Scalar mixing at the outer edge of high density turbulent jets

• Applicant: Professor Dr.-Ing. Heinz Pitsch, since 7/2015
• Subject Area: Fluid Mechanics
• Term: from 2015 to 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 266647758

Variable density round jets are among the most important mixing devices in many engineering applications. Their most prominent feature is the fact that the density ratio between that of the jet fluid and that of the surrounding fluid determines the penetration of the jet and thereby the mixing of the two fluids. The traditionally accepted scalings of the centerline decay of velocity and scalars with the square root of the density ratio were derived for small density differences of the two fluids. For large density differences the scaling with the density ratio is essentially empirical and depends on the scaling of the entrainment at the outer edge of the jet. The fluid mechanical process by which the surrounding irrotational fluid is entrained into the vortical part of the jet is not well understood. It therefore cannot not be well enough described by existing simulation methods like Large Eddy Simulations (LES). Even for constant density jets substantial differences show up between experiments and LES simulations in conditional profiles of the mean passive scalar, as well as in the Probability Density Function (PDF) of the scalar. For high density jets well resolved experimental data are missing entirely. The present proposal therefore wants to investigate by Rayleigh scatter experiments, by Direct Numerical Simulations ( DNS ) and by LES simulations the combined effect of a large density ratio and a large change of vorticity at the outer edge of turbulent round jets on scalar mixing and the resulting entrainment. The experiments will be performed for two Reynolds numbers of sulfur-hexafluoride (SF6) jets into three different coflows, namely into pure N2, into a He-N2 mixture and into pure helium, thereby attaining density ratios up to 34.5. The experiments will use high repetition planar Rayleigh scattering to resolve, based on Taylors hypothesis, the three-dimensional density field. DNS and LES simulations will be performed for these six experimental cases with the aim to reproduce the experiments. Various LES formulations for the unresolved quantities will be tested by comparing the calculated conditional profiles and the scalar PDF with the experiments and the DNS results. The tests will use the concept of optimal estimators to obtain an optimal eddy viscosity sub-filter model with intermittency corrections. For the scalar PDF different formulations of the parameters in a previously derived composite PDF model will be chosen and the errors for the different choices will be determined.

• DFG Programme: Research Grants
• Ehemaliger Antragsteller: Professor Dr.-Ing. Norbert Peters, until 7/2015 (†)