Large-eddy simulation of variable-density turbulent axisymmetric jets

Abstract

Three cases of variable-density turbulent round jets discharging from a straight circular pipe into a weakly confined low-speed co-flowing air stream are studied with the aid of large-eddy simulation. The density ratios considered are 0.14 [Helium/air], 1.0 [air/air] and 1.52 [CO 2/air], with Reynolds numbers of 7000, 21,000 and 32,000, respectively. Detailed comparisons of the statistics show good agreement with the corresponding experiments. They confirm that a lower-density jet develops more rapidly than a denser jet with the same exit momentum flux. Pseudo-similarity behavior in the three variable-density round jets is well reproduced in the simulation. The coherent structures of the three jets are investigated by visualization of the iso-surface of pressure fluctuations and vorticity. In the developing stage of the Kelvin–Helmholtz instability, large finger-shape regions of vorticity are observed for the helium jet close to the nozzle lip. This feature, however, is not found in the air and the CO 2 jet. The occurrence of strong streamwise vorticities across the shear layer in the helium jet is demonstrated by a characteristic quantity related to the orientation of the vorticity.