Identification of the effects of the nozzle-exit boundary-layer thickness and its corresponding Reynolds number in initially highly disturbed subsonic jets

Abstract

The influence of the nozzle-exit boundary-layer thickness in isothermal round jets at a Mach number of 0.9 and at diameter Reynolds numbers ReD ≃ 5 × 104 is investigated using large-eddy simulations. The originality of this work is that, contrary to previous studies on the topic, the jets are initially highly disturbed, and that the effects of the boundary-layer thickness are explored jointly on the exit turbulence, the shear-layer and jet flow characteristics, and the acoustic field. The jets originate from a pipe of radius r0, and exhibit, at the exit, peak disturbance levels of 9% of the jet velocity, and mean velocity profiles similar to laminar boundary-layer profiles of thickness δ0 = 0.09r0, 0.15r0, 0.25r0, or 0.42r0, yielding 99% velocity thicknesses between 0.07r0 and 0.34r0 and momentum thicknesses δθ(0) between 0.012r0 and 0.05r0. Two sets of computations are reported to distinguish, for the first time to the best of our knowledge, between the effects of the ratio δ0/r0 and of the Reynolds number Reθ based on δθ(0). First, four jets with a fixed diameter, hence at a constant Reynolds number ReD = 5 × 104 giving Reθ = 304, 486, 782, and 1288 depending on δ0, are considered. In this case, due to the increase in Reθ, thickening the initial shear layers mainly results in a weaker mixing-layer development with lower spreading rates and turbulence intensities, and reduced sound levels at all emission angles. Second, four jets at Reynolds numbers ReD between 1.8 × 104 and 8.3 × 104, varying so as to obtain Reθ ≃ 480 in all simulations, are examined. Here, increasing δ0/r0 has a limited impact on the mixing-layer key features, but clearly leads to a shorter potential core, a more rapid velocity decay, and higher fluctuations on the jet axis, and stronger noise in the downstream direction. Similar trends can be expected for high-Reynolds-number jets in which viscosity plays a negligible role.