Importance of the nozzle-exit boundary-layer state in subsonic turbulent jets

Guillaume A Brès , Peter Jordan (1) , Vincent Jaunet (1) , Maxime Le Rallic , André Cavalieri (1) , Aaron Towne (2) , Sanjiva K. Lele (3) , Tim Colonius (4) , Oliver Schmidt (5)

1 PPrime [Poitiers] - Institut Pprime [UPR 3346]
2 CTR - Center for Turbulence Research [Stanford]
3 Department of Aeronautics and Astronautics and Department of Mechanical Engineering
4 CALTECH - California Institute of Technology
5 IFW Dresden - Leibniz Institute for Solid State and Materials Research

To investigate the effects of the nozzle-exit conditions on jet flow and sound fields, large-eddy simulations of an isothermal Mach 0.9 jet issued from a convergent-straight nozzle are performed at diameter-based Reynolds number 1 × 10 6. The simulations feature near-wall adaptive mesh refinement, synthetic turbulence, and wall modelling inside the nozzle. This leads to fully turbulent nozzle-exit boundary layers and results in significant improvements for the flow-field and sound predictions, compared to those obtained from the typical approach based on laminar flow in the nozzle. The far-field pressure spectra for the turbulent jet match companion experimental measurements, which used a boundary layer trip to ensure a turbulent nozzle-exit boundary layer, to within 0.5 dB for all relevant angles and frequencies. By contrast, the initially laminar jet results in greater high-frequency noise. For both initially laminar and turbulent jets, decomposition of the radiated noise in azimuthal Fourier mode is performed and the results show similar azimuthal characteristics for the two jets. The axisymmetric mode is the dominant source of sound at the peak radiation angles and frequencies. The first three azimuthal modes recover more than 97% of the total acoustic energy at these angles and more than 65% (i.e., error less than 2 dB) for all angles. For the main azimuthal modes, linear stability analysis of the near-nozzle mean-velocity profiles is conducted in both jets. The analysis suggests that the differences in radiated noise between the initially laminar and turbulent jets are related to the differences in growth rate of the Kelvin-Helmholtz mode in the near-nozzle region.

Domaines: Milieux fluides et réactifs

Guillaume A Brès, Peter Jordan, Vincent Jaunet, Maxime Le Rallic, André Cavalieri, et al.. Importance of the nozzle-exit boundary-layer state in subsonic turbulent jets. Journal of Fluid Mechanics, 2018, 851, pp.83-124. ⟨10.1017/jfm.2018.476⟩. ⟨hal-02378471⟩