Page 11 - Volume 9, Issue 3

Fig. 3. Chevrons installed in the engine nozzles of an F/A-18E/F Super Hornet. Courtesy of the U.S. Naval Air Systems Command (NAVAIR).
bances in the linear hydrodynamic (i.e. acoustic) near field
decay exponentially, or evanesce, in the radial direction.
When the mean convective jet velocity is supersonic with
respect to the ambient medium, the radiation efficiency is
enhanced by Mach wave radiation. The Mach waves’ direc-
tionality is determined by trace velocity matching of the
mean convection speed of the turbulence near the shear layer
with the sound speed outside. Readers familiar with struc-
tural acoustics will ientify parallels between jet noise radia-
tion mechanisms and subsonic and supersonic wave motion
16
the overall noise from supersonic jets is directional, with the maximum levels often occurring 30-60° relative to the down- stream jet axis. This principal lobe in overall sound pressure level (OASPL) shifts upstream with increasing jet velocity
12
the spectral shape evolves from a relatively peaked “haystack” spectrum near the maximum radiation angles to a rounder spectral shape toward and upstream of the sideline.
This basic variation in spectral shape, present under a wide variety of jet conditions, prompted the empirical devel- opment of two characteristic spectral shapes, i.e. similarity spectra,17,18 and connection of these two spectra to a two-
19,20
These spectra and a schematic representation of the two-source model are shown in Figure 4. Based on a variety of experimental observations,19 the peaked spectra around the maximum radiation angle are linked to the large-scale, relatively coherent, quasi-stable tur- bulent structures in the plume. The source coherence and wave interference effects in these large-scale structures (LSS) can be used to explain the source directivity. The more rounded spectra to the sideline and upstream are believed to be associated with fine-scale structures (FSS) in the turbu- lence. According to Tam et al.,19 they are distributed through- out the plume, radiate incoherently, and are more observable
source model for jet noise.
in regions not dominated by the LSS radiation.
Because the maximum radiation appears to be tied to LSS-type turbulence, recent efforts have focused on explor-
in plates.
Because of the dominance of the Mach wave radiation,
In terms of the mixing noise spectrum, the peak frequency and the shape change as a function of angle. The characteristic Strouhal number, (St=fD/uj, where f is frequency, D is nozzle diameter, and uj is jet velocity) changes from ~0.1-0.3 in the peak radi- ation direction to higher values toward the sideline (90°) and
and broadens with increases in temperature.
Fig. 4. Left: A schematic represenation of the two-source model of jet noise, comprised of large-scale structures (LSS) and fine-scale structures (FSS). Right: The empirical
FSS (open circles) and LSS (filled circles) similarity spectra, relative to an arbitrary peak frequency 𝒇𝒑, from Tam et al.
18,19
10 Acoustics Today, July 2013