Large-eddy simulation of variable-density turbulent axisymmetric jets Ping Wang a, * , Jochen Fro ¨ hlich a , Vittorio Michelassi b , Wolfgang Rodi c a Institute of Fluid Mechanics, Technical University of Dresden, 01062 Dresden, Germany b Nuovo Pignone, Via Felipe Matteucci 2, 50127 Florence, Italy c Institute of Hydromechanics, University of Karlsruhe, 76128 Karlsruhe, Germany Received 6 November 2007; received in revised form 5 February 2008; accepted 9 February 2008 Available online 18 March 2008 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 devel- oping 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. Ó 2008 Elsevier Inc. All rights reserved. Keywords: LES; Variable-density flow; Axisymmetric jet; Pseudo-similarity; Coherent structure; Streamwise vorticity 1. Introduction Turbulent flows with variable density, which may be due to temperature variations stemming from reactions or vari- ations in the composition by fluids of different density, exist widely in nature as well as in technical devices. The ability to predict the turbulent mixing in flows with variable den- sity is vital for the modeling of the dynamics of such flows and a prerequisite for predicting turbulent combustion sit- uations. Unlike the extensively studied jets with constant density, variable-density jets are less well understood. Rel- atively few experimental studies were reported for such cases. An experiment with helium/air mixture discharging into a confined swirling flow was carried out by Ahmed et al. (1985). Sreenivasan et al. (1989) performed an exper- imental study on round jets of different densities issuing into the ambient air. Their different densities were obtained by premixing helium and air in various proportions. About the same time, Monkewitz et al. (1989) and Monkewitz and Pfizenmaier (1991) carried out an experimental investiga- tion of entrainment and mixing in transitional axisymmet- ric jets, where density difference were achieved by heating the air. Panchapakesan and Lumley (1993) conducted an experiment with helium injected into open quiescent air from a round nozzle. Later, Djeridane et al. (1996) and Amielh et al. (1996) performed experimental studies of var- iable-density turbulent jets, including helium, air and CO 2 jets exiting into a low-speed air co-flow. Numerical investi- gations of this type of flow are also relatively scarce. Jester- Zu ¨rker et al. (2005) performed a numerical study of turbu- lent non-reactive combustor flow under constant- and var- iable-density conditions using a Reynolds-stress turbulence model. They obtained good agreement between simulation and experiment for the constant-density flow, whereas the results for the variable-density flow were less satisfactory. Some large-eddy simulation (LES) of variable-density 0142-727X/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.ijheatfluidflow.2008.02.002 * Corresponding author. Tel.: +49 351 46334910; fax: +49 351 46335246. E-mail address: ping.wang@mailbox.tu-dresden.de (P. Wang). www.elsevier.com/locate/ijhff Available online at www.sciencedirect.com International Journal of Heat and Fluid Flow 29 (2008) 654–664