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- W2331036143 abstract "We present experimental results for the acoustic field of jets with Mach numbersbetween 0.35 and 0.6. An azimuthal ring array of six microphones, whose polarangle, θ, was progressively varied, allows the decomposition of the acoustic pressureinto azimuthal Fourier modes. In agreement with past observations, the sound field forlow polar angles (measured with respect to the jet axis) is found to be dominated bythe axisymmetric mode, particularly at the peak Strouhal number. The axisymmetricmode of the acoustic field can be clearly associated with an axially non-compactsource, in the form of a wavepacket: the sound pressure level for peak frequencies isfound be superdirective for all Mach numbers considered, with exponential decay as afunction of (1 – M_c cos θ)^2, where M_c is the Mach number based on the phase velocityU_c of the convected wave. While the mode m = 1 spectrum scales with Strouhalnumber, suggesting that its energy content is associated with turbulence scales,the axisymmetric mode scales with Helmholtz number – the ratio between sourcelength scale and acoustic wavelength. The axisymmetric radiation has a strongervelocity dependence than the higher-order azimuthal modes, again in agreement withpredictions of wavepacket models. We estimate the axial extent of the source ofthe axisymmetric component of the sound field to be of the order of six to eightjet diameters. This estimate is obtained in two different ways, using, respectively,the directivity shape and the velocity exponent of the sound radiation. The analysisfurthermore shows that compressibility plays a significant role in the wavepacketdynamics, even at this low Mach number. Velocity fluctuations on the jet centrelineare reduced as the Mach number is increased, an effect that must be accounted forin order to obtain a correct estimation of the velocity dependence of sound radiation.Finally, the higher-order azimuthal modes of the sound field are considered, and amodel for the low-angle sound radiation by helical wavepackets is developed. Themeasured sound for azimuthal modes 1 and 2 at low Strouhal numbers is seen tocorrespond closely to the predicted directivity shapes." @default.
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- W2331036143 date "2011-06-05" @default.
- W2331036143 modified "2023-10-16" @default.
- W2331036143 title "Axisymmetric superdirectivity in subsonic jets" @default.
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- W2331036143 doi "https://doi.org/10.2514/6.2011-2743" @default.
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