Analysis of in-cylinder flow structures and turbulence in a laboratory scale engine using direct numerical simulations

In-cylinder flow structures and turbulence characteristics are investigated using direct numerical simulations in a laboratory-scale engine at technically relevant engine speeds (1500 and 2500 rpm at full load). The data are computed for 12 compression-expansion cycles at each engine speed with initial conditions derived at intake valve closure from precursor large eddy simulations, which have been validated against experimental data. Analysis of the tumble ratio indicates significant cycle-to-cycle variation, with lower variability found at higher engine speed. The process of tumble breakdown, quantified by the evolution of mean and turbulent kinetic energy, reveals distinct features between operating conditions, with delayed turbulence peaks observed at lower engine speed. Analysis of the Reynolds stress tensor demonstrates higher stress values at higher engine speed, with pronounced anisotropy near the walls and higher values around the tumble vortex core. Examination of the anisotropic Reynolds stress invariants through Lumley triangles reveals predominantly isotropic turbulence during mid-compression, transitioning to distinct anisotropic states near top dead center. The lower engine speed exhibits a stronger tendency toward one-component turbulence due to partial tumble dissipation, while the higher speed maintains a more balanced anisotropy. These findings extend previous numerical and experimental studies on turbulence development and in-cylinder flow structures during the compression stroke, providing insights for improving turbulence modeling in practical engine simulations.