Kiyoshi Nagakura

Methods to Measure Acoustic Sources in a Closed Wind Tunnel Test Section

The aim of this work is to develop a method to localize aeroacoustic noise sources in a closed test section of a wind tunnel by using 2-D microphone arrays. In the acoustic measurement in the closed test section, it is important to reduce a boundary layer noise and a flow-induced self-noise of microphones. We calculated the cross spectrum between the outputs of the two sets of 2-D microphone arrays (Cross Spectrum Method hereinafter referred to CSM) to suppress the influence of the boundary layer noise and the flow-induced self-noise and to improve the S/N ratio. We installed a jet noise source device as an acoustic calibrator at the center of the cross section and measured the acoustic source position at the wind velocities up to 300km/h by using the CSM and the Conventional Phased Array Method (Summation Method hereinafter referred to SMM). The results indicated that the acoustic source could be identified more clearly by the CSM than by the SMM.

Experimental and Theoretical Studies on Impact Noise Generation due to Rail Joints

When a train passes over discontinuities on a rail, e.g. rail joints, impact noise due to the discontinuities is generated impulsively. In this paper, an attempt is made to understand the impact noise generation mechanism experimentally and theoretically. By performing static and running tests, vibratory properties of the track and wheel are investigated first. Second, on the basis of the measurements in the tests, a theoretical model to predict impact noise is developed.

Estimation of Aerodynamic Bogie Noise Through Field and Wind Tunnel Tests

A method has been developed to determine the aerodynamic noise from the bogie of a high-speed train using a two-dimensional microphone array in a wind tunnel. First, the flow velocity under the train car in the rail direction was measured in a field test. Second, the flow distribution was simulated in the wind tunnel. Third, aerodynamic noise generated by a bogie was estimated from the source distribution measured using a two-dimensional microphone array. Finally, the noise generated from the lower part of the car, which was obtained by adding the aerodynamic noise and the rolling noise estimated by the TWINS model, was compared with the result obtained from the field test. It was found that the estimated noise shows good agreement with the measurement obtained in the field test. The estimated lower part noise levels were consistent with the actual experimental measurements. This conclusion suggests that this method is appropriate to estimate aerodynamic bogie noise quantitatively. It is also shown that below 500 Hz the contribution of the aerodynamic bogie noise is more than those of the rolling and machinery noise.

Study on Effective Sound Barriers for High Speed Trains

In order to reduce noise along railway lines, sound barriers have been widely used. However, the effects of these barriers on railway noise have not been understood thoroughly. In this paper, the relative acoustical performances of a number of sound barriers in typical Shinkansen railway situations are first investigated through laboratory measurements using scale models. In the scale model studies, the effects of various types and shapes of the sound barriers on Shinkansen noise are investigated. The results show that the most effective barrier is Y-profile type. This is because the Y-profile barrier has noticeable effects on both mitigating the noise increase due to the multiple reflections of sound between the barrier and the vehicle surface and widening the shadow zone behind the barrier because of double diffractive effects. Secondly, the effect of the Y-profile barriers is examined by field tests with running Shinkansen vehicles. The results show similar trends to those in the laboratory measurements.

Improvement of the Source Localization Accuracy by Microphone Array Adjacent to Main Flow of Wind Tunnel

In this study, we proposed a method to improve an accuracy of source localization in a wind tunnel tests by using microphone array. Based on an acoustic path model in passing through the free shear layer, the microphone array is installed adjacent to the main flow to avoid the refraction and dissipation in the free shear layer. The flow-induced noise caused by the interference with the main flow is also effectively reduced by calculating a cross spectrum between the respective outputs of 2 pairs of 2-D microphone array. The effects of these techniques are varified by the acoustic performance test with standard source device under wind condition. As a result, firstly, it was found out that the calculation method is effective even at the uniform wind velocity of 300 km/h and enables the acoustic measurement adjacent to the main flow. Secondly, accurate noise source localization can be achieved by installing the microphone array along the line extended from the side wall of the wind tunnel nozzle adjacent to the main flow. The location error by this method is improved less than 10 mm compared with that measured outside of the free shear layer.