Wakes, blades and airfoilsan entangled trilogy Gerard van Bussel Oldenburg, 10-October 2012 Delft University of Technology Torque from Wind 2012 Wind Energy Chair Content • Evolution of rotor blade design • Airfoils and rotor control • Flat back airfoils • Airfoil design validation • (Static) stall and 3D effects • Rotor experiments • The largest blade so far • Conclusions Delft University of Technology Torque from Wind 2012 2 Wind Energy Chair Rotor design: evolution in time Delft University of Technology Torque from Wind 2012 3 Wind Energy Chair Airfoil Design considerations Airfoil parameters Thickness-to-chord ratio (1990) .24 - .21 .21 - .18 .18-.12 High maximum lift-to-drag ratio Aerodynamic Low max. and benign post stall Insensitivity to roughness Low noise Geometric compatibility Structural demands Delft University of Technology Torque from Wind 2012 4 Wind Energy Chair Airfoils and Rotor Control • In the 90ties wind turbine rotors were stall controlled • Stall control demands dedicated airfoils • And rotor blades with tuned planforms Delft University of Technology Torque from Wind 2012 5 Wind Energy Chair . 21 . The DU airfoil series (DU xx-W-yyy) TU Delft designed airfoils for use on wind turbine rotor blades Airfoil Design considerations Airfoil parameters Thickness-to-chord ratio (1990) .18-..24 . and benign post stall Insensitivity to roughness Low noise Geometric compatibility Structural demands Del ft Uni ve rsi ty of Te ch no lo g y DUWIND Rotor aerodyn am ic s 6 Wind E ne rg y C h air DU 93-W-210 DU 96-W-180 Delft University of Technology Torque from Wind 2012 6 Wind Energy Chair .18 .12 DU 91-W2-250 High maximum lift-to-drag ratio Aerodynamic Low max..21 . The DU airfoil series (DU xx-W-yyy) TU Delft designed airfoils for use on wind turbine rotor blades DU 96-W-180 DU 93-W-210 DU 91-W2-250 DU 97-W-300 Delft University of Technology Torque from Wind 2012 7 Wind Energy Chair . Airfoils and Rotor Control • In the 90ties wind turbine rotors were stall controlled • Stall control demands dedicated airfoils • And rotor blades with tuned planforms • Low drag over a large range of α ‘s • No lift overshoot at high(er angles) => low Clmax • Benign stall behaviour Delft University of Technology Torque from Wind 2012 8 Wind Energy Chair . Airfoils and Rotor Control II • Ten years later: variable pitch variable speed control getting mainstream • Airfoils better tuned to single point of operation • High l/d in a low drag pocket (limited range of α ‘s) • More optimised blade planforms • Towards much larger diameters Delft University of Technology Torque from Wind 2012 9 Wind Energy Chair . . Airfoil Design considerations Airfoil parameters Thickness-to-chord ratio (1990) .21 .21 . and benign post stall Insensitivity to roughness Low noise Geometric compatibility Structural demands Delft University of Technology Torque from Wind 2012 10 Wind Energy Chair .18 .12 High maximum lift-to-drag ratio Aerodynamic Low max.18-.24 .. 32 . Airfoil Design considerations Airfoil parameters Thickness-to-chord ratio (1990) .12 Thickness-to-chord ratio (2010) > ..24 ..18 High maximum lift-to-drag ratio Aerodynamic Increased Low design max.24 .24-.18-.32 .21 ..18 .21 . and lift post stall benign Insensitivity to roughness Low noise Geometric compatibility Structural demands Delft University of Technology Torque from Wind 2012 11 Wind Energy Chair . The DU airfoil series (DU xx-W-yyy) TU Delft designed airfoils for use on wind turbine rotor blades DU 96-W-180 DU 93-W-210 DU 91-W2-250 DU 97-W-300 Delft University of Technology Torque from Wind 2012 12 Wind Energy Chair . The DU airfoil series (DU