Abstract:To improve the bottom flow field structure and further reduce the aerodynamic drag of the high-speed train， based on the idea of bottom flow control， a triangular cross-sectional deflector located before and after the bogie cabins on the train bottom is designed， and its aerodynamic drag reduction characteristics are studied. Taking the open line running high-speed train with three coach formation CRH380B at the speed of 300 km/h as the research subject， and using the Realizable k-ε turbulence model， four typical deflector installation positions are explored， and the deflector installation position with the best drag reduction effect is selected to investigate the differences of deflector drag reduction characteristics under different combinations of five angles and five heights. The drag changes on the train bodies， bogies， and bogie cabins before and after the installation of the deflectors， the pressure distribution changes， and the flow field structure changes in the bogie area are also compared. The results show that the best drag reduction effect is achieved only when the deflectors are installed in the direction of incoming flow in front of each bogie cabin in both directions. After the installation of the deflectors， the aerodynamic drag on the train bodies and bogie cabins increases， but the drag on the bogies is significantly reduced， the flow velocity and the pressure difference in the bogie areas decreases， and the bottom flow field is significantly improved. At the same time， it is found that the 15° and 100 mm combination of the deflector has the best drag reduction effect， and the drag reduction rate of the three cars reaches 7.08%. The numerical simulation proves that the bottom deflectors can effectively reduce the train running resistance.

Abstract:In order to study the flow field characteristics of a boundary layer on non-smooth car body surface, large eddy simulation and realizable turbulence model were used for the numerical simulation and calculation of external car body flow field in both transient-state and steady-state. Then, the flow field parameters, such as velocity, thickness of viscous sub-layer, wall shear stress, surface friction coefficient, turbulence intensity and turbulence dissipation rate within the boundary layer of both non-smooth and smooth model, were compared and analyzed. The influence of the non-smooth surface upon the flow field characteristics of the car body was analyzed. The results show that the velocity within the boundary layer of the non-smooth model is obviously higher than that of the smooth model, and boundary layer thickness, wall shear stress, local frictional resistance coefficient, turbulence intensity and turbulence dissipation rate are also less than the smooth model. The introduction of non-smooth surface contributes to the mixing effect of the wake flow of the carbody, preventing the ejector effect upon outside high-speed flow to internal low-speed flow, thus reducing the loss of energy of the car body flow field.

Abstract:This paper investigated the effect of rear end deflectors on the near wake and aerodynamic drag of an Ahmed model with 25° slant angle. Drag reduction was compared for deflectors mounted on the two side faces and on the upper edge of the rear slant. The width of deflectors is 5mm, 10mm and 15 mm, respectively. The model scale is 1∶2, and the Reynolds number based oncoming flow velocity and model length is 8.7×105. The results have revealed that there is a pair of organized trailing vortices in the wake, which are accompanied by strong downwash flow. There is a D-shape flow separation zone on the slant face. Deflectors with a width of 5 mm at the two sides of the slant have negligible effect on the near wake, and slightly increase the aerodynamic drag by about 2.1%. On the other hand, all tested horizontal deflectors at the top edge of the slant and deflectors with a width of 10 mm and 15 mm at both sides of the slant considerably weaken the trailing vortices. The horizontal deflectors avoid flow re-attachment on the slant and suppress the D-shape separation zone, corresponding to the maximum drag reduction rate of 11.8%, which is higher than that of the deflectors on both sides of the slant.

Abstract:In order to study the influence of size and structure of non-smooth surface on vehicle aerodynamics, we took MIRA stepped-back model as the research object, and studied the aerodynamics of three different models by using CFD method and wind tunnel experiment. Compared with smooth surface, the principle of resistance reduction was investigated. Simulation results show that the resistance reduction effect of non-smooth units originally arranged at the trunk deck，the rear of the car and underbody, is best with the ratio of 5.90%. Non-smooth surface shows good ability to reduce model pressure drag through refining eddy of the rear of body. Meanwhile, air velocity and body frictional resistance were reduced by changing the air flow of the near wall.