AIAA-2005-565-450Acoustic Testing of the dielectric barrier(5)

推进器的声波特性

1.00E+00 1.00E-01 1.00E-02 A m p litu d e 1.00E-03 1.00E-04 1.00E-05 1.00E-06 1.00E-07 1.00E-08 1.00E-09 0 5000 10000 15000 20000 25000 30000 Frequency (Hz)1.00E+00 1.00E-01 1.00E-02 A m p litu d e 1.00E-03 1.00E-04 1.00E-05 1.00E-06 1.00E-07 1.00E-08 1.00E-09 0 5000 10000 15000 20000 25000 30000 Frequency (Hz)(a) (b) Figure 6: Frequency Spectrum in a sound booth (a) with sound absorbing material removed, (b) with material replaced (sinusoidal forcing voltage of 15 kV, at 5 kHz 45 degrees) Since the plot shown in figure 6b is quite similar to that of the free field, the booth appears to be successful in minimizing reflections as it was designed to do. Therefore, the results from tests taken in the booth are considered an accurate measurement for the direct acoustic emissions from the actuator. The next set of measurements with this first apparatus compared results using different forcing voltages. A sinusoidal forcing voltage at a frequency of 5 kHz was used to drive the 30x8 cm actuator with the glass dielectric material. Data was taken for three forcing voltages, Vppf, of 5, 7, and 10 kV. The measured microphone peak-topeak voltage, Vppm, is plotted versus the angle on a polar plot (see Figure 4) to determine the directional characteristics of the acoustic emissions from the actuator with regard to orientation along its longitudinal axis. Figure 7 shows a polar plot of the peak-to-peak microphone voltage as a function of the orientation angle. The angle at which the maximum peak-to-peak voltage occurs did not change as the voltage was increased. The amplitude difference between the three forcing voltages is large and apparently nonlinear. However, normalized plots (not shown) indicate that the pattern is fairly consistent with a large frontal lobe at approximately 40 degrees above horizontal in the direction of the flow. At the lowest voltage there appears to be a small back lobe.Figure 7: Polar plot of Vmpp, comparing acoustic emissions for different forcing voltages (Vfpp) at 5 kHz In another set of measurements with this first apparatus the size of the actuator surface was changed while maintaining the sinusoidal forcing voltage at 7 kV with a frequency of 5 kHz. Three actuator sizes were used; the original 30x8 cm, and two additional actuators (28x15 cm and 30x4 cm). The polar plot of these results, figure 8, shows the difference in the measured amplitudes of the acoustic emissions with angle. Although the directional pattern of the acoustic emissions does not appear to change much with the actuator size, the magnitude does. Interestingly enough, the smallest actuator (30x4 cm) falls between the other two actuators. 5 American Institute of Aeronautics and Astronautics