###### Q2. What future works have the authors mentioned in the paper "Flow behaviour and aeroacoustic characteristics of a simplified high-speed train bogie" ?

Such factors need to be accounted for in future work.

01 Sep 2016-Vol. 230, Iss: 7, pp 1642-1658

Abstract: Aerodynamic noise becomes significant for high-speed trains and its prediction in an industrial context is difficult to achieve. The aerodynamic and aeroacoustic behaviour of the flow past a simplified high-speed train bogie at scale 1:10 is studied using a two-stage hybrid method comprising computational fluid dynamics and acoustic analogy. The near-field unsteady flow is obtained by solving the Navier-Stokes equations numerically with the delayed detached-eddy model and the results are used to predict the far-field noise through the Ffowcs Williams-Hawkings method. The sound radiated from the same scaled bogie model is measured in an anechoic open-jet wind tunnel. The aeroacoustic characteristics of tandem wheelsets are also investigated for comparison. It is found that the unsteady flow past the bogie is characterized by coherently alternating vortex shedding from the axles and more randomly distributed vortices of various scales and orientations from the wheels and frame. The vortices formed behind the upstream geometries are convected downstream and impinge on the downstream bodies, generating a highly turbulent wake behind the bogie. The noise predictions correspond fairly well with the experimental measurements for the dominant frequency of tonal noise and the shape of spectra. Vortex shedding from the axles generates the tonal noise with the dominant peak corresponding to the vortex shedding frequency. The directivity exhibits a dipole shape for the noise radiated from the bogie. Compared to the wheelsets of the bogie, the noise contribution from the bogie frame is relatively weaker.

Topics: Bogie (58%), Vortex shedding (58%), Noise (54%), Vortex (52%), Wind tunnel (51%)

Jump to: [1. Introduction] – [4.1. Flow field] – [4.3. Wall pressure fluctuations] – [5.1. Acoustic spectra computation] and [6. Conclusions]

- Over the last few decades, researches have been conducted regarding the source mechanisms of flow-induced noise, particularly in aerospace engineering for landing gears and airframes [1,2].
- In contrast, modelling numerically some simplified geometries can reveal more details of the flow behaviour and the corresponding aeroacoustic mechanisms for some main noise-generating components of high-speed trains.
- 4 DDES is an extension of the detached-eddy simulation (DES) method which combines the large-eddy simulation (LES) in the main flow region with the Reynolds-averaged Navier-Stokes (RANS) approach in the boundary layer region close to the solid objects.
- The cell size on the axle surface is implemented as 0.42 mm around the perimeter and 0.88 mm in the spanwise direction.

- Fig. 3 visualizes the iso-surfaces of the second invariant of the velocity gradient 𝑄 to get an overview of the unsteady flow developed around the bogie.
- Distinct features are observed in different regions of the flow field.
- The flow separates from the upstream wheel front edges and interferes with the flow separated on the wheel tread; therefore, the coherent vortex shedding, seen behind the front axle, cannot be formed behind the front wheel and the wake developed there becomes fully three-dimensional.
- For the point one axle radius above and behind the front axle in the mid-plane between the wheel inner surface and axle mid-span, a tonal peak appears in the spectrum at 324 Hz, as seen in Fig. 5(a).
- A peak appears in the drag coefficient of the bogie and the front wheelset at 641 Hz, which is twice the frequency of the tonal peak in the lift coefficient while at a much lower amplitude.

- Fig. 8 displays the wall fluctuating pressure level in decibels (𝐿! = 10log 𝑝!"/𝑝!"#! , where 𝑝!" is mean-square fluctuating pressure and 𝑝!"# is reference acoustic pressure 20𝜇𝑃𝑎) on the bogie surface, which can be used to identify the potentially significant noise source regions.
- This also indicates that the massive vortex shedding generated from the front axle may potentially be a major contributor to the noise radiated from the bogie.
- Furthermore, the high pressure fluctuations can be seen around the downstream wheelset due to the flow impingement by the incoming vortex convected from the upstream geometry as well as the flow separation developed from the rear wheel front edges and the vortex shedding formed behind the rear axle.
- Based on the near-field unsteady flow data obtained from the CFD calculations, the far-field noise signals can be predicted by the FW-H acoustic analogy using equivalent acoustic sources.
- Additionally, equivalent circular-shaped receiver positions are defined in the horizontal x-z plane (the coordinates referred to Fig. 1).

- Flow statistics on lift and drag coefficients in Section 4.2 suggest that the flow transient is washed out after 0.1 s.
- Moreover, compared with the experimental data, the tonal peak has a higher amplitude from the calculations in both cases.
- The directivity of the noise radiated to far-field is calculated based on the OASPL determined from the PSD in the frequency range below 2 kHz.
- Note that the similar directivity pattern of sound radiation occurs from the two cases with the slight difference of noise amplitudes between them, which also demonstrates that the wheelsets are the dominant noise sources of the bogie and the noise contribution from the bogie frame is relatively small.

