Experimental Study of the Spectral Properties of Near-Field and Far-Field Jet Noise

Abstract

The near and far pressure fields generated by round, isothermal and cold jets of diameter D = 38 mm with Mach numbers varying over the range 0.6 ≤ M j ≤ 1.6 are investigated experimentally, and characterized in terms of sound spectra and levels. Properties of near-field jet noise, obtained in particular at 7.5 diameters from the jet centerline, are documented. They differ appreciably from properties of far-field noise, and form a database that can be used for the validation of the acoustic fields determined by compressible Navier-Stokes computations. The near pressure fields originating from simulations can thus be directly compared, without resorting to extrapolation methods which might lead to uncertainties in the far pressure fields. In the present paper, sound source localizations are also carried out from the near-field pressure signals. The experiments provide in addition far-field results evaluated at 52 diameters from the nozzle exit, in good agreement with the data of the literature. The classical dependence of jet noise features with the emission angle is observed. The level and frequency scalings of the pressure spectra obtained for subsonic jets in the sideline and downstream directions are also studied. For small radiation angles, the narrow-banded sound spectra measured are found to scale as the Strouhal number, whereas, as previously noted by Zaman & Yu [1], the one-third octave spectra seem to scale as the Helmholtz number.

Influence of the Nozzle-Exit Boundary-Layer Thickness on the Flow and Acoustic Fields of Initially Laminar Jets

“…The sound pressure levels calculated at 60r 0 from the jet nozzle exit, for St ≥ 0.1, are represented in figure 16. As previously, the strong overestimation of the acoustic levels with respect to measurements provided by Mollo-Christensen et al, 35 Lush, 36 Tanna, 37 and Bogey et al 38 for jets at high Reynolds numbers (see in table 4) indicates the presence of additional noise in the present initially laminar jets. The significant decrease of the levels when just adding inlet random noise of low amplitude in the pipe is also noticed.…”

“…For all angles, parasitic low-frequency components, certainly coming from the downstream part of the control surface as discussed in section II.D., are noted for Strouhal numbers lower than 0.15. More interestingly, compared to sound spectra provided by Tanna 37 and Bogey et al 38 for experimental jets at Reynolds numbers Re D ≥ 7.8 × 10 5 (see in table 4), the spectra contain additional high-frequency bumps. These extra noise components are especially of high magnitude for the jets simulated without inlet random disturbance.…”

“…For the aerodynamic fields, the data obtained by Hussain and Zedan, 4 and Husain and Hussain 6 in axisymmetric mixing layers, and by Lau et al, 32 Arakeri et al 33 and Fleury et al 34 in round jets at Mach number 0.9 and Reynolds numbers higher than 5 × 10 5 will be used. In the same way, for the acoustic fields, the sound pressure levels and spectra provided by Zaman 7,8 for initially laminar jets, and by Mollo-Christensen et al, 35 Lush, 36 Tanna 37 and Bogey et al 38 for jets at M ≃ 0.9 and Re D ≥ 5 × 10 5 will be considered. Some available parameters of these experiments are documented in table 4.…”

“…The inflow turbulent intensities are however higher with respect to the simulations, except for JetD005p2000 in which the peak rms axial fluctuating velocity is around 2% of the jet velocity as in Hussain and Zedan. 4 Regarding the seven jets [32][33][34][35][36][37][38] at Mach numbers around 0.9, they can be assumed to be initially turbulent because their high Reynolds numbers.…”

Axisymmetric superdirectivity in subsonic jets

“…respectively, to obtain velocity scalings of the sound radiation, as in previous work (Zaman & Yu 1985;Viswanathan 2006;Bogey et al 2007). This was done for the total values of OASPL and SPL, and also for the individual contributions of azimuthal modes 0, 1 and 2.…”