Large-eddy simulations ͑LESs͒ of isothermal round jets at a Mach number of 0.9 and a diameter-based Reynolds number Re D of 10 5 originating from a pipe are performed using low-dissipation schemes in combination with relaxation filtering. The aim is to carefully examine the capability of LES to compute the flow and acoustic fields of initially nominally turbulent jets. As in experiments on laboratory-scale jets, the boundary layers inside the pipe are tripped in order to obtain laminar mean exit velocity profiles with high perturbation levels. At the pipe outlet, their momentum thickness is ␦ ͑0͒ = 0.018 times the jet radius, yielding a Reynolds number Re = 900, and peak turbulence intensities are around 9% of the jet velocity. Two methods of boundary-layer tripping and five grids are considered. The results are found to vary negligibly with the tripping procedure but appreciably with the grid resolution. Based on analyses of the LES quality and on comparisons with measurements at high Reynolds numbers, fine discretizations appear necessary in the three coordinate directions over the entire jet flow. The final LES carried out using 252ϫ 10 6 points with minimum radial, azimuthal, and axial mesh spacings, respectively, of 0.20, 0.34, and 0.40ϫ ␦ ͑0͒ is also shown to provide shear-layer solutions that are practically grid converged and, more generally, results that can be regarded as numerically accurate as well as physically relevant. They suggest that the mixing-layer development in the present tripped jet, while exhibiting a wide range of turbulent scales, is characterized by persistent coherent vortex pairings.

Five isothermal round jets at Mach number $M= 0. 9$ and Reynolds number ${\mathit{Re}}_{D} = 1{0}^{5} $ originating from a pipe nozzle are computed by large-eddy simulations to investigate the effects of initial turbulence on flow development and noise generation. In the pipe, the boundary layers are untripped in the first case and tripped numerically in the four others in order to obtain, at the exit, mean velocity profiles similar to a Blasius laminar profile of momentum thickness equal to 1.8 % of the jet radius, yielding Reynolds number ${\mathit{Re}}_{\theta } = 900$, and peak turbulence levels ${ u}_{e}^{\ensuremath{\prime} } $ around 0, 3 %, 6 %, 9 % or 12 % of the jet velocity ${u}_{j} $. As the initial turbulence intensity increases, the shear layers develop more slowly with much lower root-mean-square (r.m.s.) fluctuating velocities, and the jet potential cores are longer. Velocity disturbances downstream of the nozzle exit also exhibit different structural characteristics. For low ${ u}_{e}^{\ensuremath{\prime} } / {u}_{j} $, they are dominated by the first azimuthal modes ${n}_{\theta } = 0$, 1 and 2, and show significant skewness and intermittency. The growth of linear instability waves and a first stage of vortex pairings occur in the shear layers for ${ u}_{e}^{\ensuremath{\prime} } / {u}_{j} \leq 6\hspace{0.167em} \% $. For higher ${ u}_{e}^{\ensuremath{\prime} } / {u}_{j} $, three-dimensional features and high azimuthal modes prevail, in particular close to the nozzle exit where the wavenumbers naturally found in turbulent wall-bounded flows clearly appear. Concerning the sound fields, strong broadband components mainly associated with mode ${n}_{\theta } = 1$ are noticed around the pairing frequency for the untripped jet. With rising ${ u}_{e}^{\ensuremath{\prime} } / {u}_{j} $, however, they become weaker, and the noise levels decrease asymptotically down to those measured for jets at ${\mathit{Re}}_{D} \geq 5\ensuremath{\times} 1{0}^{5} $, which are likely to be initially turbulent and to emit negligible vortex-pairing noise. These results correspond well to experimental observations, made separately for either mixing layers, jet flow or sound fields.