Large-eddy simulation of variable-density round and plane jets

Large-eddy simulations (LES) of heated and cooled plane and round variable-density jets were conducted using a variety of density ratios s = ρ j / ρ co , which relates the jet nozzle density ρ j to the freestream density ρ co . 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 s, there seem to be two self-similar scalings in plane jets, in the limit of small and large density ratios s. In the limit of small s or for incompressible flow, U c scales as U c ∼ 1 / x , for strongly heated jets on the other hand we find U c ∼ 1 / x . A mixed scaling is proposed and shown to work nicely for both small and large density ratios s. For the round jet simulations, on the other hand, scaling x and U c by s - 1 / 4 ( Chen and Rodi, 1980 ) collapses the round jet data. Furthermore, it is shown that the streamwise growth in the mean density or the decay of the velocity fluctuations in the quasi-self-similar region, is stronger for round jets. The round jet simulation with a density ratio of s = 0.14 is seen to develop under a global instability. To the author’s knowledge, this is the first LES of a globally unstable round jet at a density ratio of s = 0.14 . The frequency agrees excellently with experimental data and with the new scaling proposed by Hallberg and Strykowski (2006) .