Computational Study of Joint Effects of Shear, Compression and Swirl on Flow and Turbulence in a Valveless Piston-Cylinder Assembly

摘要

The potential of single-point turbulence closure models for predicting the flow aerodynamics and turbulence in internal combustion engines (IC) was investigated by computational study of idealized valveless piston/cylinder configurations. The main flow cases considered are the swirling flow in a single-stroke rapid compression machine (RCM) with flat and bowl-shaped cylinder head, as well as cyclic compression. Although still remote from a real engine, these configurations enable to analyze joint effects of major phenomena governing the aerodynamics in IC engines: shear, separation, swirl and compression/expansion. Prior to the computation of these engine-like flows, an extensive validation of applied turbulence models was performed in homogeneous and wall-bounded shear flows, each featuring separately rotation, swirl and mean flow compression effects. A second-moment (Reynolds-stress) closure model with low-Re-number and near-wall modifications, validated earlier in a variety of generic flows, was consistently applied to all cases considered. The basis for the analysis is a set of dedicated experiments and direct numerical simulations (DNS), which were performed in parallel for some of the configuration considered. In addition, other experimental results found in the literature were also used for model validation. Results of computational modeling show that the time-dependent, low-Re-number second-moment closure is capable of predicting the flow and turbulence dynamics in most cases considered, well in accord with the experimental and DNS data. In contrast, the results obtained with eddy-viscosity models, which are generally used in industrial computations, show in most cases poor quality. Some fundamental deficiencies of the latter models, related to engine flows, are also discussed.