1 Fluidic Load Control for Wind Turbine Blades C.S. Boeije, H. de Vries, I. Cleine, E. van Emden, G.G.M Zwart, H. Stobbe, A. Hirschberg, H.W.M. Hoeijmakers Engineering Fluid Dynamics

2 Overview Introduction Experimental Setup Numerical Setup Results Conclusions

3 Introduction Fatigue loads affect energy production costs –Amount of required materials for structure –Maintenance –Reliability during life span As wind turbines become larger, fatigue loads become more important Aim: Reduction of Fatigue Loads

4 Introduction (cont’d) Fatigue loads due to: –Variable wind, Turbulent inflow, Wind shear –Gravity –Tower shadow –Yaw misalignment –Wake interaction (in wind farms) State of the art load control: –Passive: through aero-elastic response of structure, e.g. tension- torsion coupling –Active: pitching of (individual) blades Pitching of blades not fast enough to control rapidly changing loads

5 Introduction (cont’d) Concepts for fast control: –Conventional trailing edge flaps Chow et al., Journal of Physics: Conference Series 75 (2007) –Flexible trailing edge flaps Barlas & Van Kuik, Journal of Physics: Conference Series 75 (2007) –MEM tabs Chow & Van Dam, J. of Aircraft, Vol. 43, No. 5, 2006, pp

6 Introduction (cont’d) Present study: load control concept using fluidic jets –Boundary layer separation control –Pitch control Concept: injection of air from multiple orifices or slits controlled individually –Slits located nearby trailing edge of blade –Continuous injection of air directed normally to blade surface

7 Introduction (cont’d) Initial steps in investigation: –Study of flow around non-rotating blade section –2D flow: Long slits: length L=O(c), width w=O(0.01c) Simultaneous operation, no control system –U inj /U ∞ =O(1) Important issues: –Is continuous injection from long slits located near trailing edge effective? –Are jet velocities of U inj /U ∞ =O(1) sufficient? –How fast can aerodynamic performance be changed, i.e. what is response time?

8 Experimental Setup Twente University’s closed loop wind tunnel: –Test section 0.7 m x 0.9 m –Maximum flow velocity 65 m/s Investigated blade section: –NACA 0018, chord length c = m –4 slits (L=0.15 m, w=0.001 m), located at x/c=0.9 Investigated cases: –Re c =6.6x10 5, M ∞ =0.176 –U inj /U ∞ =1.2 –Five angles of attack between -12° and +12°

9 Experimental Setup (cont’d) Static pressure measurements at 4 locations: x/c = 0.042, x/c = (on both sides of airfoil) Injection of air at x/c = 0.9 –Compressed air (6.5 bar) fed to Piccolo tube in aft compartment –Choked flow in holes Piccolo tube (constant mass flow) –Injection velocity determined from static pressure measurement in aft compartment

10 Numerical Setup Commercial flow solver ANSYS CFX 11.0 Reynolds Averaged Navier-Stokes equations for compressible steady and unsteady flow Shear Stress Transport eddy viscosity turbulence model Spatial discretization: blend of 2 nd - and 1 st -order accurate schemes Time discretization (unsteady flow cases): 2 nd -order accurate C-type structured grids,466x89x3 cells, y + <1

11 Numerical Setup (cont’d) In experiments sharp inner edges of slit: –Vena Contracta will form inside slit –Hot Wire Anemometry measurements show effective slit width of 70% of actual slit width Jet modeled as boundary condition at airfoil surface: constant normal velocity of 1.2U ∞ and slit width of 0.7W Quasi 2D flow: full span slit

12 Results NACA 0018 Re c =6.6x10 5 U inj /U ∞ =0.0 or 1.2 Flow on lower surface separates c l increases (less negative) due to jet by ≈0.4 (comp)

13 Results (cont’d) NACA 0018 Re c =6.6x10 5 U inj /U ∞ =0.0 or 1.2 Flow on lower surface separates c l increases due to jet by ≈0.4 (comp)

14 Results (cont’d) NACA 0018 Re c =6.6x10 5 U inj /U ∞ =0.0 or 1.2 Flow on lower surface separates c l increases due to jet by ≈0.4 (comp)

15 Results (cont’d) Comparison of calculated pressure distributions with experimental data: –In general, experiments and computations show the same trend: increase of lift –Calculations show more pronounced effect of injection on pressure on side where slit is located, clearly visible for positive angles of attack Possible causes: 3-dimensionality of flow, or applied turbulence model does not accurately predict transition

16 Results (cont’d) Instantaneous streamlines superposed on contour plot of Mach number field for α=+8°, Re c =6.6x10 5 –Left: U inj /U ∞ =0; Right: U inj /U ∞ =1.2 At TE: flow tangential L.S. At TE: flow tangential to U.S. due to entrainment jet formation of recirculation zone

17 Results (cont’d) Predicted lift curves, NACA 0018 Re c =6.6x10 5 U inj /U ∞ =0.0 or 1.2 Δc l  0.4 independent of α dc l /dα not affected

18 Results (cont’d) Transient response of lift for α=+8°, Re c =6.6x10 5 Computation started from free-stream conditions, injection starts at tU ∞ /c=15 95% of Δc l obtained in ΔtU ∞ /c  4 50% of Δc l obtained in ΔtU ∞ /c  1

19 Conclusions Studied is load control for NACA 0018 airfoil for α  [-12°, +12°] Load control by continuous injection of air from slits: –slits located near trailing edge of lower side airfoil x/c=0.9 –jet directed normal to airfoil surface, U inj /U ∞ =O(1) –slit width O(0.01c) Computations and experiments show same trend: increase of lift –Computational results show that an increase of Δc l  0.4 can be obtained, half of this in a dimensionless time of ΔtU ∞ /c  1 –Computational results predict more pronounced effect of jet than experimental results Promising option for load control