Impact of Grid Density and Turbulence Model on the Simulation of In-Cylinder Turbulent Flow Structures - Application to the Darmstadt Engine

The paper reports a wide numerical analysis of the well-known "Darmstadt engine"operated under motored condition. The engine, which features multiple optical accesses and is representative of currently made four-valve pentroof GDI production engines, is simulated using computational grids of increasing density and two widely adopted approaches to model turbulence, Reynolds Averaged Navier Stokes (RANS) and Large Eddy Simulation (LES). In the first part of the paper, attention is focused on the increase of grid density within the RANS modelling framework: both bulk-flow grid density and near-wall grid density are varied in order to analyse potentials and limitations of the different grid strategies and evaluate the trade-off between accuracy and computational cost. In the second part of the paper, an analysis is made, on equal grid density, to compare RANS and LES modelling frameworks and to highlight the improved capability of the latter in representing the spatial and temporal evolution of the in-cylinder turbulent flow structures. Results from the CFD analyses are compared with detailed PIV measurements on different planes and at different crank angle positions; furthermore, quantitative comparisons are performed using well-established quality indices.