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Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'Aerodynamics and rotor design' taking place on Wednesday, 12 March 2014 at 09:00-10:30. The meet-the-authors will take place in the poster area.

Nadia Najafi Technical University of Denmark, Denmark
Co-authors:
Nadia Najafi (1) F P Uwe Schmidt Paulsen (1) Federico Belloni (2) Gabriele Bedon (2) Jakob Mann (1)
(1) Technical University of Denmark, Roskilde, Denmark (2) Università di Padova Via Venezia, Padova, Italy

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Abstract

Dynamic behavior studies of a vertical axis wind turbine blade using operational modal analysis (OMA) and experimental modal analysis (EMA)

Introduction

Wind energy is receiving more attention in news media. Application of vertical-axis wind turbines(VAWTs)in urban environment has triggered new ideas to approach, and novel design have been realized by companies like Venco using helical shaped rotor blades. Compared with HAWTs, VAWTs can operate independently from wind direction changes and can endure temporal changes in the vertical wind speed in pitch and roll[1]. Differences exist on performance between the two concepts, in particular the structural dynamics are more complex due to the flow passing through the rotor. In structural measurements the structural dynamics of VAWTs have been started here

Approach

In this study the modal analysis is conducted with two different methods and the results are compared to validate OMA results. At first, traditional modal analysis (EMA) was used. In this method one axis accelerometers are mounted at different points on the centroid line of the blade structure. The measurement are set up in PULSE LabShop software (product of Brüel and Kjær Company), in each measurement set one point is hammered and acceleration responses are recorded in three different points, this process goes on until it covers all the points degrees of freedom. Finally the FRF is obtained for all of the points, so natural frequencies and mode shapes could be found by peak picking or other methods like Singular Value Decomposition (SVD).

Operational Modal Analysis (OMA) is the second approach used in this project in parallel with stereo vision technique. In this method the output is just required to be measured and correlation functions is considered instead of response function, actually the input is random and unknown. In this experiment some markers are fixed on the blade centroid line (the same place as the accelerometer positions). In fact the 3-D deflections of the points are monitored in time as the output, using stereo vision so the integration is not required for transforming acceleration to deflection in mode shapes identification because we will get deflection directly in this method. Two identical cameras take pictures of the blade markers while it is excited by wind forces, the cameras are programmed in LabView to take pictures at the same time with 108 fps and store them on the high speed hard disk. The correlation function between deflections of points will be analyzed to identify natural frequencies then the displacement of the markers in these natural frequencies is the blade’s mode shapes.


Main body of abstract

Dynamic behavior of a modified blade fitted onto a Venco vertical axis wind turbine is studied by two different approaches. Classical modal analysis (EMA) is carried out to validate the results of Operational Modal Analysis (OMA). Through this project, stereometric techniques are going to be used to measure 3-D deflections and characterize the dynamics in OMA test.

Traditional modal Analysis has some limitation, for instance it needs artificial excitation of the structure to measure FRF which is too difficult for large structures. In addition it is mostly conducted in the laboratory so it is difficult to use in industrial field tests. The Operational Modal Analysis input is not measured, so the structure is not shaken and it will be excited by distributed and uncorrelated wind forces [2].

Stereo vision has been used for measurements in wind turbines at Risø Campus of DTU. Paulsen et al., in their recent studies on wind turbines, have indicated that full-field optical techniques, particularly stereo photogrammetry and videogrammetry systems, have some intrinsic features and capabilities that are extremely advantageous for the present challenge of measuring the operational deflection shapes of huge rotating objects [3].
In the classical modal analysis, the structural properties are obtained by hammering the structure at a certain point in each measurement set and then measuring the acceleration response in other points. The Frequency Response Function (FRF) is investigated to identify natural frequencies and mode shapes. In this part a turbine blade is put on two supports axisymmetrically and measurements are conducted at the point on the blade centroid line. However this method has some problems which most of them originate from hammering the structure at a certain point, for instance in the current case which is a turbine helical shaped blade, recorded acceleration responses at the points in the same half with the hammering point are so affected by hammer excitation. In fact the hammer impact are added to the response so theses recorded response signals are higher than the other half while they should be approximately the same because the geometry and boundary conditions are symmetric. So sometimes just one other half of the blade with the hammering half is considered.

In OMA test, the plate is set outside in the field with the same boundary condition and supports as the EMA test, some markers are put at different places of the turbine blade centroid line and their 3-D deflections are recorded with a stereo vision system: i.e. two identical cameras which are looking at the blade while it is excited by the wind and other environmental forces. In a post process analysis, the mode shapes will be extracted from the deflection values without knowing the input. After calibrating the cameras the points deflection (also in the depth direction) is monitored in time by going in to all the pictures taken with a speed of 180 frames per seconds. By knowing deflections, the correlation function is obtained and the mode shapes and natural frequencies can be drawn by different methods like EMA method. Finally these two results are compared together to approve OMA’s capability in studying large structures.

Conclusion

Vertical Axis Wind Turbines (VAWT) is under re-newed interest for their potential use and wind direction insensibility of turbulent wind [1] at urban sites, in comparison with the Horizontal Axis Wind Turbines (HAWT). In the current study, the dynamic behavior of a modified VAWT blade intended to be mounted on a modified blade fitted onto a Venco turbine is investigated structurally by two different approaches: Traditional and Operational Modal Analysis (EMA and OMA). In EMA both input and output are used to identify the modal parameters but In OMA the structure is not shaken. On the other hand the input is random and distributed and the output is used for estimating the modal parameters [2].
For this study in the EMA test, the acceleration responses at different points on the blade’s centroid line are recorded by Pulse Labshop software while the blade is excited by the hammer impact. Afterwards, FRF has been studied to find modal parameters of the blade. There are some known difficulties in the EMA test, such as that the hammer impact affects the recorded response signals. In conclusion OMA has been proposed to avoid these kinds of difficulties. In the current OMA test the blade is put outside, excited by high speed wind and other environmental forces, then the deflections of a few points on the blade centroid line is monitored using stereo vision technique. In this technique two cameras are looking at the moving points in time, so by intersecting two sight lines 3-D coordinates are determined. The point deflection is used as the output for OMA and will be analyzed to estimate modal shapes and natural frequencies. Results of these two different approaches are compared finally.


Learning objectives
OMA solves the structural hammering problems when exciting large structures, so it is convenient for large structures. Additionally stereo vision avoids putting lots of transducers; however it increases the numbers of measured output. Acceleration transformation to displacement for finding modal shapes is not required in stereovision because the displacement is obtained directly. OMA and EMA will be done for whole turbine structure and their results are compared.


References
[1] Larsen, T. j., Madsen, H.A. (2013) On the Way to Reliable Aeroelastic Load Simulation on Vawt’s. Proceedings from EWEA conference in Vienna.
[2] Zhang, L., Brincker, R., Andersen, P. (2005) An Overview of Operational Modal Analysis: Major Development and Issues, Proceedings of the 1st International Operational Modal Analysis Conference, April 26-27, 2005, Copenhagen, Denmark.
[3] Paulsen, U. S., Schmidt, T, & Erne, O. (2010). Developments in Large Wind Turbine Modal Analysis Using Point Tracking Videogrammetry. Proceedings of the 28th IMAC, Conference on Structural Dynamics.