1. The first tangible example of an active suppression technique capable of simultaneously suppressing multiple acoustic modes (not due to excessive leading edge mass injection) was presented by McGrath and Shaw in 1997 {Ref. 2]. Figure 1 from Ref. 2 shows an acoustic spectra from a Mach .6 cavity case with no suppression invoked. Figure 2 shows the same case with a type of high frequency suppression device in use (a rod in crossflow). We will define what we mean by high frequency suppression in the next section. The technique yielded dramatic results, suppressing all tones simultaneously, and giving a tone reduction across all frequencies of over 30 dB! The same cavity was tested at Mach .8, and the results .were good, if not quite as dramatic. Attempts to duplicate this impressive performance were made in a second wind tunnel test, but unfortunately the results were not as promising. and the technique w;ls aba.ndoned for mofe consistent avenues of research [Ref. 8].
2. In 1997, Alan Cain [Ref. 9] first proposed the notion that very high frequency (in the vicinity of the so-called Kolmogorov frequency) forcing of the shear layer spanning the weapons bay could be very effective at suppressing resonant cavity tones. based upon the work of Wiltse and Glezer [ Ref. 10] in free shear layers. Cain suggested that the likely cause of the suppression in the McGrath and Shaw rod-in-crossl 1ow experiments was the same mechanism studied in detail and described by Wiltse and Glezcr.
3. Akey experiment which giVes hints at what is happening in cases of HF suppression was conducted by Wiltse and Glezer [Ref. 10] at Georgia Tech. In this experiment, a piezoceramic driven wedge (this device is shown in Figure 17 and will be further described later in the text) was used to excite the shear layer at the edge of a small rectangular jet. Figure 6 shows a velocity spectra derived from hot wire data taken at a station along thejet centerline. The gray line is the clean, unforced jet flow velocity spectra. The blaCk line shows the manipulated spectra, with a dominant HF (5 kHz) forcing tone, and a harmonic tone visible. This plot is very interesting for several reasons. First, we can see that forcing at high frequency has an effect on the spectra at all frequencies: This result was, until recently (See Ref. 26, 1995), not what was normally expected. Earlier work on the receptivity of free shear flows to external excitation (See Monkewitz & ·Huerrc (1982). Ref. 25). suggested that free shear flows are not receptive to low•level excitation at frequencies above a relatively narrow band. centered around the natural (critical or most unstable) frequency. Another important "corollary" of this prior dominant view was that the effect was also only felt in the same relatively narrow spectral range. This experimental data of Wiltse and Glezcr obviously docs not support the previous dominant view.