Complexity

Volume 2018, Article ID 6932596, 12 pages

https://doi.org/10.1155/2018/6932596

## Numerical Modeling and Investigation on Aerodynamic Noise Characteristics of Pantographs in High-Speed Trains

Correspondence should be addressed to Han Xiao; nc.ude.cuo@oaixh

Received 24 December 2017; Revised 1 February 2018; Accepted 10 February 2018; Published 20 March 2018

Academic Editor: Changzhi Wu

Copyright © 2018 Xiaoqi Sun and Han Xiao. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

Pantographs are important devices on high-speed trains. When a train runs at a high speed, concave and convex parts of the train cause serious airflow disturbances and result in flow separation, eddy shedding, and breakdown. A strong fluctuation pressure field will be caused and transformed into aerodynamic noises. When high-speed trains reach 300 km/h, aerodynamic noises become the main noise source. Aerodynamic noises of pantographs occupy a large proportion in far-field aerodynamic noises of the whole train. Therefore, the problem of aerodynamic noises for pantographs is outstanding among many aerodynamics problems. This paper applies Detached Eddy Simulation (DES) to conducting numerical simulations of flow fields around pantographs of high-speed trains which run in the open air. Time-domain characteristics, frequency-domain characteristics, and unsteady flow fields of aerodynamic noises for pantographs are obtained. The acoustic boundary element method is used to study noise radiation characteristics of pantographs. Results indicate that eddies with different rotation directions and different scales are in regions such as pantograph heads, hinge joints, bottom frames, and insulators, while larger eddies are on pantograph heads and bottom frames. These eddies affect fluctuation pressures of pantographs to form aerodynamic noise sources. Slide plates, pantograph heads, balance rods, insulators, bottom frames, and push rods are the main aerodynamic noise source of pantographs. Radiated energies of pantographs are mainly in mid-frequency and high-frequency bands. In high-frequency bands, the far-field aerodynamic noise of pantographs is mainly contributed by the pantograph head. Single-frequency noises are in the far-field aerodynamic noise of pantographs, where main frequencies are 293 Hz, 586 Hz, 880 Hz, and 1173 Hz. The farther the observed point is from the noise source, the faster the sound pressure attenuation will be. When the distance of two adjacent observed points is increased by double, the attenuation amplitude of sound pressure levels for pantographs is around 6.6 dB.

#### 1. Introduction

With the rapid development of high-speed trains, the running speed of trains is increased continuously, and train bodies are developed towards a lighter weight. Meanwhile, aerodynamic problems caused by high-speed trains become more and more significant. Especially, aerodynamic problems of pantographs have drawn immediate attention of scientific researchers.

Pantographs are an important device on the top of high-speed trains. When a train runs at a high speed, concave and convex parts on the train will cause serious disturbance on airflows and make them generate complicated flow separation, eddy shedding, and breakdown. As a result, a strong fluctuation pressure field will be caused and transformed into aerodynamic noises [1]. Studied results indicate that when the running speed of high-speed trains reaches 300 km/h, aerodynamic noises will be more than wheel-rail noises, becoming the main noise source. Aerodynamic noises of pantographs occupy a very large proportion in far-field aerodynamic noises of the complete train [2, 3]. At present, the running speeds of trains on some tracks have reached 350 km/h. Aerodynamic noises of trains not only cause noise pollution in trains, but also reduce passenger comfort and seriously affect life of residents along the track [4]. Aerodynamic noises of trains are a key factor for suppressing the increase of running speed. Designed and experimental speeds of the Japanese Shinkansen train are more than 350 km/h. However, the aerodynamic noise level reaches an oppressive level. Finally, trains of this series can only run at 300 km/h. And the designed speed of Shanghai maglev trains is more than 430 km/h. Limited by the noise standard, trains can only run under 200 km/h [5, 6].

