Study of the Aerodynamic Noise Characteristics of Bluff Bodies as a Pantograph Member

Reduction of the aerodynamic noise generated by pantographs is a matter of great important for further speedup of the high speed trains. In general, pantographs are composed of some kind of bluff bodies. Therefore, the method to reduc e the aerodynamic noise generated by bluff bodies is demanded. In this paper, we will present the noise reduction effect of cylinder by equipping intermittent holes first. Then, we will discuss the noise reduction mechanism by estimating the approximate noise source strength. Finally, we will show the results of applying this method to an elliptic cylinder. INTRODUCTION Preservation of the environment is a matter of great importance for Shinkansen (Japanese high-speed railway). Reduction of way side noise has strongly been required. As far as the present Shinkansen is concerned, the contribution of aerodynamic noise is dominant when compared with other kinds of noises, such as rolling noise, concrete structure noise and so on. A pantograph is one of the main aerodynamic noise sources of train sets. So, researches to develop a so-called low noise pantograph have been continued. The low noise pantograph is constructed to a configuration as simple as possible to reduce noise sources. The shapes of its members should be aerodynamically-smoothed to lower the aerodynamic noise. At present, some types of low noise pantographs have already been come into use (Fig. 1). They considerably contribute to the reduction of way side noise. The currently maximum speed of Shinkansen is 300km/h. In order to survive in the keen competition with other transport facilities, we have to continue researches on speed-up. Fig. 2 presents a contour map of the aerodynamic noise distribution of the low noise pantograph PEGASUS. This measurement was performed at the large scale anechoic wind tunnel of RTRI (Maihara Wind Tunnel) using an X-array microphone system. It is clear that the panhead generates larger aerodynamic noise than the articulated system does. In other words, the main noise source of a low noise pantograph is the panhead. Fig. 1 Example of Low noise pantograph (Type PS207 of JR-East ) It is well-known that a streamline body like an aerofoil generates lower aerodynamic noise than bluff bodies. Therefore, the shape of the panhead of the low noise pantograph is smoother than that of the traditional panhead. But the stability of the aerodynamic force acting on the panhead is also strongly demanded in the high speed region. If a large aerodynamic force acts on the panhead, the contact force is not able to be in the suitable range. So, completely streamlined members are difficult to apply as a panhead. Thus, measures to reduce aerodynamic noise generated from the bluff body are the main subject to reduce the noise of pantograph. REDUCTION OF AERODYNAMIC NOISE GENERATED BY CYLINDER The most typical bluff body is a cylinder. In general, a cylinder generates remarkable narrowband noise, namely Aeolian tone. Since this noise level is very high, some kinds of measures to prevent generation of this noise have been proposed. In the case of low noise pantograph, horns of the panhead are made of cylindrical members. To prevent the generation of the Aeolian tone, the horn is equipped with intermittent holes. Our research has clarified that not only the Aeolian tone but also the broadband noise are reduced by this measure. We think that it is useful to clarify this phenomenon in order to reduce the noise of bluff bodies. Therefore, we investigated the mechanism of this phenomenon experimentally first. Equipment and Models for Wind Tunnel Testing We performed wind tunnel testing by using a small scale anechoic wind tunnel of RTRI. This wind tunnel is of the Gottingen-type having an open-type test section for aerodynamic noise measurement. The nozzle size is 480mm x 400mm and the maximum wind velocity is 300km/h (83.3m/s). Fig. 3 shows the experimental apparatus. The test model is set at the test section that has end walls (450mm x 600mm). The coordinate system is defined as shown in Fig. 3. Fig. 4 shows the test models. There are four types of models, that is, (a) simple cylinder, (b) cylinder with a spiral wire of diameter 6mm, (c) cylinder with intermittent holes (20mm x 5mm R2.5mm at the corner of each holes), and (d) cylinder with a continuous slit ( thickness : 5mm ). The diameter of each models is 25mm. The Aerodynamic noise is measured by a non-directional 1/2 inch microphone which is located at x=0mm and z=-1000mm. The flow velocity at the region of wake of the models is measured Fig. 2 Contour map of aerodynamic noise generated by a low noise pantograph ( PEGASUS 2500kHz band ) wind Nozzle 480•~400 End wall 450•~600