Brussels, 14 March 2011 Boundary Layer Suction for Wind Turbine Blades : An Integral Design Approach Gael de Oliveira, Roy Campe and Eric Terry

219 Overview Boundary Layer Suction for Wind Turbine Blades : An Integral Design Approach Challenges for large blades Boundary layer control Turbulent suction physics Multi-objective airfoil optimization Design compromises –Glide ratio vs. building height –Glide ratio vs. stall Coupling blade and airfoil design Integral design process and goals Results Final note

319 Actiflow is an engineering and design company, specialized in aerodynamics Boundary Layer Suction for Wind Turbine Blades : An Integral Design Approach Actiflow is founded in 2005 as a spin-off of Delft University of Technology (NL) We have an excellent team of engineers skilled in product design, aero- & hydro-dynamics, CAE and systems & control Our services are trusted by leading OEMS spread all over Europe, active from automotive to aerospace, from wind energy to oil & gas and from the medical to the building industry Besides its normal design activities, Actiflow develops innovative idea’s and concepts which have led to different patents and software tools used in the industry

419 Growth of wind turbine size challenges classical blade technologies and design approaches Boundary Layer Suction for Wind Turbine Blades : An Integral Design Approach Wind turbines have grown considerably throughout the last years Blades are becoming larger than many scales of atmospheric turbulence – Tip and root can be exposed to different, weakly correlated, perturbations – Challenges classical design approach: single operating point assumption weakens – Active load alleviation is under consideration: requires a control vector Large blades require long thick profile regions to satisfy structural requirements – Relevance of thick profile performance will increase Conventional profiles won't experience breakthroughs in the short term Boundary layer control can tackle large blade challenges by offering: – Means to delay separation on thick profiles – A fast and robust control vector

519 Suction and Injection are naturally available and can be used to delay boundary layer separation or damp loads Boundary Layer Suction for Wind Turbine Blades : An Integral Design Approach Suction consists in removing a small amount of low momentum fluid from the boundary layer to limit its growth Injection consists in adding some fluid to the boundary layer, which can trigger controlled stall to damp loads Centrifugal forces create a pressure gradient inside the blade that is sufficient to drive suction and injection flow Mature technology applied in aviation, automotive and industry First step focuses on delaying separation near blade root, and improving glide ratio, maximum lift and roughness tolerance Suction in the pressure recovery: –Turbulent boundary layer –continuously through porous material

619 Rfoil was modified to simulate turbulent suction Boundary Layer Suction for Wind Turbine Blades : An Integral Design Approach Formally, only the impermeability condition is replaced by a normal velocity at the wall boundary condition All models and physical processes are affected: –Added convective term to integral equations (by TU Delft) –Corrections to closure relations: Skin friction Dissipation coefficient – New Term in Equilibrium Shear Stress Level From modified G-beta relation Derived approximate modified Shear Lag Equation –VI interaction Source Terms Uncertainty due to suction is lower than general Rfoil uncertainty Behavior of sucked turbulent BL's near separation is object of further research and testing

719 Competing with mature technologies and handling conflicting goals requires bold design approach Boundary Layer Suction for Wind Turbine Blades : An Integral Design Approach Suction is competing with 100 years of conventional profile design Quantifying the technology value requires objective comparison between airfoils Successful designs are meaningful compromises between conflicting goals: –Aerodynamic Performance –Stall Softness –Building Height Approach should stimulate creativity The answer: Multi–Objective Optimization

819 Multi-objective optimization is used for airfoil design Boundary Layer Suction for Wind Turbine Blades : An Integral Design Approach Design process consists of studying a compromise between goals (eg L/D vs thickness) Results are manifolds of compromises between objectives: –Set of non-inferior solutions –Images of non-inferior solutions : the Pareto Front Genetic algorithm searches for the optimal designs in the airfoil design space –NSGA-II based algorithm: gamultiobj() Airfoil shapes are represented using the CST parameterization with about 20 parameters Aerodynamic properties are evaluated with Rfoil(-suction) through a Matlab Interface Design goals consider performance over operation ranges to obtain robust designs (no single point design)

919 The compromise between glide ratio and projected thickness Boundary Layer Suction for Wind Turbine Blades : An Integral Design Approach Validation case used to assess: – Suitable shape refinement (BPO) – Ability to reach global optimum Aerodynamic goal was mean L/D: “Weighted average over Cl interval from 0.5 to 1.2” Design method outperforms existing designs, and formalizes designer insight Conditions: No suction Re = 3e6 Transition forced near leading edge: –0.01c top –0.10c bottom

