In order to reduce the aerodynamic noise generated from bogies in high-speed trains, deflectors attached at the leading and trailing edge of the bogie cavity are expected to be one of the effective mitigation measures. In past studies, the deflectors reduced the aerodynamic bogie noise, but the reduction mechanism still remains unclear. Then, we measured the flow field around a bogie with deflectors by using PIV technique. It was found that the deflectors deflect the air flow downward to the ground and the flow velocity at the bottom line of the vehicle decreased to 50% compared with the baseline condition without the deflectors. This effect of the deflectors could be the reduction mechanism of the aerodynamic bogie noise. As another mitigation measure, based on the physical understanding that a free shear layer develops along the bottom line of the vehicle in the bogie, we proposed a new design guideline for the vertical location of main motors and gear boxes in the bogie. From a wind tunnel test, it was found this bogie components arrangement could reduce the aerodynamic bogie noise by more than 2dB by avoiding the free shear layer.
