“…impinging jets, 111 the simulations suggested that structures in the wall jet could be correlated with the oscillation of the standoff shock, in agreement with the experimental observations of Henderson. 99 The simultaneous co-existence of helical and axisymmetric instability modes in the jet was observed, accompanied by the presence of two peaks in the acoustic spectra; such simultaneous peaks associated with different azimuthal instability modes have also been observed in experiment.…”
Section: Recent Developments In Understandingsupporting
confidence: 87%
“…Large toroidal vortex rings were observed at the same frequency as the jet impingement tone; the largest fluctuations in axial velocity associated with these structures occurred in the very near nozzle region. A series of LESs on a range of impinging jets has been undertaken by Gojon et al., 109–112 who considered both planar and axisymmetric jets, operating in both the ideally expanded and underexpanded conditions. For the planar 109 and axisymmetric 112 ideally expanded jets, it was demonstrated that the measured tone matched those based on a model that assumed that the upstream-propagating component was an acoustic jet mode, as suggested by Tam and Ahuja.…”
Supersonic jets, particularly shock-containing jets, often exhibit high-intensity, discrete-frequency acoustic tones. These tones are the signature of an aeroacoustic resonance loop established by the flow. This paper considers two of the classical forms of supersonic jet resonance: screech in shock-containing free jets and tones generated by the impingement of a jet against a surface. The first half of the paper provides a historical perspective on research into both forms of resonance, ranging from the seminal works of Alan Powell to recent contributions from high-fidelity numerical simulation, experiment, and stability theory. The second half of the paper provides a critical assessment of current understanding around the four processes that characterize jet resonance: a downstream propagation of energy, an acoustic generation mechanism, an upstream propagation of energy, and a receptivity mechanism.
“…impinging jets, 111 the simulations suggested that structures in the wall jet could be correlated with the oscillation of the standoff shock, in agreement with the experimental observations of Henderson. 99 The simultaneous co-existence of helical and axisymmetric instability modes in the jet was observed, accompanied by the presence of two peaks in the acoustic spectra; such simultaneous peaks associated with different azimuthal instability modes have also been observed in experiment.…”
Section: Recent Developments In Understandingsupporting
confidence: 87%
“…Large toroidal vortex rings were observed at the same frequency as the jet impingement tone; the largest fluctuations in axial velocity associated with these structures occurred in the very near nozzle region. A series of LESs on a range of impinging jets has been undertaken by Gojon et al., 109–112 who considered both planar and axisymmetric jets, operating in both the ideally expanded and underexpanded conditions. For the planar 109 and axisymmetric 112 ideally expanded jets, it was demonstrated that the measured tone matched those based on a model that assumed that the upstream-propagating component was an acoustic jet mode, as suggested by Tam and Ahuja.…”
Supersonic jets, particularly shock-containing jets, often exhibit high-intensity, discrete-frequency acoustic tones. These tones are the signature of an aeroacoustic resonance loop established by the flow. This paper considers two of the classical forms of supersonic jet resonance: screech in shock-containing free jets and tones generated by the impingement of a jet against a surface. The first half of the paper provides a historical perspective on research into both forms of resonance, ranging from the seminal works of Alan Powell to recent contributions from high-fidelity numerical simulation, experiment, and stability theory. The second half of the paper provides a critical assessment of current understanding around the four processes that characterize jet resonance: a downstream propagation of energy, an acoustic generation mechanism, an upstream propagation of energy, and a receptivity mechanism.
“…In all cases, the spectra look like those near the nozzle in figure 12, suggesting that the spectral content of the sound waves computed using the analogy will be similar to that of the LES sound fields. The resemblance between the pressure spectra on the plate and those in the acoustic field has also been noted for supersonic impinging jets in the numerical study of Gojon & Bogey (2018). Figure 22.Pressure spectra on the plate at and for jetnohole (black solid line), jetsmallhole (red solid line), jetmediumhole (blue solid line) and jetlargehole (green solid line).…”
The generation of acoustic tones in four round jets at a Mach number of 0.9 impinging on a plate at a distance
$L=6r_0$
from the nozzle exit, where
$r_0$
is the nozzle radius, has been investigated by large-eddy simulation. Three plates are perforated by holes of diameters
$h=2r_0$
,
$3r_0$
and
$4.4r_0$
, centred on the jet axis, whereas the fourth plate has no hole, in order to study the effects of the hole on the jet flow and acoustic fields. In all cases, acoustic tones emerge in the jet near field, upstream but also downstream of the plate for the perforated plates. Their frequencies are similar for all jets and close to those predicted for aeroacoustic feedback loops establishing between the nozzle and the plate, involving flow disturbances convected downstream and waves propagating upstream at the ambient speed of sound. Their levels, however, decrease with the hole diameter, by a few dB for
$h\leqslant 3r_0$
but approximately by 40 dB for
$h=4.4r_0$
. The features of the feedback loops are identified by computing two-dimensional space–time correlations and frequency–wavenumber spectra of the pressure fluctuations in the jet mixing layers. These loops are found to be closed by free-stream upstream-propagating guided jet waves, produced by the impingement of vortical structures on the plate for
$h\leqslant 3r_0$
and by the scattering of the jet aerodynamic pressure fluctuations by the hole edges for
$h=4.4r_0$
. Finally, an acoustic analogy is used to show that the contributions of the pressure fluctuations on the plate to the radiated noise are dominant.
“…9 More recently, the authors studied in depth the tone production mechanisms in underexpanded and ideally expanded impinging round jet using compressible large eddy simulation (LES). 10–12 Planar jets impinging on a flat plate normally also produce intense tone frequencies. However, compared to round jets, Arthurs and Ziada 13 noted that tones are visible at lower flow velocities.…”
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