xx-W-yyy) TU Delft designed airfoils for use on wind turbine rotor blades DU 00-W-212 DU 96-W-180 DU 00-W2-350 DU 93-W-210 DU 00-W2-401 DU 91-W2-250 DU 08-W-180 DU 97-W-300 Delft University of Technology Torque from Wind 2012 13 Wind Energy Chair . Airfoils and Rotor Control II • Ten years later: variable pitch variable speed control getting main stream • Airfoils better tuned to single point of operation • High l/d in a low drag pocket (limited range of α ‘s) • More optimised blade planforms • Towards much larger diameters BUT • Big issue for upscaling: square cube law!! Delft University of Technology Torque from Wind 2012 14 Wind Energy Chair . Rotors (so far) did not follow the square cube law!! Molly.Gasch M ~ R3 Garrad Hassan M ~ R2.1 PhD Turaj Ashuri 2012 Delft University of Technology Torque from Wind 2012 15 Wind Energy Chair .4 Ashuri (2012) M ~ R2.6 Jamieson M ~ R2. Escapes from R2-R3 law through: • Thicker airfoils • Higher cl design • More optimised planforms (thicker stem. slender tips) • Design for slightly lower induction factors BUT • Drawback: more pronounced (abrupt) stall behaviour Delft University of Technology Torque from Wind 2012 16 Wind Energy Chair . 2 0 2 4 6 8 10 12 14 ΩR tip speed ratio [-] TSR = λ = U Delft University of Technology Torque from Wind 2012 17 Wind Energy Chair .8 Cp. Ct [-] 0.Relation between CP-λ and CP-λ curve 1.2 T 1 CT = 3 2 1 2 ρU π R 0.6 P 0.4 CP = 3 2 1 2 ρU πR 0. More recent airfoil developments Measurements in Delft • increase of Reynolds number to 5.0 x 106 (via increase in chord and wind speed) …… and in Göttingen (op to Re 12 x 106) DU 08-W-180 compared to DU 96-W-180 • 5% increased max lift-to-drag • 20% higher (design) lift Suite of Flat back DU airfoils (designed using RFOIL calculations) • aerodynamic shaping of the “root” based upon DU97-W-300 Delft University of Technology Torque from Wind 2012 18 Wind Energy Chair . 6t/c (60%) • Challenge: generate “stable” lift DU-A 400-050 against limited drag (pres. Papadakis) DU-A 501-100 DU-A 600-180 Delft University of Technology Torque from Wind 2012 19 Wind Energy Chair .Flatback airfoils to brigde the gap • Transition between root airfoil and circular hub connection • Based upon DU97-W-300 • And even thicker dedicated “airfoils” up to 0. Root solutions may vary • Flat back airfoils DU-A 400-050 DU-A 501-100 DU-A 600-180 • Extended blade chords (fitted on the erection spot) • Huge circular cross sections (with some hidden features) • Multi element airfoils (slats/flaps) (poster Zahle) Delft University of Technology Torque from Wind 2012 20 Wind Energy Chair . We don’t know how to validate airfoil designs! • Flow around wind turbine blade is extremely complex • There is no location where the flow is 2 dimensional Delft University of Technology Torque from Wind 2012 21 Wind Energy Chair .. But…. We don’t know how to validate airfoil designs! • ANSYS CFX calculations • Watch “particles: carefully • Look at radial displacements (radial velocities => BEM not valid) • See how complex root flow is Source: KARI Korea Delft University of Technology Torque from Wind 2012 22 Wind Energy Chair . grid issues. etc etc) Delft University of Technology Torque from Wind 2012 23 Wind Energy Chair . We don’t know how to validate airfoil designs! • Flow around wind turbine blade is extremely complex • There is no location where the flow is 2 dimensional • What is an angle of attack in rotor flow? (pres Guntur) • Strip theory (BEM theory) is not valid (pres Soerensen) • CFD calculations are time consuming and can be ambiguous (turbulence modeling. numerical dissipation.. transition modeling. But…. CFD