Turbulent flow mechanisms in mixing T-junctions by Large Eddy Simulations

Abstract

We consider the turbulent mixing process in two T-junction geometries as simplified models for mixing in the intake manifolds of Internal Combustion (IC) engines. These junctions have square and circular cross-sections, respectively. The turbulent flow structures and modes are analyzed by Large Eddy Simulations (LES). A grid sensitivity study is performed and the velocity field and the mixing scalar are compared to experimental data. The agreement is good for high enough mesh resolutions. Furthermore, the LES results are compared to unsteady Reynolds averaged Navier–Stokes (URANS) results, in order to gain an understanding of the shortcomings associated with URANS. The secondary structures found in both geometries include Dean-like vortices due to flow curvature in the region of the junction. Further downstream of the junction, these vortices are dissipated and due to an upward motion of the bulk flow, new vortical structures are generated. These downstream vortical structures rotate in the opposite direction relative to the upstream ones and govern the mean scalar distribution far downstream of the junction. We find also that the URANS results show qualitatively different flow structures leading to different scalar distributions as compared to experimental and LES results. The mixing quality is studied using a uniformity index showing a more uniform and faster mixing in the circular cross-section case. Spectral analysis of the LES data show for both geometries a shear layer instability with a dimensionless frequency in the order of unity. Additionally to that, vortex-shedding phenomena are observed in the circular case at St≈0.5.