Direct numerical simulation of turbulent flows in the slip flow regime

Direct numerical simulation was carried out to study the effect of weak rarefaction on the characteristics of turbulent flows in microchannels. Knudsen numbers, which are used to characterize the degree of rarefaction, were selected to be 0.0 (non-rarefied), 0.001, 0.005, 0.01 and 0.02, and slip-flow boundary conditions were introduced in the simulation of fluid flow to account for the effect of weak rarefaction. Turbulent statistics, including mean streamwise velocity, turbulence intensities and Reynolds shear stress, was investigated for the rarefied flow conditions. It can be concluded that the weakly rarefied turbulent flow has higher streamwise velocities over the entire channel width. Rarefaction may also lead to higher turbulence intensities and higher Reynolds shear stress in the vicinity of the wall. The budgets for the Reynolds stresses were also computed for different Knudsen numbers. The budget data reveal that most of the terms in the budget were significantly affected in the near-wall region of the channel due to the rarefaction effect.