At present, researches on aerodynamic noises of pantographs are relatively underdeveloped compared with researches on the complete train structure and system. Researches on aerodynamic noises of pantographs are mainly completed by experimental test and numerical simulation. Experimental tests are divided into wind tunnel tests and full-scale model with real trains. Noger et al. [7] tested aerodynamic noises of the TGV pantograph system using wind tunnels, finding that the perpendicular face on the back of pantographs was one of the most important aerodynamic noise sources. Kitagawa and Nagakura [8, 9] tested aerodynamic noises of high-speed trains using wind tunnel and real-train tests, finding that aerodynamic noise sources included pantographs, bogies, nose tips, pilots, train heads, train tails, train connectors, and skirt plates. Mellet et al. [1] used the real-train experiment to test aerodynamic noises outside trains, finding that sound pressure levels of far-field aerodynamic noises were in approximate linear relation with the logarithm of running speed, and the contribution of pantographs to the total far-field noise ranked the second position. Gao et al. [10] used the wind tunnel test to study aerodynamic noises of high-speed trains with scaled ratio of 1 : 8, finding that noises of bogies and pantographs are major noise sources of the model, and the pantograph had large noise energies at 500 Hz~800 Hz, 2~4 kHz, and 6 kHz. In respect of numerical researches, combining computational fluid dynamics with computational acoustics was used to study aerodynamic noises, noise generation mechanism, and noise radiation characteristics of high-speed trains or main components (such as bogies, pantographs, and joints). King III [11] equalized the pantograph as a cylinder and used a dipole sound source to describe aerodynamic noises induced by cylinder eddy shedding, further analyzing far-field aerodynamic noise of pantographs and pointing out that sound pressures of pantographs were in direct proportion to the sixth power of the running speed, while the sound pressure level was in linear relation with the logarithm of the running speed. Liu et al. [12] adopted a hybrid method which obtained an equivalent aerodynamic noise source using large eddy simulation and then loaded the source on acoustic boundary elements. Characteristics of dipole noise sources on the surface of single-arm pantographs were studied in detail. Studied results indicate that main energies of the single-arm pantograph were concentrated within 100~700 Hz. When the running speed was stable, with the increased frequency, the amplitude of dipole noise source on the surface of pantographs would be decreased. When the frequency was increased from 20 Hz to 5000 Hz, the amplitude of dipole noise source under different running speeds was decreased by 30 dB. Du et al. [13] conducted a numerical analysis on aerodynamic noises of pantographs. Analyzed results indicate that carbon slide plate was a major control factor in aerodynamic noses, while the bottom frame structure was the second factor. In order to reduce aerodynamic noises of the pantograph, Ikeda et al. [14] adopted porous materials to pantographs and proposed a novel pantograph, achieving obvious noise reduction effects. Xiao and Shi [15] conducted a simulation computation for different cross section shapes of pantograph insulators. They found that insulators with elliptic cross sections whose long axis consists with the airflow direction are optimal. Yu et al. [16] designed three kinds of pantograph guide guards and conducted a noise reduction analysis based on opened running mode of pantographs, finding that noise reduction effects were obvious and sound pressure levels were decreased by 3 dB adopting this pantograph guide guard similar to air barriers.

In those published papers, just using wind tunnel or real-train tests has a high cost and low efficiency, while the repeatability of experimental results is poor. The problem has been solved very effectively by the reported numerical simulation. However, most researches fail to verify the numerical model using experimental test, and the reliability of studied results cannot be ensured. The approach of noise numerical simulation mainly depends on acoustic analogy theory and cannot conduct systematic researches on noise radiation characteristics. Aiming at these problems, this paper conducted an in-depth research on generation mechanism, sources, and radiation characteristics of aerodynamic noises for pantographs based on acoustic analogy theory and boundary element method. The reliability of numerical models is also verified by experimental test. Studied results prove that, in high-frequency bands, the far-field aerodynamic noise of pantographs is mainly contributed by the pantograph head. Single-frequency noises are in the far-field aerodynamic noise of pantographs, where main frequencies are 293 Hz, 586 Hz, 880 Hz, and 1173 Hz.