Model comparison NREL rotor in NASA-Ames wind tunnel 3D Full Navier-Stokes Phase VI. 10 m/s Limited Streamlines Suction side ANSYS/ CFX Transition en k-ω SST EllipSys3D Fully turbulent Unsteady DES BL: k-ω SST FLUENT Fully turbulent Steady k-ω SST Delft University of Technology Torque from Wind 2012 24 Wind Energy Chair . The lift coefficient across the span NREL rotor in NASA-Ames tunnel => α not measured. derived with vortex wake code => cl and cd from α.max Delft University of Technology Torque from Wind 2012 25 Wind Energy Chair .max à Near root: equal d(cl)/dα in attached conditions higher cl. cn and ct Comparison with 2d: à Near tip: d(cl)/dα reduced lower cl-level lower cl. - Tip view + + + + Front view Delft University of Technology Torque from Wind 2012 26 Wind Energy Chair . . Three-dimensional tip flow • Three-dimensional flow in tip region (pres Guntur) • Analogous to (translating) wing Wake induction Added local induction due to tip vortex • This leads to a virtual de-camber (lower d(cl)/dα) near the tip . . β : airfoil twist • Tuning parameters (a.4.2) Snel et al.h)=(3.3d = cl . 2000 • Engineering modification to integral boundary layer formulation: • Better capture of cp-distribution • XFOIL (2d) à RFOIL (rotational) Delft University of Technology Torque from Wind 2012 27 Wind Energy Chair . 1993 (2.1) Chaviaropoulos & Hansen.inv : inviscid airfoil cl at given α (no rotation) • c : chord.2.0.2 d + a ⎜ ⎟ ( cos β ) ( cl .n.. r: radius.2d : ‘true’ airfoil cl at given α (no rotation) • cl.inv − cl .2 d ) ⎝ r ⎠ • cl.Stall delay models (semi empirical) • Basis: 3D (rotational) boundary layer equations • Engineering modelshcorrecting Cl: ⎛ c ⎞ n cl . two-dimensional • Viscous with Re = 5·106.RFOIL demonstration – results • Airfoil: DU91-W2-250 • Cases: • Potential flow • Viscous with Re = 5·106. rotating with c/r = 0.3 Delft University of Technology Torque from Wind 2012 28 Wind Energy Chair . E L.5 degrees) Delft University of Technology Torque from Wind 2012 29 Wind Energy Chair .a.o. Stall delay models need 2d input => measurements • 2d flow beyond stall is not 2d!! => stall cells occur above a certain a. (pres Manolesos) • How good is Rfoil then?? Flow T.E DU 91-W2-250 at 12 degrees (stall angle 9. 5 Re = 3.0x106 0. the larger the problem! Delft University of Technology Torque from Wind 2012 30 Wind Energy Chair . RFOIL.0 0. + 20% l.000 0.0 -1.s.5 smooth -0.5 1.s.045 -10 0 10 ķ (o) 20 cd -0.s.s.0 DU 97-W-300 0. -1.0 cl cl 1.0 1.0 0.030 0.015 0.5 zztape 5% u.0 2.E • The thicker the airfoil. Stall delay models need 2d input => measurements • 2d flow beyond stall is not 2d!! • How good is Rfoil then?? 2. + 15% l.5 1.5 0.0 • Drag is under predicted • At heavy separation/stall Rfoil (in 2d mode) is wrong T.E L. 1% u. 0 100.0 150.0 VG's at x/c= 0.0 -5.0 5.2 0.5 max.5 50.5 Sharp kink in P-V curve VG's at x/c= 0.0 20. power 1.6 Re = 3.0x106 Clean -0. through vortex generators Distance to stall power (VG’s) (Delay of rated wind speed) 2.0 Rapid drop => 0.4 DU 93-W-250 VG's at x/c= 0. Tweak 2d measurements around stall e.0 Cl/Cd Angle (deg.g.0 0.0 10.0 25.0 15.0 Increase Cl in 1.) BUT: leads to drag increase at all α‘s Delft University of Technology Torque from Wind 2012 31 Wind Energy Chair . use 2d measurements .So what is the situation: • At the real root region flow is highly 3 dimensional: no 2d equivalent at all .2d measurements of de-cambered airfoil Delft University of Technology Torque from Wind 2012 32 Wind Energy Chair .first determine effect of tip vortex DU-A 501-100 .trust Rfoil for stall behaviour . Rfoil .and/or tweak stall behaviour using VG’s • Midspan & outboard: .design using e.use 2d measurements • At the tip region: DU-A 400-050 .3D measurements or CFD • Inboard: .g.design for de-cambering DU-A 600-180 . Blade design through: • Coupling of RFoil with BEM 3D panel code . midspan and outboard .using genetic/gradient based optimisers (pres Grasso) . manufacturing and costs • Validation of 2-3 airfoils in windtunnel • Check final design using CFD DU-A 400-050 => Experiments on real rotors needed for validation!! DU-A 501-100 DU-A 600-180 Delft University of Technology Torque from Wind 2012 33 Wind Energy Chair .with cost functions for structure.to capture 3D effects from planform and wake • Coupled design of inboard. Will CFD become a winner in the end?? • if issues regarding turbulence modeling. grid issues. numerical dissipation are tackled • and calculation time is speeded up • and validation has taken place in more operational cases • then the answer may become yes Validation experiments on real rotors!! DU-A 400-050 DU-A 501-100 DU-A 600-180 Delft University of Technology Torque from Wind 2012 34 Wind Energy Chair . transition modeling. Experiments for validation of codes Rotor experiments in controlled conditions: • The NASA-Ames experiments DU 91-W2-250 • The MEXICO experiments Risø A1-21 (Gerard Schepers presentation Thursday afternoon) 2.25 m NACA 64-418 • The TU Delft OJF experiments Mexico rotor blade Delft University of Technology Torque from Wind 2012 35 Wind Energy Chair . rotating blade Delft University of Technology Torque from Wind 2012 36 Wind Energy Chair . 2 m O Chord wise measurement set-up RPM: 0-20 Hz. λ=7 at Vwind= 17 m/s Laser 2 cameras • measurements not inside BL • axis-symmetric flow conditions • stationary laser.Root flow rotor: Chord wise SPIV experiments 2 bladed rotor. Stereo PIV (SPIV) results Spanwise velocities: brown => outboard blue => inboard Panel code calculations Stereo PIV experimental results PhD research Busra Akay. Daniel Micallef Delft University of Technology Torque from Wind 2012 37 Wind Energy Chair . Daniel Micallef Delft University of Technology Torque from Wind 2012 38 Wind Energy Chair . Comparison measurements (SPIV) to calculations (3D panel code) SPIV Panel code Axial velocities λ=7 top: SPIV measurements bottom: panel code calculations PhD research Busra Akay. and root flow • Detailed information about radial component of flow (outside BL) • Forces on the blade through contour integration of velocities In plane velocities station 2 (mid span) and force integration contours Radial velocities in root region PhD research Busra Akay Together with Carlos Ferreira and Daniele Ragni Delft University of Technology Torque from Wind 2012 39 Wind Energy Chair . including tip. From SPIV experimental results: • 3D velocity field around rotor. Details of the tip flow SPIV Panel code Tip vorticity from SPIV measurements Axial velocities station at tip PhD research Daniel Micallef Delft University of Technology Torque from Wind 2012 40 Wind Energy Chair . LM Alstom Haliade (150m O) blade VG’s Gurney flap (BL fence) Vortex generators Ozlem Ceyhan ECN Delft University of Technology Torque from Wind 2012 41 Wind Energy Chair . Conclusions • 2d wind tunnel measurement still basis for major part of blade design • 3D wake model inevitable for a.. root flow and tip (vortex) flow • Stall modeling needs improvement (stall delay + dynamic stall) • 2d measurements +Rfoil+3D panel code current design vehicle • CFD is runner up for airfoil and for blade design • 3D rotating experiments are key in further understanding and validation Delft University of Technology Torque from Wind 2012 42 Wind Energy Chair .a.o.