Aerodynamic Noise Reduction using Porous Materials and their Application to High-speed Pantographs

In Japan, it is essential to reduce wayside noise along railway lines to enable the introduction of even faster highspeed trains. Aerodynamic noise generated from such trains represents the major noise source in the high-speed range, because the acoustic power of this kind of noise is proportional to the sixth power of the train speed [1]. In general, noise barriers are placed along high-speed railway lines in Japan to prevent propagation of noise from the lower part of the car body. However, these barriers have little effect in terms of preventing noise from the upper part of the body. Aerodynamic noise caused by pantographs installed on train roofs is one of the dominant sources of noise generated by high-speed trains, and it has therefore become increasingly important to reduce this kind of noise. There are a number of methods of reducing the degree of aerodynamic noise, one of which involves modifying the shape of the structure in question. As aerodynamic noise is caused by unsteady vortices that occur in the flow [2], various structures with streamlined shapes that prevent unsteady vortex shedding have prevailed. As an example, attempts have been made to modify the shape of a panhead that forms part of a pantograph [3]. Such efforts have demonstrated that a streamlined panhead shape is effective in reducing aerodynamic noise. Another vital method is flow control. Although the modification of structural shapes is effective in reducing aerodynamic noise, such measures are limited to a certain extent due to the various functions that must be maintained. The reduction of such noise has been achieved by covering the surface of objects with fur materials [4], [5] a technique based on the reduction of vorticity. However, it is difficult with the application of fur materials to satisfy the requirements of durability against weathering in railway vehicles. We have proposed the application of porous materials as a new method of airflow control that is broadly applicable to railway vehicles. To investigate the effects of such materials, we first applied them to cylinders and measured the aerodynamic noise radiated in wind tunnel tests. Next, we applied the materials to a high-speed pantograph and evaluated noise levels and noise-source distribution in wind tunnel tests. This paper outlines the results of these tests and discusses the effects of porous materials.