A general form of the stress-strain constitutive relation was introduced for the application of two nonlinear k-ε turbulence models, namely, the ARS model of Gatski and Speziale ([1]) and the Cubic model of Lien et al. ([2]), to the numerical analysis of flow fields in a test engine with flat-piston and bowl-in-piston arrangements, under swirling or no-swirling flow motored conditions. The model capabilities in capturing turbulent flow features were compared to those of the upgraded linear RNG k-ε model which was previously indicated as a good compromise between accuracy and computational cost ([3]). Evaluations were made on the basis of the predicted flow evolution throughout the whole engine cycle, as well as of the comparison between numerical and experimental results. Furthermore, the effect of the stress-strain relationship on the predicted averaged turbulence quantities and anisotropy invariant values were examined, in addition to the sensitivity of the nonlinear models to the mesh quality. Finally, prospects concerning possible improvements of turbulence Eddy Viscosity Models (EVM) were presented. The predictions were made by a newly developed CFD code embedding various accuracy-order finite-volume discretization schemes. Modified wall boundary conditions with respect to the conventional logarithmic-function approach were used, so as to give negligible importance to the local equilibrium hypothesis.

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