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Holger Foysi
Aerodynamic Institute RWTH Aachen; Fachgebiet Stromungsmechanik TU Munchen, Boltzmannstr. 15, D-85748 Garching, Germany; UC San Diego, 9500 Gilman Drive, La Jolla, CA 92093, USA

Juan Pedro Mellado
Institute for Combustion Technology RWTH Aachen, 52062 Aachen, Germany; Max Planck Institute for Meteorology, Bundesstr. 53, 20146 Hamburg, Germany

Sutanu Sarkar
Mechanical and Aerospace Engineering, University of California at San Diego, 9500 Gilman Drive, La Jolla, CA 92093


Large-eddy simulations of plane and round variable density jets, as well as direct numerical simulations of plane jets were conducted using a variety of density ratios s = ρjco, which relates the jet nozzle density ρj to the freestream density ρco. The initial momentum flux was kept constant for better comparison of the resulting data. Both simulations confirm experimental results, in that the jet half-width grows linearly with streamwise coordinate x and the lighter jets decay much faster than the heavy ones. The centerline velocity decay is however different between the plane and round geometries. Whereas the round jets exhibits a decay with 1/x for all density ratios, there seem to be two self-similar scalings in plane jets, in the limit of small and large density ratios. In the limit of small density ratios or incompressible flow, Uc scales as Uc ~ 1/√x, for strongly heated jets on the other hand we find Uc ~ √(ρco/(xρc)) ~ 1/x. Using nondimensional values for x and Uc (Chen and Rodi [1980]) collapses the round jet data. Furthermore, the streamwise growth in mean density or the decay of the velocity fluctuations in the self-similar region is stronger for round jets. The round jet simulation with a density ratio of s = 0.14 shows additionally a global instability, whose frequency agrees excellently with experimental data.