Application of periodic boundaries in freight train aerodynamic performance simulations

Abstract

The length of long marshalling freight train and the non-streamline shape of wagon lead to more difficulties in simulating the aerodynamic performances. To simplify the simulation process of long freight trains and conserve computational resources, periodic boundary conditions and the improved delayed detached eddy simulation based on the shear-stress transport k-ω turbulence model were employed in this study. To better analyze the aerodynamic coefficients of full-scale, nine-boxcar, and periodic-boundary simulations, the pressure and viscous drag were analyzed separately. Aerodynamic coefficients and surrounding flows of freight trains obtained from periodic-boundary, full-scale, and nine-boxcar simulation methods are compared in detail to investigate the feasibility of the periodic-boundary condition for numerical simulations of long freight trains. The dependence of the computational mesh was verified in the STAR-CCM + software. Simulation results show that the deviations of drag coefficient under the periodic-boundary and full-scale conditions is 7.6 %, and that of the nine-boxcar and full-scale conditions is 5.7 %. Based on the simulated flow fields, the variations of pressure and velocity fields around the test wagon are similar in these three simulations. Besides, the distribution of detached vortices around the test wagon are similar in simulations. Moreover, locomotive has no significant influence on aerodynamic performances of the fifth wagon in the simulation of one locomotive with nine boxcars. In this study, the computing time of full-scale condition is 58 hours, and that of periodic-boundary condition is one-tenth of full-scale condition. Consequently, it can be concluded that the periodic boundary is a potential choice in aerodynamic simulations of a long freight train, better balancing the simulation accuracy and efficiency.