How does a high density ratio affect the near- and intermediate-field of high-Re hydrogen jets?

A comprehensive investigation of the mixing properties, in the near- and intermediate-field, of fully turbulent hydrogen round jets with high density ratios has been carried out by using an in-house 3D Large Eddy Simulation model. The model employs a non dissipative sixth-order compact finite difference scheme for space discretization and a fourth-order Runge-Kutta scheme for time integration. A Localized Artificial Diffusivity model has been used both to account for unresolved sub-grid scales and to avoid numerical instabilities. The model has been validated by comparing both the centerline mean velocity properties and the turbulence statistics of a low-Mach air into air jet with experimental results. A qualitative view of the jet coherent structures and a quantitative analysis of the turbulence scales have also been provided. The model has been used to investigate the structure of two low-Mach number hydrogen jets with the same momentum flux and very high ambient to jet density ratios, up to 50, and high Reynolds number, up to about 10^5. As expected, the centerline velocity decay rate is much larger than that of the air jet, however in the near- and intermediate-field the use of the classical definition of the effective diameter fails to collapse the velocity profiles onto a single slope and a recent definition of an effective diameter given by Sautet and Stepowski is required. Furthermore, the spreading rates of the two hydrogen jets are less affected by the density ratio with respect to the velocity decay rate