Numerical analysis of aerodynamic noise of a high-speed pantograph

Shi Lei1, Zhang Chengchun1*, Wang Jing1,2, Ren Luquan1

1 Key Laboratory of Bionic Engineering, Ministry of Education, Jilin University, Changchun 130022, China; 2Faculty of Agriculture, Jilin University, Changchun 130062, China

*Corresponding author: jluzccc@163.com

Abstract—The aerodynamic noise of pantograph is one of the most important noise sources of high speed trains. Therefore, the reduction of the aerodynamic noise of pantograph becomes very important for reducing the total noise level of the high-speed trains. In this paper, the distribution of dipole sound sources at the surface of a single-arm pantograph and the noise radiation from it are predicted numerically by the combination of incompressible fluid flow analysis using the finite element method (FEM) and acoustic analysis using the boundary element method (BEM) at speed of 300km/h. The objectives of this calculation are to locate the noise sources on the surface of this type of pantograph and to understand the mechanism of the aerodynamic noise radiation. The analysis results indicate that: the aerodynamic noise of the high-speed pantograph is induced by the vortex shedding; the main noise sources of this single-arm pantograph are the pantograph head, knee, arms around knee and foot region; the aerodynamic noise of this type of pantograph is mainly the low-frequency and mid-frequency noises.

Keywords- high speed train, pantograph, aerodynamic noise, computational fluid dynamics CFD , boundary element method BEM

I. INTRODUCTION As the maximum train speed increases, aerodynamic

noise from high speed trains causes a significant environmental problem. Some studies have shown that aerodynamic noise becomes significant above 300 km/h and can become predominant with the reduction of the contribution of rolling noise[1-3]. It became known that one of the major noise sources was the aerodynamic noise generated by pantographs in the case of high-speed trains. The aerodynamic noise of pantograph increases dramatically with the train speed and it is one of the most important noise sources at 300 km/h. If the speed of train increases further, the pantograph aerodynamic noise will become even more important[2-4]. Therefore, it is of great important to implement low-noise pantographs on future high-speed trains. In order to reduce the aerodynamic noise of pantograph, a number of improvements to pantograph itself have been made in Germany [5-7] and in Japan [8-11].

Before carrying out pantograph optimizations to reduce the aerodynamic noise, it is necessary to localization of the noise sources and clarify the characteristics of the aerodynamic noise generated by high-speed pantographs. The wind tunnel test is one of the most useful measures for the purpose. The noise sources of pantographs were therefore investigated through aeroacoustic measurements on a full-scale pantograph in the German Dutch Wind

Tunnel (DNW) by means of the acoustic mirror technique [12], and two other type of full-scale pantographs were measured at wind velocity of 300km/h(83.3m/s) and 360 km/h(100m/s) in the Japan Railway Technical Research Institute’s(RTRI) large scale anechoic wind-tunnel by using a microphone array system[9,10]. The results all indicated that the strong sources of aerodynamic noise are concentrated at the foot and the pantograph head with another compact source at the pantograph knee. The oblique arms do not con tr ibu te s ign if ican t ly to the noise rad ia t ion .

Though the wind tunnel tests have been carried out for some years, to investigate the distribution of sound pressure at the pantograph surface is limited. In order to carry out effective countermeasures against future single-arm pantograph noise, the pantograph surface noise sources must be located and the noise radiated from it must be investigated. There are few applications of the acoustic analysis to the aerodynamic noise radiation problem from a high-speed pantograph. In this paper, the distribution of dipole sound sources at the surface of a single-arm pantograph and the noise radiation from it are predicted numerically by the combination of incompressible fluid flow analysis using the finite element method (FEM) and acoustic analysis using the boundary element method (BEM) at a wind speed of 83.3m/s (300km/h). The objectives of this calculation are to identify the aerodynamic noise sources on the surface of high-speed pantograph and to understand the noise generation mechanism.

II. FLUID FLOW ANALYSIS The aerodynamic noise emitted from a pantograph in

uniform flow is predicted by the following two step method ignoring coupling between acoustics and the flow field. First, the unsteady incompressible fluid flow analysis (FEM) of a noise source on a pantograph surface, and next, the acoustic analysis (BEM) of noise propagation from the pantograph surface to far field. The fluid flow can be treated as incompressible and the quadrupole noise from the turbulent flow of the surface can be neglected because the Mach number is small enough in this calculation. The dipole noise radiated from a pantograph surface is the principal noise source.

