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RESEARCH PAPERS

Potential Load Reduction Using Airfoils with Variable Trailing Edge Geometry

[+] Author and Article Information
Thomas Buhl

Wind Energy Department, Risø National Laboratory, P.O. Box 49-DK-4000 Roskilde, Denmarkthomas.buhl@risoe.dk

Mac Gaunaa

Wind Energy Department, Risø National Laboratory, P.O. Box 49-DK-4000 Roskilde, Denmarkmac.gaunaa@risoe.dk

Christian Bak

Wind Energy Department, Risø National Laboratory, P.O. Box 49-DK-4000 Roskilde, Denmarkchristian.bak@risoe.dk

J. Sol. Energy Eng 127(4), 503-516 (Jun 23, 2005) (14 pages) doi:10.1115/1.2037094 History: Received March 21, 2005; Revised June 23, 2005

This paper presents an investigation of the potential for reduction of fluctuating loads on wind turbine blades with the use of flaplike deflectable trailing edges. More specifically, the aeroelastic response of an elastically mounted airfoil section with a deflectable trailing edge is investigated. This is done by coupling a model for the aerodynamic forces on a deforming airfoil with a linear spring/damper model for the elastic deformation of a rigid airfoil to which the forces associated with the deflection of the trailing edge are added. The analysis showed that when the airfoil experienced a wind step from 10to12ms the standard deviation of the normal force could be reduced by up to 85% when the flap was controlled by the reading of the airfoil flapwise position and velocity, while reductions of up to 95% could be obtained when the flap was controlled by the reading of the angle of attack. When the airfoil experienced a turbulent wind field, the standard deviation of the normal force could be reduced by 81% for control based on measured angle of attack. The maximum reduction using a combination of flapwise position and velocity was 75%. The maximum deflection of the trailing edge geometry was, in all the considered cases, small enough to justify the use of a potential flow code for calculation of the aerodynamic forces. Calculations showed that the effect of a time lag in the actuators and sensors may drastically reduce the efficiency of the control algorithm. Likewise, the effect of a low maximum actuation velocity reduces the efficiency of the control algorithm. The analysis of the two-dimensional (2D) aeroservoelastic system shown in this paper indicates that the potential of using trailing edge flaps for reduction of fluctuating loads is significant.

Copyright © 2005 by American Society of Mechanical Engineers
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Figures

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Figure 1

The notation and direction for the profile

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Figure 2

Flowchart of the MATLAB code used in the following

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Figure 3

Step response approximations for the flat plate and the Risø B1-18 airfoil

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Figure 4

The unit-depth local mass used in the present work

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Figure 5

Inflow setup. The y=0 plane corresponds to the rotor plane. U corresponds to the rotational velocity, while V(t) corresponds to the free-stream velocity.

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Figure 6

Deflection corresponding to β=5deg

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Figure 7

Wind step response. Std(N) as a function of control parameter Ay.

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Figure 8

CN as a function of time. The solid black line is the response without any control (β is equal zero).

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Figure 9

CN as a function of time for different values of By

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Figure 10

Standard deviation of the normal force as a function of control value Aα

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Figure 11

Time series of CN for different values of Aα

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Figure 12

Standard deviation of the normal force as a function of time lag

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Figure 13

Standard deviation of the normal force as a function of actuator velocity

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Figure 14

Flap angle β as a function of time for different values of control parameter Aα

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Figure 15

Flap angle β as a function of time for different time lags

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Figure 16

Flap angle β as a function of time for different actuator velocities

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Figure 17

CN as a function of time for different combinations of control strategies

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Figure 18

Flap angle β as a function of time for different combinations of control strategies

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Figure 19

Time series of the y position of the airfoil for different control strategies

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Figure 20

Time series of the pitch angle θ

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Figure 21

Turbulent wind input response. Iso-curves of the reduction of Std(N) for the Aα control.

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Figure 22

Turbulent wind input response. Iso-curves of the reduction of Std(N) as a function of control parameter Ay and By.

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Figure 23

Turbulent wind input response. The reduction of Std(N) as a function of the time lag.

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Figure 24

Turbulent wind input response. Reduction of Std(N) as a function of maximum actuation velocity.

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Figure 25

Relative lift magnitude error, em (upper) and lift phase angle error, ep (lower) as functions of the reduced frequency, k, resulting from a constant chordwise upwash velocity in a sinusoidal gust

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