Influence of nozzle-exit boundary-layer conditions on the flow and acoustic fields of initially laminar jets

Abstract

International audienceRound jets originating from a pipe nozzle are computed by large-eddy simulations (LES) to investigate the effects of the nozzle-exit conditions on the flow and sound fields of initially laminar jets. The jets are at Mach number 0.9 and Reynolds number 105, and exhibit exit boundary layers characterized by Blasius velocity profiles, maximum root-mean-square (r.m.s.) axial velocity fluctuations between 0.2 and 1.9% of the jet velocity, and momentum thicknesses varying from 0.003 to 0.023 times the jet radius. The far-field noise is determined from the LES data on a cylindrical surface by solving the acoustic equations. Jets with a thinner boundary layer develop earlier but at a slower rate, yielding longer potential cores and lower centreline turbulent intensities. Adding random pressure disturbances of low magnitude in the nozzle also increases the potential core length and reduces peak r.m.s. radial velocity fluctuations in the shear layer. In all the jets, the shear-layer transition is dominated by vortex rolling-ups and pairings, which generate strong additional acoustic components, but also amplify the downstream-dominant low-frequency noise component when the exit boundary layer is thick. The introduction of inlet noise however results in weaker pairings, thus spectacularly reducing their contributions to the sound field. This high sensitivity to the initial conditions is in good agreement with experimental observations

Initial condition effects on large scale structure in numerical simulations of plane mixing layers

“…In Case WN the peak in the velocity fluctuation is predicted further downstream than in the experiment, which is typical in other simulations of shear flows. 23,39 In Case RRM, however, the location of this peak is well-predicted, although the fluctuation levels in the initial region are higher than those recorded in the experiment. At x ⇡ 0.32m both simulations attain, to within experimental error, the fluctuation level recorded in the experiment, but they attain a di↵erent self-similar state.…”

Simulation of Flow Control with Microjets for Subsonic Jet Noise Reduction

“…Numerical spectra nevertheless qualitatively well collapse with experiments especially at low angles, for which the frequency of maximum level is particularly very well reproduced. The important levels observed numerically around 5 kHz at 90 degrees, particularly visible for the isothermal jet, are explained by the initially laminar jet, which generates an additional sound source compared to an initially turbulent jet (Bogey & Bailly, 2010). As a consequence of the observations made on PSD, simulated OASPL, represented on Fig.…”

“…These very low computed turbulence levels are a consequence of the unperturbed inflow imposed on the nozzle inlet boundary and may be the reason of the discrepancies observed. Bogey (Bogey & Bailly, 2010) indeed numerically demonstrated for a M j = 0.9 and Re D = 10 5 jet that the presence of disturbances in the nozzle increase the potential core length and lower the turbulence levels on the jet axis.…”

Mach-number scaling of individual azimuthal modes of subsonic co-flowing jets

“…It is well known that several different sources contribute to the overall sound radiation from subsonic jets: (i) large scale structures mainly occurring close to the potential core region, (ii) breakdown of large scale structures into fine-scale turbulence near the end of the potential core, (iii) fine-scale turbulence within the initial shear layers of fully turbulent jets, and (iv) trailing-edge noise resulting from the interaction between flow and the solid wall at the nozzle exit. Furthermore, the importance of the initial conditions on the jet development and noise has been well documented (Hussain & Zedan 1978;Gutmark & Ho 1983;Zaman 1985;Raman, Zaman & Rice 1989;Bogey & Bailly 2010). Thus, to capture the above mentioned noise generation mechanisms and to consider realistic initial conditions for the jet, simulations in which the nozzle is included and the flow exiting the nozzle is fully turbulent are required.…”