A. Fluid flow analysis model The SST-based Scale-Adaptive Simulation model

(SST-SAS model)[13] is adopted for the flow simulation. The Scale-Adaptive Simulation (SST) concept is based on the introduction of the von Karman length-scale into the turbulence scale equation. The information provided by the von Karman length-scale allows SAS models to

2013 Fourth International Conference on Digital Manufacturing & Automation

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DOI 10.1109/ICDMA.2013.198

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dynamically adjust to resolved structures in a URANS simulation, which results in a LES-like behavior in unsteady regions of the flowfield. At the same time, the model provides standard RANS capabilities in stable flow regions.

The governing equations of the SST-SAS model are

���

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��

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jk

t

jkj

j xk

xkcPkU

xtk

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� �

� �jjj

t

j

SASkjj

xxkF

xx

QPk

aUxt

��

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��������

��

1212

1

2

(2)

Where � is the turbulence eddy frequency, 2�� is the

�� value for the ��k regime of the SST model, SASQ is the SAS source term

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���

�

��

��

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�� 0,1,1max2max 22

Φ

22

2jjjjK

SAS xk

xk

kxxkC

LLSQ ��

������

�

(3)

Where ,51.32 �� 3/2�� , 2�C , L is the length scale of the modeled turbulence

� ��� �� 41/ ckL (4)

and the von Karman length scale KL� given by

USL K ��

��

� (5)

is a three-dimensional generalization of the classic boundary layer definition

� � � �yUyULBLK ���� /�� (6)

The first velocity derivation � �yU � is represented in

kL� by S , which is a scalar invariant of the strain rate

tensor ijS :

ijijSSS 2� ,���

�

���

�

��

���

�i

j

j

iij x

UxUS

21 (7)

The same S also directly participates in SASQ (Equation 3)

and in the turbulence production term 2SP tk �� .

The second velocity derivative � �yU �� is generalized to 3-D using the magnitude of the velocity Laplacian:

� ��

���

�

���

���i jj

i

xxUU

22

(8)

As a result, L and KL� are both equal to � �y� in the logarithmic part of the boundary layer, where 41.0�� is the von Karman constant.

B. Computational procedure In order to obtain results of a high level of quality, a

detailed geometry of a single-arm pantograph with only simplifying the foot region was used, and the unsteady simulation was carried out at full scale. The fluid flow

around a pantograph model is numerically solved in the region shown in Figure 1.

Figure 1. Schematic diagram of the computational domain around a pantograph model

The computational domain (L W H) is 8 4 3m.

The pantograph was placed on the computational domain bottom boundary, and the computational domain top boundary was located at 4m from the bottom boundary. The inlet and outlet exit were placed, respectively, at -3.5m and 4.5m from the pantograph. The boundaries in z-direction were placed, respectively, at -2m and 2m from the pantograph. At the upstream boundary of the inlet, a uniform velocity 83.3 m/s (300 km/h) without turbulent fluctuation was prescribed. At the downstream boundary, the fluid traction was assumed to be zero (traction free condition). On the pantograph surface, non-slip condition was prescribed to simulate pantograph model.

Figure 2. Grids around the pantograph

Figure 2 shows the grids around the pantograph. The flow calculation of the pantograph was carried out on unstructured Cartesian hybrid grids with prismy elements and tetrahedral elements. There are 3 layers of prisms over the surface of the pantograph model. The mesh behind the pantograph is refined. The number of nodes is 2 543 964, and the number of elements is 10 637 988. In the present work, the flow simulation was obtained utilizing the software package, ANSYS CFX. The solution is time-advanced using an implicit and second-order accurate scheme. A time-step of Δt = 5 10-5s was used for the physical timescale. Starting from the zero initial flow field for the velocities and pressure, the flow were computed until it reached to a statistical equilibrium state. After that, data were sampled for the BEM acoustic analysis.

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C. Fluid flow analysis results

Figure 3 Vortex shedding from the pantograph

Figure 3 shows the complicated structure of the pantograph head, knee and foot lead to strong unsteady flow separations. As pantograph aerodynamic noise generation is mainly due to vortex shedding around components of the pantograph, the unsteady flow separation from the pantograph could be isolated as a particularly strong and compact source of aerodynamic noise.