1019 Suction improves performance over wide range of thicknesses Boundary Layer Suction for Wind Turbine Blades : An Integral Design Approach Studied the compromise between glide ratio and performance for various volumetric suction rates Ideal airfoil would be on upper right corner Same conditions as in previous slide Suction improves performance over wide range of thicknesses Thick BLS powered airfoils with t/c=0.35 have same glide ratio as conventional t/c=0.20 airfoils Proper optimization convergence in very thick regions is hindered by the current analysis code limitations, but will be solved by setting more sophisticated goals

1119 The application of suction changes the shape of the optimal airfoil Study case: the 30% thick airfoils from the previous slide Optimal airfoils with suction are more cambered as steeper pressure recovery can be handled Double curvature means more efficiency, but steeper stall, specially with suction Boundary Layer Suction for Wind Turbine Blades : An Integral Design Approach

1219 It seems smooth stalls can also be achieved with suction Optimizer was used to quantify compromises between stall harshness and glide ratio Goals were mean L/D and stall harshness: “Squared sum of deviations in neighborhood of maximum lift point” There is an almost universally desirable stall/performance compromise Ideal airfoil would be at lower right corner Boundary Layer Suction for Wind Turbine Blades : An Integral Design Approach Further wind tunnel testing is planned in order to validate the simulation of suction in separated flows

1319 Multi-objective optimization provides foundation for an Integral Blade Design Approach Boundary Layer Suction for Wind Turbine Blades : An Integral Design Approach r/R Importance Aerodynamics Structure A WT blade design is a varying compromise between demands A complete blade geometry can be conceived as a set of non-inferior solutions to a multi-objective optimization problem This enables to generate airfoil families tailored for specific blades and sites, in a single optimization case By postulating a desirable thickness distribution we can assign each section a blade location and account for variation of flow conditions along blade radius:  Solidity  Reynolds  Available suction  Operational ranges

1419 Specify –Diameter –Tip speed ratio –Turbulence intensity –Characteristic yaw and tilt misalignment –Reference wind speed Postulate a desirable distribution of: –Axial induction factor (‘a’) –Structural parameter (eg. building height) –Bounds for maximum and minimum chord –Suction volume coefficient Integral Design Flow Boundary Layer Suction for Wind Turbine Blades : An Integral Design Approach 2 Run Case Obtain Optimized blade: –Airfoil family –Suction panel location –Operational Cl –α - ɸ - twist angles 1 3 4

1519 Integral design goals should maximize power production and minimize loads Boundary Layer Suction for Wind Turbine Blades : An Integral Design Approach The BEM equations indicate that D/L is a measure of a section's distance to ideality Independent variables: – Profile shape – Suction panel location – chord Evolutive and simplified rotor model based on non-inferior individuals performance is used to predict the required operating profile lift Unsteadiness of flow varies along blade and statistical treatment of perturbations provides approximate operational ranges

1619 Integral design results Boundary Layer Suction for Wind Turbine Blades : An Integral Design Approach Blade Radius: 63m Tip Speed Ratio: 7 Reference Wind Speed: 9 m/s Suction Cq = Turbulence Intensity: 15% Tilt Angle: 10 deg Characteristic Yaw Error Angle: 5 deg Forced Transition: 0.05c (upper) / 0.10c (lower) Minimum and maximum chord distribution: roughly 80% and 160% of NREL reference blade Thickness distribution: same as NREL 5MW reference blade

1719 Integral design results – airfoil shapes Boundary Layer Suction for Wind Turbine Blades : An Integral Design Approach Building Height r/R (Location) Mean L/D rough Max L/D rough Max L/D clean Design Cl

1819 Final Note Boundary Layer Suction for Wind Turbine Blades : An Integral Design Approach Boundary Layer Suction brings significant glide ratio and maximum lift increases on thick profiles Multi-objective optimization techniques respond to major blade design challenges Design method is under constant development. Evolutive Rotor Concept is very powerful and provides solid framework to couple disciplines: – Using a structural performance index instead of building height – Control strategies Research on sucked turbulent boundary layers near separation proceeds with theoretical efforts and experimental campaign in TU-Delft's boundary layer tunnel This project is supported by Delft University of Technology, and by the Energy Research Subsidy of the Dutch ministry of Economic affairs

19 Thank you for your attention – Questions Boundary Layer Suction for Wind Turbine Blades : An Integral Design Approach Wind tunnel experiment of airfoil with boundary layer suction, Low Speed Laboratory, Delft University of Technology

Actiflow BV Zinkstraat AD Breda The Netherlands Tel: