Direct Computation of the Noise Generated by Subsonic Jets Originating from a Straight Pipe Nozzle

Abstract

The noise generated by isothermal round jets at Mach number M = 0.9 and at the diameter-based Reynolds number ReD = 5 × 105, originating from a straight pipe nozzle, is computed directly using Large Eddy Simulation (LES), in order to highlight the potential influence of the inlet boundary conditions on the acoustic predictions. Two jets are simulated, displaying levels of fluctuating axial velocity at the nozzle exit respectively of 1.6% and 9% of the jet velocity, while the momentum thickness of the shear layers is nearly the same. The shear-layer development and the radiated sound fields obtained for the two jets are found to differ significantly. The shear layer of the jet with low initial turbulence levels develops for instance with higher turbulence intensities and a velocity flow field that is more correlated. In addition coherent annular vortices and pairings are clearly observed in this jet. Regarding the radiated noise, the jet with high initial turbulence levels provides sound levels and spectra in fairly good agreement with experimental data obtained for jets at Reynolds numbers ReD ≥ 5 × 105. The jet with low nozzle-exit turbulence levels is shown to generate more noise, which might result from vortex pairings in the shear layer as it was suggested by Zaman [1].