Large Eddy Simulation of In-Cylinder Flows

The internal combustion engine is an important part of the power generation world wide. In order to improve its efficiency and to decrease the generated emissions, a better understanding about the air flow within the cylinders of the engine is required. Large eddy simulation (LES), a computational method that has become feasible in the study of the internal combustion engines only during last decade, allows one to obtain a detailed view of different in-cylinder flow phenomena. Another advantage of LES is that it can be used to study unwanted variations in the mean quantities between different cycles, a phenomenon called cycle-to-cycle variation (CCV). The present dissertation considers the use of LES in the study of fluid flow within a single cylinder of an engine. The dissertation belongs to the field of computational physics, or more specifically, to computational fluid dynamics. The objectives of the dissertation are to study the requirements for the reliable LES of the in-cylinder flow and to demonstrate how to utilise the obtained data in order to study the CCV. In order to reach the objectives, four different flow cases were considered: a static valve-cylinder assembly, a realistic single-cylinder configuration, a turbulent pipe flow, and a simplified engine in the form of a non-static valve-cylinder assembly. The requirements for the reliable LES of in-cylinder flows were studied by concentrating on the resolution of the computational mesh, the suitability of the selected subgrid scale modelling approaches, and the effects of the non-stationary mesh on the outcome of the simulation. The overall in-cylinder flow features and the CCV were studied using velocity statistics, flow modifications, and nonlinear time series analysis. The results of the dissertation indicate that LES of an in-cylinder flow can be carried out reliably if the simulation is set up correctly. However, a very fine mesh is required especially in the near-wall region and the mesh motion can affect the results in a manner that is not consistent with mesh resolution. As a result of the present study, a new method has been demonstrated for evaluating the quality of moving mesh simulations. The results of the non-static valve-cylinder assembly simulations indicate that both the intake and the in-cylinder regions affect the CCV and that the CCV in the studied flow is a chaotic phenomenon.