Characterizing the Evolution of Boundary Layers in IC Engines by Combined Direct Numerical and Large-Eddy Simulations

The structure of boundary layers (BLs) and wall heat flux is investigated as they evolve during the compression stroke in an optically accessible, single-cylinder research engine of passenger-car dimensions with a typical four-valve pent-roof design operated at motored and throttled conditions. Three-dimensional direct numerical simulations (DNS) of the compression stroke were carried out, which enable full resolution in space and time of all flow and temperature field structures in the entire domain, including the BLs. Since the high computational cost precludes calculation of the scavenging cycle, scale-resolving simulations were employed to provide initial fields for the DNS at intake valve closure. The analysis revealed that BLs deviate from ideal scaling laws commonly adopted in algebraic wall models, and that the non-zero streamwise pressure gradient correlates with changes in the near-wall profiles. Phenomenologically, such deviations are similar to those for developing BLs, and in particular for impinging flows. The momentum BL structure was found to be affected by the large-scale bulk flow motion, in contrast to the thermal BLs which exhibit a more structured behavior following the density increase due to compression. Inspection of the heat flux distribution confirmed the similarity between the flow and heat flux patterns and identified regions of intense heat flux, mainly in locations of strong directed flow towards the wall. The improved characterization of the boundary layer structure and its evolution during the compression stroke not only constitutes an important step towards improved understanding of near-wall phenomena in internal combustion engines, but the vast dataset also serves as a database for development of improved wall models.