AIAA-2005-565-450Acoustic Testing of the dielectric barrier(2)

推进器的声波特性

The DBD plasma actuator consists of two thin electrodes placed asymmetrically on an aerodynamic surface. The upper electrode is exposed to the air while the lower electrode is encapsulated by a dielectric material and a substrate material as shown in Figure 1. A high AC voltage (5-10 kV amplitude, with frequency in the range of 1-10 kHz) is applied to the electrodes. The plasma then forms in the region above the insulated electrode. The appearance of the plasma is accompanied by a coupling of directed momentum into the surrounding air.Figure 1: Diagram of the plasma actuator configurationValue of acoustic testing The plasma when energized emits a purple glow that is accompanied by a very distinct tone. It is evident that the acoustic tone changes based on orientation, forcing voltage, and forcing frequency. This change in the emitted sound with various parameters led to the thought that the acoustic waves may provide an indication of the interaction between the actuator and the air. As Enloe et. al.13-14 points out, there are two major contenders for momentum coupling mechanisms for the plasma: either the air is given an amount of momentum on one half of the forcing cycle and a similar or smaller amount in the same direction on the other half; or there is a larger momentum increase followed by a smaller decrease of momentum over the course of a cycle. We refer to these as the PUSHpush or the PUSH-pull options. The former mechanism would be associated by a net force on the bulk plasma, while the latter would tend to be the result of the current carriers in the gap interacting with the air. The authors believed that the plasma actuator’s acoustic emissions had the potential of sorting out these two possibilities, and hence contributing to understanding the underlying mechanism.II.Experimental Set-UpA. Overview of set-up Two sets of acoustic experiments were completed at the US Air Force Academy each using a different apparatus. The first set provided directional characteristics and a fast Fourier transform of the average waveform. The second set was designed to address concerns about possible reflections during the first set, the frequency response of the microphone used, and to determine and examine the acoustic waveform in more detail. The first set of experiments was completed in a sound-absorbing structure, designed to suppress reflections that would interfere with the recording of the actuator’s acoustic emissions. Figure 2 shows the configuration of the booth used. The plasma actuator with its accompanying electrical equipment was placed on a stand on the bottom of the booth. The acoustic sensor was suspended from wires approximately one meter above the actuator. The sensor’s output was fed through an oscilloscope which is connected to a computer-based data acquisition system (DAQ). 2 American Institute of Aeronautics and Astronautics