Influence of initial turbulence level on the flow and sound fields of a subsonic jet at a diameter-based Reynolds number of 105

Five isothermal round jets at Mach number M = 0.9 and Reynolds number Re-D = 10(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 Re-theta = 900, and peak turbulence levels u(e)' 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)'/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)'/u(j) <= 6 %. For higher u(e)'/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)'/u(j), however, they become weaker, and the noise levels decrease asymptotically down to those measured for jets at Re-D >= 5 x 10(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.