#### 2. Acoustic Analogy Theory

With the development of computer technologies, computational aeroacoustics have gradually developed into an important tool which explores aerodynamic noise mechanism, finds noise source positions, and predicts noises. The method of combining CFD and acoustic analog theory [17] (FW-H equation [18] and free space green function) is the most popular aerodynamic noise prediction method in current engineering applications. The basic idea divides computation of sound fields into two steps: firstly, CFD is used to compute near-field parts, and sound source information data is obtained; then, acoustic analog theory is used to solve propagation of sound waves from near field to far field.

Aerodynamic noises are the result of interactions between fluid and structure when fluids flow through the solid surface. As universal fluid software, Fluent integrates strong computation ability for aerodynamic noises. By solving fluid dynamics equations, Fluent can directly achieve generation and propagation of acoustic waves. The direct computation method is called CAA (Computational Aero Acoustics). Viscidity and turbulence effects are simulated accurately through directly solving nonsteady N-S equations and Reynolds average RANS equations [19–21]. CAA method requires a numerical solution method with high precision, refined meshes, and nonreflective boundary conditions, so that the computational cost is very high. At present, the method cannot be used to solve aerodynamic noise problems of high-speed trains. Another computation method in Fluent is the Lighthill acoustic analogy which is widely used and can also be called AAA (Aero-Acoustic Analogy) method [22–24]. Different from CAA method, “acoustic analogy” method decouples wave equations and flow equations. A nonsteady flow equation is solved at first. Then, solved results are taken as a noise source. An acoustic wave solution is solved by a wave equation. In this way, the acoustic wave solution is detached from flowing solution process, so that the computational efficiency is improved, and complicated aerodynamic acoustic problems can be solved. Based on mass conservation and momentum conservation equations, Lighthill obtained a wave equation of aerodynamic noises generated from turbulent flows with a small scale enclosed by static fluids, as follows:where is the disturbance quantity of fluid density, , and and are density before and after disturbance; is the Lighthill stress, and ; is the viscosity stress; is the symbol of Kronecker delta; is sound velocity. The left end of (1) is the same with those of common acoustic equations, and the right end is equivalent to a sound source item. In fact, the right end of (1) contains a variable , so (1) is not an acoustic wave equation in fact. Essentially, it is still a fluid flow equation. As pointed out by Lighthill, if the right end of equation is deemed as a quadrupole source item, (1) will become a typical acoustic wave equation, and the method can be called “acoustic analogy” method.

Based on the Lighthill equation, FW-H (Ffowcs Williams and Hawkings) applied the generalized Green function to generalize the Lighthill acoustic analogy theory into a flow noise issue with arbitrary solid boundaries, obtaining a FW-H equation which is widely applied at present [18]. The equation is as follows:In this equation, the right end of FW-H equation can also be deemed as sound source items, where the first item is a Lighthill sound source item, namely, a quadrupole sound source; the second item is a sound source caused by surface fluctuation pressure (force distribution), namely, a dipole sound source; the third item is a sound source caused by surface acceleration (fluid displacement distribution), namely, a monopole sound source. The Lighthill sound source item only exists outside the surface of a movable solid, while it is equal to zero inside the surface; the second and third sound source items are only formed on the solid surface.

#### 3. Computational Model of Pantographs and Experimental Verification

Figure 1 presents a pantograph in high-speed trains adopted in this paper. A pantograph of high-speed trains is generally composed of a pantograph head, a frame, a bottom frame, and a transmission mechanism. A frame is composed of components such as a sway rod, an upper arm rod, a lower arm rod, a supporting rod, and a balance rod. All the components are connected by hinges. The frame is supported by the bottom frame. The bottom frame is fixed on the train top by insulators. The pantograph head is supported by the frame with a lifting device. The transmission mechanism acts on the lower arm rod to realize lifting actions. An aerodynamic lifting device is installed on the base and acts on a sector plate which is located on the lower part of the lower arm rod by a steel wire rope, so that pantograph lifting is achieved. Lower arm rod, upper frame, and pantograph head are made of stainless steel. The carbon slide plate is installed on the pantograph head support. The pantograph head support is suspended under 4 pull springs. Two torsional springs are installed between the pantograph head and upper frame.