In-cylinder flows in internal combustion engines are highly turbulent in nature. An important property of turbulence that plays a key role in mixture formation is anisotropy; it also influences ignition, combustion and emission formation. Thus, understanding the turbulence anisotropy of in-cylinder flows is critical. Since the most widely used two-equation Reynolds-averaged Navier-Stokes (RANS) turbulence models assume isotropic turbulence, they are not suitable for correctly capturing the anisotropic behavior of turbulence. However, large eddy simulation (LES) can account for the anisotropic behavior of turbulence.
In this paper, the Reynolds stress tensor (RST) is analyzed to assess the predictive capability of RANS and LES with regard to turbulence anisotropy. The influence of mesh size on turbulence anisotropy is also looked into for multi-cycle LES. Investigations were conducted to understand the anisotropic behavior of turbulence during the intake stroke of a single-cylinder optical research engine. Turbulence anisotropy has been visualized directly in the physical domain of engine with the help of componentality contours. The turbulence anisotropy predicted by RANS simulations confirms that it is completely influenced by the gradients of mean flow velocity as per the definition of RST according to the Boussinesq approximation, whereas LES captured the anisotropic behavior of turbulence. During the intake stroke, large structures such as the intake jet and the recirculation region are shown to have an effect on in-cylinder turbulence anisotropy. In addition, a comparison of coarse mesh LES with fine mesh LES revealed that the latter predicts more isotropic turbulence due to sub-grid modeling assumptions.