Computation of the Noise Radiated by Jets with Laminar/Turbulent Nozzle-Exit Conditions

Abstract

Subsonic isothermal round jets at Mach number Mj = 0.9 and at the diameter-based Reynolds number ReD = 5 × 10 5 are computed using compressible Large Eddy Simulation (LES), in order to investigate the effects of nozzle-exit turbulence levels on jet noise. A pipe nozzle is included in the computational domain, and the development of the boundary layer inside the nozzle is calculated. In this way, two jets displaying respectively low and high turbulence levels at the nozzle exit are considered. In the two cases, the levels of fluctuating axial velocity at the nozzle exit are indeed of 0.016 and 0.090 with respect of the jet velocity, while the momentum thickness of the boundary layers is nearly the same. The shear-layer developments and the radiated sound fields obtained for the two jets are found to differ significantly. The shear layer of the jet with low nozzle-exit turbulence levels develops with higher turbulence intensities and a velocity flow field that is more correlated. Coherent annular vortices are also clearly observed only in this jet. Regarding the radiated noise, the jet with high turbulence levels at the nozzle exit provides sound levels and spectra in very good agreement with experimental data obtained for jets at high Reynolds numbers ReD � 5 × 10 5 , which are expected to be initially turbulent. The computed jet with low exit turbulence levels is shown to generate more noise, which results from vortex pairings in the shear layer.