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Investigation of Boundary Layer Suction on a Wind Turbine Airfoil Using CFD

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Author: Apostolos Tentolouris Piperas Investigation of Boundary Layer Suction on a Wind Turbine Airfoil using CFD Supervisor: Martin O.L. Hansen Wind Energy Building 403 Kongens Lyngby Master’s Thesis 5th August 2010 Acknowledgements There is not much to acknowledge really. People without whom I would never have survived my studies and who helped me realize that there is plenty of beauty to be shared despite the smothering workload of these past two years, a period that eventually lead to the present document, do not need this page to be aware of it and most likely will never read it in the first place. I would like however to express my gratitude to my supervisor Martin O.L. Hansen, probably the smartest person on the planet, for taking the time to bother with me and my questions, and Dalibor Cavar with Juan Pablo Murcia without whom my work would have taken twice the time and effort. Finally, I would like to thank whoever was responsible for my admittance to DTU. I may not have become a better engineer in the direction I was hoping, but I ended up becoming a better person, which is something I could have never hoped for. i Preface This report is part of the requirements to achieve the Master of Science in Engineering (M.Sc.Eng.) at the Technical University of Denmark. It represents 30 ECTS points and was carried out at the Department of Mechanical Engineering at the Technical University of Denmark from Fe- bruary until August 2010. iii Abstract The present Master Thesis deals with the investigation of suction as a mean of boundary layer control on a wind turbine root airfoil using CFD. Flow around a NACA 4415 airfoil is simula- ted in ANSYS CFX 12.1 environment and transition to turbulence as well as flow separation are studied for various arrangements of suction. The coefficients of lift and drag are computed for different angles of attack and the lift and drag curves after applying suction are compared with the corresponding values of the clean airfoil. Finally, a simplistic analysis is carried out in order to evaluate the impact and the usability of boundary layer suction on a wind turbine blade. v Table of Contents List of Figures xi List of Symbols 1 1 Introduction 1 1.1 General . ................................... 1 1.2 Previous Work .................................. 1 1.3 Scope . ................................... 2 2 Boundary Layer Theory 3 2.1 Boundary Layer Basics ............................. 3 2.2 Laminar and Turbulent flows.......................... 5 2.3 Boundary Layer Thickness - Drag ....................... 7 2.4 Transition . ................................... 10 2.5 External Pressure Gradient . ........................ 14 2.6 Boundary Layer Separation . ........................ 15 2.7 Separation Bubbles . ............................. 17 2.8 Boundary Layer Control ............................ 18 2.9 Boundary Layer Suction ............................ 21 3 CFD Implementation 25 3.1 Setting up the model . ............................. 25 3.1.1 Geometry . ............................. 25 3.1.2 Mesh .................................. 25 3.1.3 Setup .................................. 27 3.1.4 Solver .................................. 30 4 Results 37 4.1 Suction Location . ............................. 38 4.2 Discrete Suction versus Distributed Suction . 39 4.3 Suction Quantity . ............................. 45 4.4 Finer Analysis .................................. 47 4.5 Wind Turbine Performance Enhancement . 51 vii viii TABLE OF CONTENTS 4.5.1 Blade Element Momentum Method . 52 4.5.2 BEM algorithm . ............................ 52 4.5.3 BEM results . ............................ 55 4.6 The Blade as a Centrifugal Pump . ...................... 59 5 Conclusions and Perspectives 63 5.1 Conclusions . ................................. 63 5.2 Suggestions for Further Work .......................... 63 A Appendix 69 List of Figures 2.1 Boundary layer development. ........................ 4 2.2 Thickness and shear variation . ........................ 5 2.3 Laminar and turbulent non-dimensionalised velocity profile. 6 2.4 Turbulent boundary layer profile. 6 2.5 Turbulent boundary layer structure. ....................... 7 2.6 Diplacement thickness. ............................. 8 2.7 Boundary layer thicknesses. ........................ 9 2.8 Shear stress coefficient. ............................. 9 2.9 Shear stress distribution. ............................ 10 2.10 Transition from laminar flow to turbulent. 11 2.11 Tollmien Schlichting waves. ........................ 11 2.12 Ribbon frequency effect on boundary layer response. 12 2.13 Neutral Stability Curve. ............................. 13 2.14 Pressure distribution on an airfoil. ....................... 13 2.15 Effect of pressure gradient on boundary layer. 