Flow and sound fields of low-Reynolds-number temporal jets at Mach numbers from 0.3 to 2

Abstract : Five temporally-developing isothermal round jets at a diameter-based Reynolds number ReD of 3,125 and Mach numbers M of 0.3, 0.6, 0.9, 1.3 and 2 have been computed using direct numerical simulation. The flow and near acoustic fields of the jets are described in detail, and cross-correlations between the two fields are presented in order to identify noise generation mechanisms, and especially that associated with the jet potential-core closing. That closing is found to occur later at a higher Mach number. Apart from that, the jet flow properties, including velocity spectra, are similar, and even nearly identical for M ≤ 1.3. The jets appear to radiate acoustic waves mainly in the downstream direction, with amplitude increasing and wavelength decreasing with the Mach number. More quantitatively, the noise spectra nearly collapse when they are plotted as as a function of kzr0M −1 , where kz is the axial wavenumber and r0 is the initial jet radius, and scaled in amplitude using a M 8 power law. The angle of sound emission is noted to decrease monotonically as time passes, except for the jet at M = 2 for which it is close to the angle expected for Mach waves over a long time period. Finally, significant values of correlations are obtained between the pressure waves propagating downstream in the jet near field and the flow fluctuations on the jet axis near the time of potential-core closing for M ≥ 0.6, with levels strengthening with the Mach number. The correlations calculated from the flow fluctuations at r = r0 are much weaker. For the jet at M = 2, however, they suggest the generation of Mach waves in the mixing layers. These results are comparable to those reported for spatially-developing jets.
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Christophe Bogey, Pierre Pineau. Flow and sound fields of low-Reynolds-number temporal jets at Mach numbers from 0.3 to 2. 2018 AIAA/CEAS Aeroacoustics Conference, Jun 2018, Atlanta, United States. ⟨10.2514/6.2018-3615⟩. ⟨hal-02352088⟩

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