Direct numerical simulation of the effect of compression on the flow, temperature and composition under engine-like conditions

The effect of compression on the flow, temperature and composition inside a cylinder is investigated using direct numerical simulation (DNS). The initial conditions are obtained by a separate DNS of the intake stroke in an open-valve setup which includes thermal and species mixing. The results show significant changes of the turbulence and temperature fields during compression: The decrease of kinematic viscosity resulting from the increasing pressure results in smaller turbulent length scales and higher dissipation rates. Temperature fluctuations away from the walls decrease slightly during the first half but increase strongly during the second half of the compression stroke towards the Top Dead Center (TDC) due to heat transfer to and from the walls and turbulent transport. At TDC the turbulent flow field is anisotropic, and the axial fluctuation velocity is approximately 30% smaller than the fluctuation velocities in the radial and azimuthal directions. The integral length scale of temperature is approximately 25% higher than the integral length scale of turbulent kinetic energy. The stratification in the species concentration is found to be practically negligible.