Identification of the effects of the nozzle-exit boundary-layer thickness and its corresponding Reynolds number in initially highly disturbed subsonic jets - Archive ouverte HAL Access content directly
Journal Articles Physics of Fluids Year : 2013

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 Re D ≃ 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 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 δ0 = 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 δθ(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 δ0/r 0 and of the Reynolds number Reθ based on δθ(0). First, four jets with a fixed diameter, hence at a constant Reynolds number Re D = 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 Re D between 1.8 × 104 and 8.3 × 104, varying so as to obtain Reθ ≃ 480 in all simulations, are examined. Here, increasing δ0/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.
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hal-00931418 , version 1 (08-04-2016)

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Christophe Bogey, Olivier Marsden. Identification of the effects of the nozzle-exit boundary-layer thickness and its corresponding Reynolds number in initially highly disturbed subsonic jets. Physics of Fluids, 2013, 25, pp.055106. ⟨10.1063/1.4807071⟩. ⟨hal-00931418⟩
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