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.

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Fluidic Load Control for Wind Turbine BladesC.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

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Overview

• Introduction

• Experimental Setup

• Numerical Setup

• Results

• Conclusions

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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

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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

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Introduction (cont’d)

• Concepts for fast control:– Conventional trailing edge flaps

• Chow et al., Journal of Physics:Conference Series 75 (2007) 012027

– Flexible trailing edge flaps• Barlas & Van Kuik, Journal of Physics:

Conference Series 75 (2007) 012080

– MEM tabs• Chow & Van Dam, J. of Aircraft, Vol. 43,

No. 5, 2006, pp. 1458-1469

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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

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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

– Uinj/U∞=O(1)

• Important issues:– Is continuous injection from long slits located near trailing edge

effective?– Are jet velocities of Uinj/U∞=O(1) sufficient?– How fast can aerodynamic performance be changed, i.e. what is

response time?

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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 = 0.165 m

– 4 slits (L=0.15 m, w=0.001 m),located at x/c=0.9

• Investigated cases:– Rec=6.6x105, M∞=0.176

– Uinj/U∞=1.2

– Five angles of attack between -12° and +12°

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Experimental Setup (cont’d)

• Static pressure measurements at 4 locations: x/c = 0.042, x/c = 0.221 (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

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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 2nd- and 1st-order accurate

schemes• Time discretization (unsteady flow cases): 2nd-order accurate• C-type structured grids,466x89x3 cells, y+<1

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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

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Results

NACA 0018 Rec=6.6x105 Uinj/U∞=0.0 or 1.2

• Flow on lower surface separates

• cl increases (less negative) due to jet by ≈0.4 (comp)

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Results (cont’d)

NACA 0018 Rec=6.6x105 Uinj/U∞=0.0 or 1.2

• Flow on lower surface separates

• cl increases due to jet by ≈0.4 (comp)

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Results (cont’d)

NACA 0018 Rec=6.6x105 Uinj/U∞=0.0 or 1.2

• Flow on lower surface separates

• cl increases due to jet by ≈0.4 (comp)

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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

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Results (cont’d)

• Instantaneous streamlines superposed on contour plot of Mach number field for α=+8°, Rec=6.6x105

– Left: Uinj/U∞=0; Right: Uinj/U∞=1.2

At TE: flow tangential L.S. At TE: flow tangential to U.S. due to entrainment jet formation of recirculation zone

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Results (cont’d)

• Predicted lift curves, NACA 0018 Rec=6.6x105 Uinj/U∞=0.0 or 1.2

• Δcl0.4 independent of α • dcl/dα not affected

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Results (cont’d)

• Transient response of lift for α=+8°, Rec=6.6x105

• Computation started from free-stream conditions, injection starts at tU∞/c=15

• 95% of Δcl obtained in ΔtU∞/c4

• 50% of Δcl obtained in ΔtU∞/c1

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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, Uinj/U∞=O(1)– slit width O(0.01c)

• Computations and experiments show same trend: increase of lift – Computational results show that an increase of Δcl0.4 can be

obtained, half of this in a dimensionless time of ΔtU∞/c1– Computational results predict more pronounced effect of jet than

experimental results

• Promising option for load control