Acoustic Sources and Far-Field Noise of Chevron and Round Jets

Abstract

This paper investigates numerically the acoustic sources and far-field noise of chevron and round jets. The acoustic sources are described by the fourth-order space-time velocity cross correlations, which are calculated based on a large-eddy simulation flowfield. Gaussian functions are found to fit the axial, radial, and azimuthal cross correlations reasonably well. The axial length scales are three to four times the radial and azimuthal length scales. For the chevron jet, the cross-correlation scales vary with azimuthal angle up to six jet diameters downstream; beyond that, they become axisymmetric like those for a round jet. The fourth-order space-time cross correlation of the axial velocity R 1111 is the dominant source component, and there are considerable contributions from other source components such as R 2222 , R 3333 , R 1212 , R 1313 , and R 2323 cross correlations where 1, 2, and 3 represent axial, radial, and azimuthal directions, respectively. For the chevron jet, these cross correlations decay rapidly with axial distance whereas for the round jet, they remain roughly constant over the first 10 jet diameters. The chevron jet intensifies both the R 2222 and R 3333 cross correlations within two jet diameters of the jet exit. The amplitude, length, and time scales of the crosscorrelations of a large-eddy simulation velocity field are investigated as functions of position and are found to be proportional to the turbulence amplitude, length, and time scales that are determined from a Reynolds-averaged Navier-Stokes calculation. The constants of proportionality are found to be independent of position within the jet, and they are quite close for chevron and round jets. The scales derived from Reynolds-averaged Navier-Stokes are used for source description, and an acoustic analogy is used for sound propagation. There is an excellent agreement between the far-field noise predictions and measurements. At low frequencies, the chevron nozzle significantly reduces the far-field noise by 5-6 dB at 30 deg and 2-3 dB at 90 deg to the jet axis. However, the chevron nozzle slightly increases high-frequency noise. It was found that R 1212 and R 1313 cross correlations have the largest contribution to the jet noise at 30 deg to the jet axis, whereas the R 2323 cross correlation has the largest contribution to the jet noise at 90 deg to the jet axis. The Reynolds-averaged Navier-Stokes calculations are repeated with different turbulence models, and the noise prediction is found to be almost insensitive to the turbulence model. The results indicate that the modeling approach is capable of assessing advanced noise-reduction concepts.

On the Space-Time Correlation of Heat Release Rate in Turbulent Premixed Flames

“…It is obviously shown in Figure 7 of the peak correlation are determined by ∆ + x =ū + τ + , which is in agreement with the convection characteristics observed in Figure 6(c) as well as the spatio-temporal correlations of jet noise sources (e.g. Karabasov et al, 2010Karabasov et al, , 2011Depuru Mohan et al, 2015), local burning area fluctuations (Hemchandra and Lieuwen, 2010), and local chemiluminescence fluctuations (Rajaram and Lieuwen, 2009). The envelope of the peak correlation values indicates the decay of Ω p (∆ + x , τ + ) with the separation distance ∆ + x and/or time delay τ + .…”

“…In a series of recent studies on modelling the two-point space-time correlation of jet noise sources (e.g. Karabasov et al, 2010Karabasov et al, , 2011Depuru Mohan et al, 2015), it was observed that this correlation can be well represented by the Gaussian-type function commonly used in classical turbulence. Tam (2015) recently proposed that the spectral shape of combustion noise from hydrocarbon fuels is the same as the similarity spectrum of the large turbulence structure noise from high-speed jets, regardless of the turbulence level or fuel properties.…”

“…One kind of temporal correlation of heat release rate fluctuations is the two-point space-time correlation which has been successfully applied to model the spectrum of fluid dynamic noise in non-reacting flows, for example jet noise spectrum (e.g. Karabasov et al, 2010Karabasov et al, , 2011Depuru Mohan et al, 2015). However, the two-point space-time correlation of the heat release rate fluctuation in turbulent flames has scarcely been reported in prior literature (Liu et al, 2011) primarily because reliable heat release rate data are absent in previous numerical simulations or measurements.…”

Linear Stability Implications of Chevron Geometry Modifications for Turbulent Jets

“…38,39,40 In our preliminary RANS simulations reported here, we have adopted the standard k − model; 41 it was also found to be the most appropriate for RANS simulations of serrated jets required for noise prediction using the acoustic analogy approach. 9 All simulations reported here were performed using the open-source CFD package OpenFOAM. 42 The rhoSimpleFoam solver was used, which is a steady-state SIMPLE solver for RANS of compressible flows.…”

“…5,6,7 Another approach that is sometimes favored is empirical noise models that invoke an acoustic analogy and take as input an estimate of the scales of turbulence fluctuations in the jet plume, usually using Reynolds-averaged Navier Stokes (RANS) simulations. 8,9 Here, we propose to build the foundation for a third alternative that is explained now.…”