AIAA-2005-565-450Acoustic Testing of the dielectric barrier(8)

推进器的声波特性

Figure 12: Microphone output waveform for different angles for 8 kHz at 5 kV.IV.Analysis and DiscussionThese acoustical emission spectral measurements (FFT’s), polar plots and waveforms from a plasma actuator as presented in the previous section can provide insight into how the plasma actuator is coupling momentum into the surrounding air. The FFT’s showed two roughly equal peaks, one at the driving frequency and one at twice the driving frequency. Some basic modeling gives insight into the interpretation of this Fourier decomposition of the acoustic emissions. Two assumptions made in this modeling are: 1) the Fourier components of the air acceleration are preserved in the Fourier components of the sound, and 2) that the net force used to obtain the time varying response can be roughly averaged over time. Based on these assumptions, we assumed various inputs to the model of such a system and compared the spectral content of the response in this model to that observed experimentally. Applying a square wave-like input function for force vs. time is a reasonable choice based on our previous observations13,14. If the air is being given a large push and a small push, the second harmonic would be expected to have a strong presence in the acoustic emissions. Assuming the negativegoing half of the cycle is not as effective in producing the force (and therefore acoustic signal) as the positive-going half (a reasonable assumption based on previous results13,14) ; the time history and resulting FFT plot shown in figure 13 is obtained. This matches our measured FFT (Figure 5) supporting the idea that the plasma actuator is in fact interacting with the air by giving it a big “push” and then a second but smaller “push”. 8 American Institute of Aeronautics and Astronautics