Page 16 - Volume 9, Issue 3

Fig. 12. NAH reconstruction in the vicinity of the F-22A for military power for (a) 125 Hz and (b) 500 Hz. Levels are shown on a half conical surface at the approximate location of the shear layer edge, and over a plane at y = 1.9 m, the height of the centerline of the jet.
Fig. 13. LES simulations of a highly heated supersonic jet issued from a military-style nozzle using the Charles solver. Left: Contours of temperature (yellow scale) and pres-
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sure (blue scale). Right: Skewness of the unsteady pressure field inside and outside the shear layer.
tion of these sources, the nature of the military jet noise sources is being investigated more deeply with near-field acoustical holography and equivalent source models.
Near-field Acoustical Holography
Radiation from large-scale turbulent structures domi- nates the total energy from all but the most modest flows45 and thus, has become the focus of current measurement and noise reduction studies. Since large-scale turbulence is high- ly structured and characterized by high spatial coherence, its radiation can be represented by relatively few, properly selected, low-order basis functions. Consequently, many studies utilize equivalent source models (ESMs) of the large- scale structures in conjunction with application of inverse methods in the jet near field. ESMs make assumptions about the source properties, such as size and distribution, shape, structure, and spatial coherence. These range from develop- ing wave packet models,46-48 to space-time correlations around the jet, to simple source models50,51,52 of the jet noise source region. These methods can be used to predict levels at maintainer locations and thus quantify noise exposure.
To study the noise generation without explicit source assumptions, near-field acoustical holography53,54 has been used to characterize the noise environment around the F- 22A.55,56 The measurement “holograms” are the individual planes of data in the F-22A OASPL maps in Figure 7, con- structed from the rectangular array in Figure 6 and the sta- tionary, ground-based linear “reference” array. By matching wave functions to the measured holograph pressures, a model of the field is generated, and the predicted pressures at any other location can be calculated. Figure 12 shows NAH reconstruction of the field for two frequencies. Note that there is a 10-20° forward shift in the directionality of the main lobe from 250 Hz to 500 Hz and that the source region, estimated by the white portions of the conical surface, signif- icantly contracts as it moves toward the nozzle.
Ties to Concurrent Work
Some of the military jet noise analyses described thus far
were conducted as part of an on-going jet noise reduction
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program sponsored by ONR and NASA. Given the wide
scope of independent research by program participants, it is
Jet Noise from Military Aircraft 15