Near-Field and Far-Field Acoustics of Laminar and Turbulent Nozzle-Jet Flows

摘要

Using a direct numerical simulation setup we investigate the effects of nozzle exit flow conditions on jet flow development and sound radiation at a diameter-based Reynolds number of Re_D = 18100 and Mach number Ma = 0.9. The flow within a short cylindrical nozzle is modeled by a potential flow core and a laminar, transitional or developing turbulent wall boundary layer. For laminar and transitional nozzle exit conditions, transition to turbulence in the jet shear layer is governed by the well-known development of Kelvin-Helmholtz instabilities, for which growth rates and dominant frequencies agree well with most amplified modes of linear stability theory. For turbulent nozzle exit conditions, the jet flow development is characterized by a rapid changeover of the turbulent wall boundary layer to the turbulent free shear layer within about one nozzle diameter. The near-field sound is computed within a domain spanning fifteen jet diameters in the radial direction. In agreement with previous results, we obtain enhanced acoustic radiation when the nozzle exit turbulence level is reduced. At low emission angles, characteristic peaks in the spectra are shifted towards higher frequencies for lower nozzle exit turbulence levels. Acoustic results are found to be in reasonable agreement with computational and experimental results from literature. The pressure field is analyzed in frequency and frequency-wavenumber space, where acoustic and hydrodynamic pressure waves are separated by the circle representing the dispersion relation of an acoustic wave. For the extension of the near-field results to the far-field the Spectral Lighthill Method is applied (SLM). SLM sound spectra show a strong low-frequency peak at low emission angles with peak levels independent of the specific nozzle exit conditions, in contrast to near-field spectra.