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

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 similar or equal to 5x10(4) 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 r(0), 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 delta(0) = 0.09r(0), 0.15r(0), 0.25r(0), or 0.42r(0), yielding 99% velocity thicknesses between 0.07r(0) and 0.34r(0) and momentum thicknesses delta(theta)(0) between 0.012r(0) and 0.05r(0). Two sets of computations are reported to distinguish, for the first time to the best of our knowledge, between the effects of the ratio d(0)/r(0) and of the Reynolds number Re-theta based on delta(theta) (0). First, four jets with a fixed diameter, hence at a constant Reynolds number Re-D = 5 x 10(4) giving Re-theta = 304, 486, 782, and 1288 depending on d(0), are considered. In this case, due to the increase in Re-theta, 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 x 10(4) and 8.3 x 10(4), varying so as to obtain Re-theta similar or equal to 480 in all simulations, are examined. Here, increasing d(0)/ r(0) 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. (C) 2013 AIP Publishing LLC.