Large-Eddy Simulations of isothermal round jets at a Mach number of 0.9 are performed in order to investigate the influence of the nozzle-exit boundary-layer thickness on initially highly disturbed subsonic jets at moderate Reynolds numbers. The jets are originating from a pipe nozzle of radius r_0, and exhibit, at the exit section, peak disturbance levels of 9 per cent of the jet velocity, and mean velocity profiles similar to laminar boundary-layer profiles of thickness δ_0 = 0.09r_0, 0.15r_0, 0.25r_0 or 0.42r_0, yielding momentum thicknesses δ_θ(0) between 0.012r_0 and 0.05r_0. Four jets at a diameter Reynolds number Re_d = 5 × 10~4, providing momentum-thickness Reynolds numbers Reg = 304, 486, 782 and 1288 depending on δ_0, are first considered. Four jets at Reynolds numbers Re_D = 8.3 × 10~4, 5 × 10~4, 3 × 10~4 and 1.8 × 10~4, with δ_0 = 0.09r_0, 0.15r_0, 0.25r_0 and 0.42r_0, respectively, giving Re_θ ～ 480 in all cases, are then examined. The effects of δ_0/r_0 and Re_θ on the jet flow and sound fields can thus be distinguished. At a constant Re_D, thickening the initial shear layers mainly results in lower turbulence intensities in the mixing layers and weaker sound levels at all emission angles due to the variations of Re_θ. Different trends are therefore obtained at a nearly identical Re_θ. Increasing the ratio δ_0/r_0 in this case leads to a shorter potential core, higher centerline velocity fluctuations, and stronger noise in the downstream direction.
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