A numerical investigation of inter-carriage gap configurations on the aerodynamic performance of a wind-tunnel train model

The influence of different inter-carriage gap configurations, including end wall geometries (3 cases) and gap spacings (0, 5, 8, 10, 15, 20, and 30 mm), on the aerodynamic characteristics of a wind-tunnel train was investigated. The shear stress transport (SST) k-omega turbulence model was employed to determine the airflow features of the train at Re = 2.25 x 10(6). For validation, the numerical drag force and pressure distributions on the streamlined heads were compared with the experimental benchmark of wind tunnel experiment. The numerical data show that substantial variations in the flow fields, pressure distributions and aerodynamic forces are observed between the trains with and without gap spacings, no matter which configuration is employed. As the gap spacing increases, the airflow along train body rushes into the gap easily, causing the formation of vortices at the gap between the internal and external windshields. The decreasing restriction of flow in the gap also contributes to the pressure differences on the end walls. With the increase of gap spacings, the pressure on both of the first and second inter-carriage gaps is decreased, and it on the first one is a little higher than that on the second at each gap spacing. The end wall geometry affects the flow structures around the train, especially in the region below half-height of the train. This results in a difference in the boundary layer thicknesses and drag contribution in all cases. The discrepancy of end wall geometry causes a substantial variation in the aerodynamic drag between different cases. As gap spacing increases, the aerodynamic drag of the head car decreases, while those of the middle and rear cars increase significantly. When the three cases are compared, the discrepancy of the total aerodynamic drag of Case 1 is the smallest when compared to the base case with a minimum of 0.03% at 10 mm gap spacing and followed by 0.05% at 8 mm. Therefore, to determine the aerodynamic forces for high-speed trains with fully enclosed inter-carriage configuration in wind tunnel test, having a high comparative value as the actual trains, the end wall geometry in Case 1 is recommended with a gap spacing of 10 mm or 8 mm.