Page 12 - Volume 9, Issue 3

Fig. 5. Left: Waveforms from an F-22A Raptor ground run-up measured at the specified distances along the 55° radial. Right: Measured and predicted spectra at a distance of 305 m. See Ref. 29 for additional details.
ing its properties. Azimuthal modal decompositions21,22 of numerical and experimental data have revealed relatively axisymmetric radiation at low frequencies, with the signifi- cance of higher-order helical modes increasing as a function of frequency and toward the sideline direction. These modes can be described in terms of wave packets, wave-like func- tions that mimic the growth and decay of turbulence insta- bilities with downstream distance (see Figure 4), and whose coherence extends over multiple characteristic wavelengths. Jordan and Colonius23 have recently reviewed the use of wave packets to characterize LSS radiation from jets; wave packets’ tie to jet noise reduction efforts are described near the con- clusion of this article.
A characteristic of high-power military jet noise that is particularly important to human perception is the presence of acoustic shocks that produce a highly audible irregular popping sound in the waveform. Historically, Ffowcs Williams et al.24 described jet “crackle” as being due to sharp shock-like compressive features in the waveform. Although they believed the shocks were radiated from the jet directly, shocks may also form and steepen as high-amplitude noise propagates nonlinearly. Nonlinear acoustic wave propaga- tion25,26 in air results when the source characteristics (ampli- tude, frequency, spatial extent) are such that an amplitude- dependent sound speed occurs, resulting in an alteration of waveform shape and possible shock formation. Recent stud- ies27-29 on the nonlinear propagation on noise from military jets have shown that the evolution of these shocks has a sig- nificant impact on the high-frequency portion of the spec- trum. Nonlinear effects in jet noise have also been observed in supersonic laboratory30,31 scale jets.
As an example28 of acoustic shocks in jet noise propaga- tion, Figure 5 displays measured waveforms from a static, afterburning F-22A Raptor at locations between 23 and 305 m along the maximum radiation angle 55° from the down- stream jet axis. Highlighted is the steepening of acoustic shocks around 3.21 and 3.23 s over the propagation range. Also shown in Figure 5 is the measured one-third octave band spectrum at 305 m, along with numerical predictions
based on propagating 23 m measured data to 305 m using both linear and nonlinear propagation models. Note the impact of the shocks on the measured and nonlinearly pre- dicted high-frequency spectra, where the levels at 20 kHz are about 80 dB at a distance of 305 m (1000 ft) from the aircraft! Although the increase in high-frequency energy does not sig- nificantly impact level-based loudness metrics,32,33 the pres- ence of shocks affects perception34 in the near and far fields, making their study important. These effects are the subject of ongoing work, some of which is summarized in this article.
The remainder of this article describes static engine run- up measurements of the F-22A Raptor and F-35AA-1 Lightning II Joint Strike Fighter and the results of recent data analyses. Because important insights about jet-noise source and radiation characteristics can be obtained from near-field measurements, these analyses help to demonstrate how the body of knowledge regarding supersonic jet aeroacoustics – much of which has been gained using laboratory-scale jets and numerical simulations – applies to actual military jet air- craft noise. Also included are reports on concurrent ONR and NASA-sponsored10 efforts that target characterization or reduction of the noise generation.
Recent Military Jet Noise Investigations
Measurements
Near-field measurements35 of military jet aircraft noise are challenging. High levels (peak levels exceeding 170 dB) and large signal bandwidth (from 10 Hz to more than 20 kHz) require low-sensitivity 6.35 or 3.18 mm Type 1 micro- phones with appropriate peak-handling capability for the microphone, preamplifier, cables, and the data acquisition system. Furthermore, sampling rates of at least ~100 kHz are required to capture shock-like features of the waveform, and excessive vibration of the data acquisition system must be avoided. The combination of instrumentation demands and harsh measurement environment makes extensive datasets of military jet noise relatively rare.
The F-35AA-1 static run-up measurements were con- ducted in 2008 at Edwards Air Force Base (AFB) by a Joint
Jet Noise from Military Aircraft 11