- The aerodynamic and aeroacoustic behaviour of the flow past a simplified bogie has been studied using the DDES model and FW-H acoustic analogy.
- It is found that both streamwise and spanwise vortices are generated due to flow separation and vortex shedding around the bogie.
- Furthermore, a vertical dipole pattern of noise radiation is predicted for the upstream wheelset; whereas the downstream wheelset has a multi-directional directivity pattern due to the lift and drag dipoles being aligned perpendicular to each other and its sound generation is relatively weaker.
- These findings are helpful to understand the aerodynamic noise generating mechanisms from the bogie at full scale.
- The turbulent inflow and the complex geometry will lead to complex flow structures and these will also affect the noise generation.

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Abstract: The operation of high-speed trains in ice and snow weather results in a large amount of snow accumulation with ice on the bogies, which will pose a risk to the safety of high-speed trains. In this paper, the snow accumulating on the bogie has been investigated using a numerical simulation method based on the Realizable k - e turbulence model and Discrete Phase Model (DPM). The accuracy of mesh resolution and methodology of CFD was validated by the experimental results of wind tunnel tests. The DPM was used to investigate the mechanism of snow accumulation on the bogie by analysing the characteristics of movement of snow particles. Based on this analysis, two deflectors with the angles of 2.58° and 5.14° were designed, and the anti-snow effect of deflectors for the bogies was compared with numerical results. The results show that lots of snow particles underneath the bogie have direct impacts on the equipment of the bogie, causing massive snow accumulation on the bottom surface. A small amount of snow particles turn back to the region above the bogie from the rear cabin cover, which leads to little snow accumulation on the upper surface of the bogie. The number of particles accumulating on the bottom surface of the bogie is much more than that on the top. Application of deflectors with different angles can improve the anti-snow performance in the bogie region of high-speed trains. The deflector with an angle of 5.14° has the better anti-snow performance. It can reduce the snow accumulation on the whole bogie surfaces by 49.34%, while on the primary heat-producing devices, such as calipers and motors, by 42.47% and 47.40%, respectively.

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Abstract: In this paper, the snow accumulation on the bogies of high-speed trains has been investigated using a numerical simulation method based on the unsteady Reynolds-Averaged Navier-Stokes simulations (URANS) coupled with the Discrete Phase Model (DPM). The effects of bogie cut outs’ shape, running speed of high-speed trains and snow particle density and diameter on the snow accumulation and particle movement characteristics are discussed. The results show that the bogie installation region with inclined plates shows better anti-snow performance than the configuration with straight plates, which greatly affects the flow structure and snow concentration distribution in the upper space of bogie regions. The running speed of high-speed trains has dominant effect on the snow accumulation on the bogies, and the snow accumulation issue of bogie becomes more serious with increasing running speed. Furthermore, the snow particle density and diameter also have large influence on the snow accumulation on the bogies. With the increase of snow particle density and diameter, the flow range at height direction around bogie region of snow particles become lower and the quality of snow accumulation decrease significantly.

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Abstract: The aerodynamic behaviour of flow past a simplified high-speed train bogie including the ground underneath with ballast particles at scale 1:10 is studied numerically. It is found that the flow around the bogie is highly unsteady due to strong flow separations and flow interactions developed there. Generally, the ballast particles distributed inside the wheels are situated in the stronger turbulent flow and are subject to much higher aerodynamic forces than the particles located outside the wheels. Moreover, these aerodynamic forces increase when the ballast particles are located downstream of the bogie cavity and reach the peak values close to the bogie cavity trailing edge. The force time-series are produced based on the simulations of an array of the ballast particles in a wind-tunnel setup and it shows that the ballast flight is apt to happen as the rear part of the bogie cavity passing by the ballast bed. When the ballast particles become airborne, the fluctuating forces generated increase significantly. Therefore, the stronger unsteady flow developed around the bogie cavity, especially in the cavity trailing edge region, will produce larger fluctuating forces on the ballast particles, which will be more likely to cause ballast flights for high-speed railways.

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17 Apr 2018Abstract: Aerodynamic noise is a significant source for high-speed trains but its prediction in an industrial context is difficult to achieve. In this article, the flow and aerodynamic noise behaviour of a s...

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...A similar meshing strategy has been previously employed for the isolated wheelset and bogie cases in which good agreements were achieved between numerical simulations and experimental measurements for the radiated far-field noise.(8,9,12) The boundary conditions applied are as follows: the upstream inlet flow is represented as a steady uniform flow (U11⁄4 30m/s) with a low turbulence intensity....

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TL;DR: A numerical study using IDDES is carried out to investigate the aerodynamic and the aeroacoustic response of a simplified ICE3 high-speed train model, focusing on the front part of the train and, in particular, on the first bogie cavity.

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Abstract: Railways are an environmentally friendly means of transport well suited to modern society. However, noise and vibration are key obstacles to further development of the railway networks for high-speed intercity traffic, for freight and for suburban metros and light-rail. All too often noise problems are dealt with inefficiently due to lack of understanding of the problem. This book brings together coverage of the theory of railway noise and vibration with practical applications of noise control technology at source to solve noise and vibration problems from railways.

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...It is generally accepted that aerodynamic noise becomes a significant problem for high-speed trains running at speeds over 300 km/h.(5,6) Considerable progress has been made in understanding the aerodynamic phenomena associated with high-speed trains....

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Such factors need to be accounted for in future work.