCO
International Training Network - Grant No.642682
Computational resources provided by Flemish Supercomputer Center
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Thank you