1 fluidic load control for wind turbine blades c.s. boeije, h. de vries, i. cleine, e. van emden,...
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![Page 1: 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](https://reader036.vdocument.in/reader036/viewer/2022062407/56649f475503460f94c69995/html5/thumbnails/1.jpg)
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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