III. ACOUSTIC ANALYSIS

A. Acoustic analysis model The noise propagation problem with time-harmonic

load is described by the Helmholtz equation � � � � � �zyxωqρzyxpkzyxp ,,j,,,, 0

22 ���! (9) with boundary conditions � � � � ppp Ωr rr "� (10)

� � � � � � vn0

n Ωr rnrjr "�

��

� ���

� p (11)

� � � � � � � � � �Z

pZZp Ωrnrrjrrr

0n "

��

�����

� (12)

� � � � 0rjrrrlim

r�

���

��

��

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kpp (13)

where 2

2

2

2

2

22

zyx ��

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�! , p is the acoustic

pressure, cfck /2/ %� �� is wave number, f%� 2� is angular frequency, f is frequency Hzcorresponding wave length fcck //2/2 ��� �%%& , p٠is

the acoustic pressure boundary condition, � �rp is the given acoustic pressure value on pΩ , vΩ is the velocity boundary

condition , n is normal direction on boundary, � �rn� is

normal direction velocity on vΩ , ZΩ is the acoustic

impedance boundary condition � �rZ is given acoustic impedance value on ZΩ .

B. Computational procedure

Figure 4 Acoustic boundary elements of pantograph (portion)

Figure 5 Acoustic analysis model

This calculation was carried out using the Virtual.Lab Acoustic code. We impose the boundary condition of prescribed pressure ( � � � �rr pp � condition) on the pantograph surface by applying a discrete Fourier transformation to the pressure fluctuation data of fluid flow analysis. Figure 4 shows the acoustic boundary elements of pantograph. The element size L is 8mm, satisfy the condition:

max6 fcL ' (14)

Where, c is the speed of sound, maxf is the highest frequency . This calculation was performed at FFT frequencies up to 2 kHz. Acoustic analysis model and observation points of sound pressure are provided on surfaces shown in Figure 5.

C. Acoustic analysis results Figure 6(a-f) shows the sound pressure levels on the

pantograph surface in 63 Hz band, 125 Hz band, 250Hz band, 500Hz band, 1000Hz band and 2000Hz band respectively. From these results, we observe that the aerodynamic noise is mainly radiated from the pantograph head, the pantograph knee, the oblique arms around the pantograph knee and the foot region. Good agreement is obtained between the analysis results and the wind tunnel measured data, which supports the validity of our analysis. Besides, the analysis results indicate that the oblique arms around the pantograph knee of this single-arm pantograph are also the main noise source in low frequency.

The main noise source is created where the strong vortices that are shed from. The pantograph head, the pantograph knee, the oblique arms around the pantograph knee and the foot region being the most complex and turbulent region and thus the origin of the most important noise generation, as shown in Figure 3. In order to reduce the aerodynamic noise, it is important to find out the best configuration of the pantograph for reducing the vortex

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shedding from these places and weakening the vortex interaction with the pantograph surface.

(a) 63Hz band

(b) 125Hz band

(c) 250Hz band

(d) 500Hz band

(e) 1000Hz band

(f) 2000Hz band Figure 6 Distribution of noise sources on pantograph dB(RMS)

Figure 7(a-f) shows the sound pressure levels on

acoustic field in in 63 Hz band, 125 Hz band, 250Hz band, 500Hz band, 1000Hz band and 2000Hz band respectively. From these results, we observe that the low-frequency and mid-frequency components of aerodynamic noise radiated from the pantograph are larger than that of the high-frequency components. The pantograph therefore needs acoustic optimization in order to limit the low-frequency and the mid-frequency noises.

(a) 63Hz band

(b) 125Hz band

840

(c) 250Hz band

(d) 500Hz band

(e) 1000Hz band

(f) 2000Hz band

Figure7 Acoustic field sound pressure dB(RMS)

IV. CONCLUSIONS In this paper, numerical simulation was used to predict

the aerodynamic noise from a single-arm pantograph at a wind speed of 83.3m/s (300km/h). The approach used was to divide the calculation in to an unsteady incompressible flow analysis using the finite element method (FEM) and an acoustic analysis using the boundary element method (BEM). We analyzed the results and the following major conclusions were obtained

(1) The aerodynamic noise of the high-speed pantograph is mainly induced by the vortex shedding.

(2) The main noise sources of this single-arm pantograph are the pantograph head, knee, arms around knee and foot region.

(3) The aerodynamic noise radiated from this single-arm pantograph is mainly the low-frequency and mid-frequency noises.

ACKNOWLEDGEMENTS

The authors gratefully acknowledge the support of NSFC Projects of Joint fund of high-speed Train Basic Research (Grant No. U1134109), the National Natural Science Foundation of China (Grant No.51106062), the National Natural Science Foundation of China (Grant No.51206058), and Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20110061120048).

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