Grid Independence Validation and Numerical Simulation of an Opposed-Piston Free-Piston Engine

To evaluate the grid independence of the in-cylinder numerical simulation, the combustion chamber model of an opposed-piston free-piston engine was divided into three styles: structured, unstructured, and hybrid meshes. Furthermore, the grid independence of the hybrid mesh was analyzed using different grid resolutions. In order to avoid the influence of subjective judgments, the grid resolution is introduced to characterize the degree of grid refinement, combined with the grid convergence index (GCI) to assess the convergence of simulation. The results show that there are differences in the flow fields for the different grid styles. In addition, the hybrid mesh captured more small-scale swirls around the intake and exhaust ports. When the resolution reached 12, the GCI was less than 6%, indicating that the numerical simulation was impervious to the grid resolution. Moreover, the mixing distribution hardly changed when the resolution reached 12.