Objective
A method has been developed for measuring the airload distribution on blades by means of simple strain gauge instrumentation. Deflection and twist angles can theoretically be calculated by a double integration of strains. Apart from requiring local blade characteristics, this needs a large number of measuring stations and even then the results obtained are unreliable because the smallest experimental errors in strain can cause large errors in the integrated result. A quite different and much simpler method has been developed.
Blade deflections under load can now be measured reliably with strain gauges through a strain pattern identification procedure. Reliable results depend to a large extent on placing gauges through a strain pattern identification procedure. Reliable results depend to a large extent on placing gauges so that no 2 model strain patterns resemble each other. A high degree of precision in strain measurement is also recommended. The number of good strain gauge measurements necessary is less than double the number of modes considered and for wind turbine blades a total of 4 to 6 modes should suffice.
Load distribution measurements based on deflections are theoretically valid but for similar accuracy a larger number of modes may need to be considered than in the case of deflection measurements. Moreover, measurements at higher mode orders tend to get less accurate as the measured strain levels become very small. Tests on helicopter rotors in a wind tunnel have shown that reasonably good results can be obtained for spanwise loads.
In the case of teetered or hinged rotors the strain gauge measurements must be completed by the blade root angular deflection and this angle must be corrected to give the rigid body motion which does not appear in the gauge measurements.
The deflections and loads obtained give an immediate assessment of rotor behaviour in real conditions thus guiding designers and manufacturers during the development of turbines.
The main objective of the research was to determine if the spanwise wake measurements can give the spanwise load distribution. An experimental setup enabling the measurements of the airfoil characteristics at 3 spanwise stations on the full scale horizontal axis wind turbine (HAWT) was developed. This was done in order to identify the main mechanisms controlling the aerodynamic forces on a rotating HAWT blade in natural turbulent conditions. In particular the measurements were aimed at quantifying the importance of 3-dimensional flow effects, unsteady effects and rotary wing effects.
The measurements of the near wake have been performed by 3 anemometers, a wind vane and a fast response 5-hole pitot tube. All the sensors are mounted on one of the main poles of the turbine tower. The distance from the velocity sensors to the rotor disc depends on the yaw position of the turbine. For the wind directions used in the present data analysis the distance is typically about 1 chord length.
The model used to obtain the theoretical coupling between the rotor load distribution and the near wake deficit is an integrated rotor/wake model. It is a 2-dimensional model which uses the actuator disc concept. The governing equations are the Euler equations plus the main turbulent stress tern in the wake derived on the basis of the eddy viscosity concept. The input to the model is the rotor loading which is computed using blade element theory.
Computed and measured wake deficits at 4 radial stations and at wind speeds of 8, 12 and 15 ms{-1} have been compared. For the rotor in the stall region and poststall region it appears that the computed wake close to the blade root coincides best with experiment when using the 3-dimensional data as input. On the other hand, the deloading at the tip, which is also typical for the 3-dimensional airfoil data, resulting in less deceleration of the flow cannot be confirmed by measurement.
THE MAJOR CAUSE OF WIND TURBINE FAILURE IS FATIGUE. THE DIFFICULTY IN PREDICTING FATIGUE IS IN LARGE PART DUE TO AN INSUFFICIENT KNOWLEDGE OF THE DYNAMIC INPUT THROUGH THE ROTOR. THERE IS A NEED FOR A BETTER UNDERSTANDING OF REAL BLADE LOADING AND THIS CALLS FOR AN EXPERIMENTAL TECHNIQUE.
TWO THEORETICALLY SOUND METHODS BASED ON STRAIN GAUGE MEASUREMENTS WERE DEVELOPED IN TURN, BUT BOTH PROVED TO BE FAR TOO SENSITIVE TO THE SLIGHTEST SCATTER IN THE MEASUREMENTS FOR THEM TO HAVE ANY PRACTICAL APPLICATIONS.
A MORE ROBUST METHOD THAT ALSO USES STRAIN MEASUREMENTS HAS NOW BEEN DEVELOPED. NUMERICAL SIMULATIONS SHOW THE DIFFICULTIES AND LIMITATIONS OF THIS TECHNIQUE. LABORATORY TESTS ON A LOADED BEAM HAVE BEGUN AND RESULTS TEND TO CONFIRM THE NUMERICAL SIMULATIONS. MEASUREMENTS OF AERODYNAMIC LOADS ON A WIND TURBINE ARE BEING PREPARED.
THE MAJOR DIFFICULTIES THAT WERE FOUND TO BE INHERENT IN THE MEASURING OF BLADE LOADS THROUGH LOCAL STRAINS HAVE SERIOUSLY CURTAILED THE SCOPE OF THIS RESEARCH PROJECT.
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92322 Châtillon
France