A LES Study on the Evolution of Turbulent Structures in Moving Engine Geometries by an Open-Source CFD Code

The dynamics and evolution of turbulent structures inside an engine-like geometry are investigated by means of Large Eddy Simulation. A simplified configuration consisting of a flat-top cylinder head with a fixed, axis-centered valve and low-speed piston has been simulated by the finite volume CFD code OpenFOAM^®; the standard version of the software has been extended to include the compressible WALE subgrid-scale model, models for the generation of synthetic turbulence, some improvements to the mesh motion strategy and algorithms for LES data post-processing. In order to study both the initial transient and the quasi- steady operating conditions, ten complete engine cycles have been simulated. Phase and spatial averages have been performed over cycles three to ten in order to extract first and second moment of velocity; these quantities have then been used to validate the numerical procedure by comparison against experimental data. Complex unsteady features of turbulent fields like laminar-to-turbulent transition and tumble vortexes evolution have been studied either by time-resolved analysis and Proper Orthogonal Decomposition (POD). In addition, the cycle-to-cycle variations of flow field due to turbulent unsteadiness has been investigated by statistical analysis. Despite the present study has been carried out on a simplified geometry, it shows that LES can be a reliable tool not only for predicting averaged quantities (mean velocity and Reynolds stresses), but also to reproduce the dynamic behavior of complex turbulent structures in IC engines.