Generation of Excess Noise by Jets with Highly Disturbed Laminar Boundary-Layer Profiles

The generation of noise by jets with highly disturbed laminar boundary-layer profiles at the nozzle exit, also referred to as initially nominally laminar jets in the literature, is investigated using large-eddy simulation and linear stability analysis. Four jets at a Mach number of 0.9 and a Reynolds number of 5x104, one with a nonlaminar boundary-layer profile and three others with laminar profiles, are considered for exit peak turbulence intensities equal to 6% in the nonlaminar case and to 9% in the laminar ones. The jets with laminar boundary-layer profiles all radiate greater sound pressure levels than the jet with a nonlaminar profile but weaker initial disturbances. This particularly appears at high frequencies for the jets with a thinner boundary layer compared with the nonlaminar case. These results are shown to be related to the dependence on the shape of the boundary-layer profile of the most unstable frequencies downstream of the nozzle. For a laminar profile, these frequencies are similar to those obtained downstream in the mixing-layer profiles, whereas they are higher for a nonlaminar profile. Despite larger nozzle-exit flow disturbances, this leads to longer-term persistence of coherent large-scale structures in the shear layers, hence stronger velocity fluctuations and noise levels, for the present initially nominally laminar jets than for the other one.