29.2.1 VAWT Modal Impact Test No. 1
Experimental modal analysis was conducted to classify the free response characteristics of the turbine system. The first modal impact test on the wind turbine was conducted on a small test stand bolted to the concrete floor. The test stand was constructed from 1/8’’thick drawn-over-mandrel steel tubing, and had an overall height of 31’’, which provided a rigid base on which to test the modes of vibration of the turbine. A FARO Arm®three dimensional coordinate measurement machine was used to consistently place the impact locations on each blade as well as to record their locations in space and the surface.
Normal vectors for accurate mode shape animation. Four points were spaced 5 in. apart from the leading edge to the trailing edge of each blade on the low-pressure, outer surface of the blade, in 18 planes separated by 4 in vertically, for a total of 72 impact points per blade. Each of the six struts supporting the blades had 17 impact points along the top edges, and there were 12 points on the rotor shaft:6 evenly spaced along the axis of the shaft, and another six rotated 120°from the first group. One triaxial, AC-coupled accelerometer was placed on the top, trailing edge of each blade using adhesive, and one was placed near the top of the rotor shaft. Figure29.1shows impact locations and accelerometer placement.
2.2 Modal Impact Test No. 2: VAWT on Telescopic Roof Fixture
VAWTs installed on or near buildings are subjected to a much different wind environment than in open areas, high off the ground where wind turbines are usually located. Wind flow accelerates over and around buildings, which can be advantageous in terms of energy capture, but also subjects the turbines to a high level of wind shear and turbulence.Figure29.2demonstrates this effect in a velocity vector field around a rectangular building using a k-epsilon turbulence computational fluid dynamics model [4], as well as a preliminary CFD model of the lab building on which the turbine will be installed.A telescopic roof fixture was designed and fabricated to facilitate the observation of different intensities of wind shear over the building, as the wind field varies with height above the roofline. The fixture is constructed from two sleeved, drawn-over-mandrel steel tubes separated by Delron plastic bushings. A hand-winch and pulley system is used to raise and lower the inner tube, and 3/4’’ through-holes in the outer tube allow a locking pin to be inserted through the inner tube in 6’’ increments. The total vertical travel is limited to 5.5 ft, or about one turbine height. Figure29.3shows the roof fixture with the turbine installed in the lab, the pulley mechanism, and a dimensioned drawing of how the turbine will be installed on the lab roof in future operational testing.
2.3 Modal Analysis Results
The first modal test (Sect.29.2.1) was conducted with high spatial resolution: 330 total impact locations. After observing the mode shapes, subsequent tests were completed with fewer inputs. The complex mode indicator function (CMIF) was used to determine the modal frequencies from the experimentally measured frequency response functions 本文来自优/文(论"文\网，
毕业论文 www.youerw.com 加7位QQ324~9114找原文(FRFs), and the modal vectors were extracted from the left singular vectors corresponding to the peaks in the CMIF. Some of the resulting mode shapes are represented in Fig.29.4and are described in Table29.1. For clarity in the images, the blades are not displayed, but in general they moved in a motion similar to that of their supporting struts.For the testing conducted on the telescopic mast (Sect.29.2.2), the complex mode indicator function was used to observe changes in the modal response among the three extended heights for one azimuth position. The bandwidth of interest in the tests was 0–100 Hz, and many of the resonant frequencies in this range exhibited a significant decrease in modal frequency as the height of the tower increased. This effect was especially evident in the 3.8Hz mode indicated in Table29.2, which was predominantly a mode of vibration of the tower. The mode shape of the turbine around 18 Hz appeared to be coupled to the tower motion, and also exhibited a significant shift down in frequency with increased tower height, as is expected due to the reduced stiffness. There were also several modes of vibration that involved little tower response, such as those near 30.6 Hz and 100.3 Hz, and these frequencies had a much smaller change in relation to the height of the tower. The effects are evidenced in the plot of the top line of the CMIF for each of the tower heights at one azimuth position in Fig.29.5. The results of the tests indicated that there was little change in modal response with respect to azimuth position. This may be due to the excitation being applied to the tower rather than to the blades and struts directly, which is likely less effective in exciting the blades and struts in the prescribed direction as compared to a direct impact. Figure29.6displays the top line of the CMIF for each of the azimuth angles on a single plot that reveals the similarity in the results. Notably, the 3.8 Hz tower mode had nearly the identical result in the CMIF for each azimuth angle, verifying that that particular resonant frequency is largely dependent on the tower height and not the rotor azimuth position. There were some subtle changes in rotor modes,such as a resonance around 138 Hz, which was present for azimuth positions 60 through 120°, but not 0, 20, and 40°.The mode shape involved the second bending mode of the struts attached to the two blades closest to the impact location when rotated from 60 through 120°, which may not have been well-excited in the first three blade orientations.
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