3 Operational Testing
With the modal frequencies and vectors determined, the next step was to use that information to gain insight into the structural response of the turbine in operation. This study used an in-house wind dynamics test bed to gather baseline data in a controlled airflow environment prior to future testing of the turbine in real rooftop operating conditions. Separate tests were conducted for a range of wind speeds from 3.4 to 5.4 m/s(7.6–12.1 miles per hour), which starts near the idle speed of the turbine and increases into the power-generating region. Additionally, four separate wind shear conditions were tested to explore the effect of wind shear on operational response: (1) a uniform flow with no wind shear; (2) a vertically sheared flow like that present in the atmospheric boundary layer or seen in the profile of wind flow over a building; and (3, 4) left and right side-shear, whichmay be due to obstructions, wind flow around the sides of a building, or upstream wind turbines.
3.1 Experimental Setup
The experimental test bed consists of an enclosed test section which generates a simulated wind flow using four large axial exhaust fans to pull air through a honeycomb, laminarizing core (see Fig.29.7below). Accelerometers were placed in the same locations on the rotor as during the first modal test (Sect.29.2.1); however, unlike the modal impact tests, DC-coupled sensors were used to better capture the low frequency, per-revolution harmonics. A data acquisition device and wireless transmitter were statically balanced and fastened to the base of the rotor shaft. Because of the added mass and the change in boundary conditions inside the test bed, another modal impact test was conducted to obtain the modal parameters. Wind shear was generated by placing window screening material on the inlet of the test bed. The resulting inlet wind plane was measured using an ultrasonic anemometer at 100 measurement points, 2 ft inward from the honeycomb core face,and the results were interpolated to generate the images in Fig.29.8. Leading edge shear (hereafter denoted LS) indicates that the wind was slowed on the side of the leading, advancing edge of the blade. Trailing edge shear (TS) indicates the wind was slowed on the trailing, retreating edge of the blade.
3.2 Operational Modal Analysis
The wind velocity fields were experimentally measured, but because the force input on the turbine is unknown, operational modal analysis (OMA) was applied to relate the operational response of the turbine to the free response dynamics measured in the experimental modal analysis. The two primary assumptions in OMA are: (1) the power spectrum of the input force is broadband and smooth (i.e. has no poles or zeroes in the frequency range of interest), and (2) the forcing function is spatially distributed in a uniform manner [5]. Assumption (1) is not particularly well-fulfilled for wind-excited structures since the power spectrum of wind is generally dominated by low-frequency components [6]. In general, however, rotating machinery is self-excited at harmonics of the operating speed [7], and this was evident in the OMA frequency response functions measured on the VAWT. Assumption (2) is reasonable for the rotating wind turbine, and the measured wind planes in Fig.29.8demonstrate the spatial uniformity.本文来自优/文(论"文\网，
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The data processing involved time-synchronously averaging the accelerometer responses, then performing OMA by first computing the autocorrelation of the time response, which is the time-domain equivalent of the auto power spectrum. The result is two-sided, containing a positive exponential portion corresponding to the negative poles of the power spectrum, and a decaying exponential portion corresponding to the positive poles [5]. Since both parts of the result contain the same information, the positive exponential part is set to zero, essentially zero-padding the time signal. The resulting function is treated as an impulse response function, the discrete Fourier transform of which is the OMA FRF. Figure29.9schematically describes the process.
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