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.

Automated summary

The study investigates the generation of noise by jets with highly disturbed laminar boundary-layer profiles at the nozzle exit. Jets with laminar boundary-layer profiles radiate greater sound pressure levels than the jet with a nonlaminar profile, but with weaker initial disturbances. The results demonstrate the dependence on the shape of the boundary-layer profile of the most unstable frequencies downstream of the nozzle.