Acoustics of excited jets—a historical perspective

Brown, Clifford A.

doi:10.1121/1.4778975

Cited by 5 publications

(2 citation statements)

References 51 publications

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“…The pronounced sensitivity of jets to variations of the inflow condition has been demonstrated experimentally 13,14 as well as numerically. 15,16 Hence, investigating the sensitivity of the simulations to changes in disturbance amplitudes seems appropriate.…”

Section: Introductionmentioning

confidence: 83%

Numerical study of eigenmode forcing effects on jet flow development and noise generation mechanisms

2009

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The effect of nonlinear interaction of instability eigenmodes on jet flow transition and its near acoustic field for a high-subsonic round jet at a Reynolds number of Re= 4.5ϫ 10 5 and a Mach number of Ma= 0.9 is investigated using large-eddy simulations. At the inflow, helical perturbations of azimuthal wavenumbers ͉n͉ = 4 , . . . , 8 determined from linear stability theory are superimposed on a laminar base flow in order to trigger transition to turbulence. The disturbance amplitude is varied parametrically in the range from 1.5% to 4.5% of the jet exit velocity U j . Thereby we aim to characterize sources of noise generation and, in particular, underlying mode interactions. With increasing forcing amplitude, the transitional behavior of the jet changes which affects the mean flow and also the acoustic near-field, which are both analyzed in detail. As the forcing amplitude is increased, the axial root-mean-square peak levels along the jet centerline are reduced by approximately 7%. Simultaneously, pronounced dual-peak distributions are generated along the jet lip line which are related to the localization of vortex pairings of the jet column mode. For low-amplitude excitation the azimuthal turbulent kinetic energy spectra show that the unexcited, naturally least stable axisymmetric mode n = 0 and the helical mode n = 1 dominate the early nonlinear regimes between z Ϸ 6r 0 and 9r 0 where r 0 is the jet radius. An analysis of the Fourier mode amplitude clarifies that this energy rise is linked to the helical mode n = 1. For higher forcing amplitudes, in addition to the varicose mode n = 0 interactions between the excited even mode n = 4 and higher azimuthal harmonics thereof dominate the azimuthal energy spectra. These differences in the early nonlinear development of the eigenmodes are found to alter the acoustic near-field. At small angles from the downstream jet axis, the peak acoustic frequency occurs at a Strouhal number based on the angular frequency and the jet diameter D j of St= D j / ͑2U j ͒ Ϸ 0.4. For low-amplitude forcing sound pressure levels are slightly enhanced which can be linked to the dominant low azimuthal wavenumbers identified in the transitional region. In the sideline direction, regardless of the excitation level, broadbanded spectra with maxima in the band 0.7 Յ StՅ 0.8 are found which is maintained at intermediate observer angles. For high forcing amplitude, however, a tonal component outside the initially excited frequency range is observed. This peak at StϷ 0.88 can be explained by weakly nonlinear interactions of initially forced eigenmodes n = 4 and n = 8 together with the jet column mode.

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Section: Introductionmentioning

confidence: 83%

Numerical study of eigenmode forcing effects on jet flow development and noise generation mechanisms

2009

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“…Apart from the direct propagation of core noise to the far field, core noise fluctuations can play a pivotal role in the jet noise amplification. 4 Experimental studies have shown that tonal and broadband excitations upstream of the nozzle exit can amplify the jet noise by as much as 5-10 dB compared to the unforced condition. [5][6][7] In particular, Bechert & Pfizenmaier 7,8 reported a pronounced broadband shift of the radiated sound for excitations at the preferred shear-layer frequency.…”

Section: Introductionmentioning

confidence: 99%

Jet Noise Receptivity to Nozzle-upstream Perturbations in Compressible Heated Jets

See

Amini

Koh

et al. 2012

18th AIAA/CEAS Aeroacoustics Conference (33rd AIAA Aeroacoustics Conference)

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Effects of nozzle-upstream entropy perturbations on the acoustic radiation from heated jets are investigated. For this, a model problem is considered, in which a gas-turbine combustor discharges reaction products through a converging nozzle into the ambient environment. The turbulent reacting flow field in the combustor is computed using large-eddy simulation (LES), and the unsteady flow-field at the combustor exit is extracted to provide realistic inflow conditions to the jet-flow simulation. To study the indirect coupling process, arising from the interaction of the combustion-generated entropy fluctuations with the adverse pressure gradient through the nozzle, a linearized Euler formulation is employed. Parametric studies are performed to investigate effects of frequency and amplitude of the nozzle-upstream entropy perturbations on the jet instability and the jet noise directivity. Simulation results show that the directivity is dependent on the perturbation frequency. Excitation near the preferred shear-layer instability leads to strong acoustic radiation in the 45 • forward direction, and the radiation angle decreases with decreasing excitation frequency.

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