14 2.16 Velocity profiles and gradients. ........................ 15 2.17 Boundarly layer profiles and point of inflection. 16 2.18 Flow separation on an airfoil. ........................ 16 2.19 Effect of adverse pressure gradient on the boundary layer. 17 2.20 Separation bubbles. ............................. 18 2.21 Vortex generators. ............................. 20 2.22 Boundary layer aceleration. ........................ 20 2.23 Boundary layer control via suction. ....................... 21 2.24 Comparison between continuous and discrete suction. 22 2.25 Critical value of suction coefficient. ...................... 22 2.26 Skin friction variation under optimum suction . 23 3.1 Relative error. .................................. 26 3.2 Generated Mesh. ............................. 29 3.3 Image of the domain for the LE suction case prior the import into the Solver.. 30 3.4 No suction for 0 degrees angle of attack. 31 3.5 No suction for 15 degrees angle of attack. 33 ix x LIST OF FIGURES 3.6 Transient simulation for no suction case at 15 degrees angle of attack. 33 3.7 Leading edge distributed suction (Cq = 0.03) for 15 degrees angle of attack. 34 3.8 Leading edge distribution - Tight convergence, higher number of iterrations . 34 3.9 Transient simulation for (Cq = 0.03) at 15 degrees angle of attack . 35 4.1 Eddy viscosity for 10o angle of attack ..................... 37 4.2 Point of transition ................................ 38 4.3 Non dimensionalized eddy viscosity and shear stress . 39 4.4 Application of suction at maximum thickness point and at leading edge . 40 4.5 Point of transition to turbulent flow at different angles of attack . 41 4.6 Location of distributed suction . ...................... 41 4.7 Velocity gradient for clean airfoil and distributes suction . 42 4.8 Flow separation at 17o angle of attack for a clean airfoil. 42 4.9 Pressure coefficients for different angles of attack . 43 4.10 Pressure coefficient at 17o angle of attack for different suction cases. 43 4.11 Flow separation for different distributed suction cases . 44 4.12 Lift and drag curves for discrete suction . 45 4.13 Lift and drag curves for distributed suction . 45 o 4.14 CLand CD values for different suction coefficients at 15 angle of attack . 46 4.15 CL ratio for different suction coefficients at 15o angle of attack . 46 CD 4.16 Pressure coefficient for different suction coefficients at 15o angle of attack . 47 4.17 Eddy viscosity for different suction coefficients at 15o angle of attack . 47 du o 4.18 Velocity gradient dy for different suction coefficients at 15 angle of attack . 50 4.19 Streamlines for different suction coefficients at 15o angle of attack . 50 4.20 Lift coefficient response for different angles of attack . 50 4.21 Lift and drag curves for the clean airfoil and Cq = 0.08 case . 51 4.22 Suction effect on aerodynamic coefficients . 51 4.23 Velocities at rotor plane. ............................ 52 4.24 Tjaereborg wind turbine characteristics ..................... 55 4.25 Angle of attack variation ............................ 56 4.26 Chord distribution of the Tjaereborg blade . 56 4.27 Lift and drag curves of root segment ...................... 57 4.28 Power contribution of each of the three first segments . 58 4.29 Power curve of the Tjaereborg wind turbine after suction . 58 4.30 Power ratio between the clean blade and suction cases . 59 4.31 Weibull distribution with A = 8 and k = 2................... 59 4.32 Power contribution of each of the two close to hub segments for reduced chord 60 4.33 Tjaereborg power curve after suction and 25%chord reduction . 60 4.34 Tjaereborg power coefficient curve after suction and 25%chord reduction . 61 4.35 Thrust on the rotor for a range of wind speeds from cut-in speed to rated power 61 5.1 Mass flow distribution at suction location. 64 LIST OF FIGURES xi A.1 Distributed suction location . ........................ 69 A.2 Eddy viscosity for different turbulence models. 70 A.3 Eddy viscosity for normal and 45o inclined suction for different angles of attack 71 A.4 Sensitivity check for steady state simulations . 72 A.5 FFT of lift coefficient response. ........................ 72 A.6 Lift coefficient response at 60o angle of attack. 73 A.7 Suction arrangement for pump driven suction on a glider plane . 73 List of Symbols a .................................................axial induction coefficient [] a! ............................................tangential induction coefficient [] A .............................................................rotor area [m 2] AEO................................................annual energy output [GWh] CD ..........................................................drag coefficient [] C fx ...............................................local shear stress coefficient [] C fL ...........................................averaged
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