AIAA-2005-565-450Acoustic Testing of the dielectric barrier(9)

推进器的声波特性

As a comparison, if instead the force were a large push and a small pull, there would be a very small contribution from the second harmonic, as shown in figure 14. Since this is contrary to what we observe, we believe that the acoustic data suggest that a “PUSH-push” mechanism is operating in the plasma actuator. The positive-going stroke is considerably less effective at moving air than the negative-going stroke. This is consistent with previous data showing that the discharge is reasonably uniform on the positive going cycle while on the negative going cycle the discharge is more “patchy”13. The polar plots also support a model with a “push” in the direction of momentum coupling (higher acoustic emissions in the direction of the flow). The waveform data and its change with frequency and direction further support this concept of a PUSH-push model with a strong initial PUSH and then a smaller secondary push for each cycle. The data in Figure 10 for 5 KHz suggests a strong positive compression, followed by a rarefaction, then a second smaller compression followed by a stronger rarefaction. At 5 KHz we see little effect of direction on this waveform (see Figure 11). However, at 8 KHz, not only is the waveform changed from the 5 KHz case (Figure 12), but there is also a directional effect in the waveform as well. The Figure 14: Force versus time for a PUSH-pull situation, with a relatively waveform viewed from the direction in small contribution by the “back stroke.” Again, the frequency spectrum is which the flow accelerates now shows also shown. a much smaller second compression followed by larger rarefaction. This does not appear to occur in the acoustic emissions observed upstream of the actuator for the 8 KHz case. This suggests the possibility that at the higher frequency the first compression does not have time to move significantly out of the way before the second compression occurs. If so, the compressible nature of the air is a factor in the momentum coupling.V.ConclusionThe directional and spectral acoustic emissions from a plasma actuator have been measured using two different experimental setups, microphones and procedures. The results from these measurements provide insight into the nature of the momentum coupling of the actuator and the air. It appears from these results that the dielectric barrier discharge plasma actuator is interacting with the air by adding a larger push (compression and acceleration) followed by a second but smaller push (“PUSH-push”) during each cycle. It also appears that compressibility effects of the air during the acceleration may be an important part of the momentum coupling process.AcknowledgmentsThis work was sponsored by the Air Force Office of Scientific Research. Dr. John Schmisseur, program manager, under project order plasma theme funding. This support is gratefully acknowledged. The authors also gratefully acknowledge the assistance of Cadets First Class Timothy Sutphen and Joshua Olson who conducted the measurements with the second apparatus and performed data reduction as part of their AE471 laboratory project under the direction of the authors.9 American Institute of Aeronautics and Astronautics