Role of Anisotropy in Turbulent Mixing Noise

Khavaran, Abbas

doi:10.2514/2.7531

Cited by 93 publications

(41 citation statements)

References 15 publications

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“…Predictions and measurements were in a reasonably good agreement, though there were discrepancies at high frequencies. Khavaran [10] has reported two additional changes to the existing MGBK model. First, an axisymmetric turbulence model was used rather than an isotropic description of acoustic sources.…”

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confidence: 99%

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

et al. 2015

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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.

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confidence: 99%

“…With these updates, they showed better noise predictions compared to the MGBK model [11]. Viswanathan [12] compared the MGBK model [13] and the fine-scale turbulence mixing noise model proposed by Tam and Aurialt [14] with his measurements. Tam and Auriault's model predicted the sound power spectral density better than the MGBK model.…”

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confidence: 99%

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

et al. 2015

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“…Furthermore, by choosing to use the coefficient matrix A ijlm , the potential to include anisotropic effects of turbulence is possible. Inclusion of these effects can be beneficial, as shown by Khavaran [44].…”

Section: Modeling the Two-point Cross Correlation Of The Lighthill Stmentioning

confidence: 93%

Prediction of Near-Field Jet Cross Spectra

Miller

2015

AIAA Journal

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A prediction method is developed based on the acoustic analogy for the cross-power spectral density in the convecting near field of compressible fluid turbulence. Equivalent source near-field, midfield, and far-field terms within the model integrand create corresponding near-field, midfield, and far-field radiating waves. These equivalent sources are modeled with a single equation for the two-point cross correlation of the Lighthill stress tensor that is dependent on the jet operating conditions. An alternative equivalent source model based on steady Reynoldsaveraged Navier-Stokes solutions is proposed. The cross-power spectral density model automatically reduces to a traditional autopower spectral density model when observers are at the same location. Predictions of radiation intensity and coherence compare favorably with measurements in the near field, midfield, and far field for a wide range of jet Mach numbers and temperature ratios.fully expanded jet diameter e s = number of ensembles F t = far-field integrand terms f = frequency G = cross-power spectral density g = Green's function k = turbulent kinetic energy l si = turbulent length scale M = Mach number M c = convective Mach number coefficient M d = design Mach number M j = fully expanded Mach number M t = midfield integrand terms M ∞;i = component of the vector freestream Mach number N t = near-field integrand terms P f = prefactor Pr = Prandtl number Pr t = turbulent Prandtl number p = pressure R = magnitude of x R g = gas constant R ijlm = two-point cross correlation of T ij r = source vector S = spectral density S ij = source tensor S y = equivalent spatial source distribution St = Strouhal number T ij = Lighthill stress tensor T j = fully expanded temperature t = time u = velocity vector u j = fully expanded jet velocity x = observer spatial coordinate y = source spatial coordinate y c = core length α = constant length scale coefficient β s= ratio of specific heats δ = Dirac delta function δ ij = Kronecker delta function ϵ = dissipation of turbulent kinetic energy ηξ; η; ζ = vector within source region ρ = density σ = anisotropic dependence parameter σ f = anisotropic amplification exponent τ = retarded time τ s = turbulent time scale ϕ = azimuthal observer angle Ψ = inlet observer angle ω = radial frequency

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“…This solution usually deteriorates close to the boundary of the so-called "zone of silence" that forms near the downstream jet axis. A more-rigorous high-frequency solution, referred to as an asymmetric approximation [12], produces better agreement with the exact solution and is under consideration for future improvement of the MGBK code.…”

Section: Acoustical Predictionsmentioning

confidence: 99%

Sound transmission loss of tiled brick walls

et al. 2005

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An investigation was conducted at the NASA Glenn Research Center using a set of three round jets operating under unheated subsonic conditions to address the question: "How close is too close?" Although sound sources are distributed at various distances throughout a jet plume downstream of the nozzle exit, at great distances from the nozzle the sound will appear to emanate from a point and the inverse-square law can be properly applied. Examination of normalized sound spectra at different distances from a jet, from experiments and from computational tools, established the required minimum distance for valid far-field measurements of the sound from subsonic round jets. Experimental data were acquired in the Aeroacoustic Propulsion Laboratory at the NASA Glenn Research Center. The WIND computer program solved the Reynolds-Averaged Navier-Stokes equations for aerodynamic computations; the MGBK jet-noise prediction computer code was used to predict the sound pressure levels. Results from both the experiments and the analytical exercises indicated that while the shortest measurement arc (with radius ~8 nozzle diameters) was already in the geometric far field for high-frequency sound (Strouhal number >5), low-frequency sound (Strouhal number <0.2) reached the geometric far field at a measurement radius of at least 50 nozzle diameters because of its extended source distribution. © 2005 Institute of Noise Control Engineering.

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Role of Anisotropy in Turbulent Mixing Noise

Cited by 93 publications

References 15 publications

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

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

Prediction of Near-Field Jet Cross Spectra

Sound transmission loss of tiled brick walls

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