Takeshi Sueki

Application of porous material to reduce aerodynamic sound from bluff bodies

Aerodynamic sound derived from bluff bodies can be considerably reduced by flow control. In this paper, the authors propose a new method in which porous material covers a body surface as one of the flow control methods. From wind tunnel tests on flows around a bare cylinder and a cylinder with porous material, it has been clarified that the application of porous materials is effective in reducing aerodynamic sound. Correlation between aerodynamic sound and aerodynamic force fluctuation, and a surface pressure distribution of cylinders are measured to investigate a mechanism of aerodynamic sound reduction. As a result, the correlation between aerodynamic sound and aerodynamic force fluctuation exists in the flow around the bare cylinder and disappears in the flow around the cylinder with porous material. Moreover, the aerodynamic force fluctuation of the cylinder with porous material is less than that of the bare cylinder. The surface pressure distribution of the cylinder with porous material is quite different from that of the bare cylinder. These facts indicate that aerodynamic sound is reduced by suppressing the motion of vortices because aerodynamic sound is induced by the unstable motion of vortices. In addition, an instantaneous flow field in the wake of the cylinder is measured by application of the PIV technique. Vortices that are shed alternately from the bare cylinder disappear by application of porous material, and the region of zero velocity spreads widely behind the cylinder with porous material. Shear layers between the stationary region and the uniform flow become thin and stable. These results suggest that porous material mainly affects the flow field adjacent to bluff bodies and reduces aerodynamic sound by depriving momentum of the wake and suppressing the unsteady motion of vortices.

Aerodynamic Noise Reduction of a Pantograph by Shape-Smoothing of Panhead and Its Support and by the Surface Covering with Porous Material

To reduce aerodynamic noise generated by a pantograph, which is one of the dominant noise sources of high-speed trains, the authors proposed some noise reduction techniques, that is, shape-optimization of a panhead, relaxation of aerodynamic interference between panhead and articulated frame and surface covering with porous material. To evaluate total noise reduction effect of them, wind tunnel tests were performed with a prototype pantograph to which these techniques were applied. The test results show that noise level of the prototype pantograph is lower than that of the currently-used pantograph by about 4 dB. Furthermore, it was also confirmed that the prototype pantograph has enough aerodynamic stability against change of attack angle.

Aerodynamic Noise Reduction of a Pantograph Panhead by Applying a Flow Control Method

To reduce aerodynamic noise generated by a pantograph panhead, a flow control method is applied to the panhead. First, a plasma actuator is applied to the panhead surface to verify that flow control can control flow separation and reduce turbulence intensity behind the panhead. Second, based on the flow control mechanism derived from the experiments applying the plasma actuator, a more practical flow control method is proposed using air suction as an alternative. Experimental results show that air suction near the separation point can reduce narrow band aerodynamic noise from the panhead.

Recent Studies on Aerodynamic Noise Reduction at RTRI

As the maximum speed of high-speed trains increases, the effect of aerodynamic noise on the sound level at the wayside of the track becomes important. When the surface of bluff bodies such as pantographs is covered with porous materials, the aerodynamic noise generated by unsteady motion of vortices is significantly reduced. Experimental evaluation techniques of instantaneous flow fields using time-resolved PIV enable prediction of sound in the far field based on the theory of vortex sound. Aerodynamic noise emitted from a partial model of a pantograph is predicted numerically by coupling the calculation of unsteady flow with the evaluation of acoustical behaviour. The simulation succeeds in giving detailed information on the structure of aerodynamic sound sources.