PAPERS
Numerical Analysis of Mechanism of Aerodynamic Noise Reduction in Bogie Area by Rounding Corners of Bogie Cavity
2025 年 66 巻 3 号 p. 195-200
詳細
2025 年 66 巻 3 号 p. 195-200
Aerodynamic noise radiated from high-speed trains in operation is attracting attention from an environmental point of view. Bogie areas are known to be the main sources of the aerodynamic noise. Rounding the four corners of the bogie cavity has been proposed as a measure to reduce bogie aerodynamic noise, and wind tunnel tests have confirmed its effectiveness. However, detailed flow fields around the bogie area have not been identified and the mechanism of noise reduction by such measures remains unclear. In this study, numerical analyses of the flow field in the vicinity of the bogie area were carried out to investigate the changes in the flow field caused by the proposed measure and to discuss the reduction mechanism.
Aerodynamic noise generated by trains travelling at high speeds is a wayside environmental issue. As train speeds increase, the contribution of aerodynamic noise to overall noise at points used for environmental standard evaluations increases. This may lead to wayside noise exceeding these environmental standards. In addition, low-frequency components of aerodynamic noise below 100 Hz and infrasound (pressure fluctuation) below 20 Hz may cause rattling of house fittings and a sense of psychological or physiological oppression in residents, which may affect the wayside living environment.
One of the main sources of such aerodynamic noise from Shinkansen trains during high-speed operation is considered to be the bogies. It is estimated that aerodynamic noise from bogie areas accounts for about 30% of the overall noise at 320 km/h and increases to about 40% when the speed is increased to 360 km/h [1]. In order to increase the speed of commercial trains, it is necessary to reduce the aerodynamic noise from the bogie areas.
There are two ways to reduce aerodynamic noise from bogies: through measures taken on ground equipment and measures on vehicles. From a cost point of view, it is preferable to implement measures on vehicles. Vehicle measures which change the shape of the bogie section, such as adding deflectors or rounding the corners of the bogie cavity, have been proposed, and wind tunnel tests have confirmed their effectiveness [2]. However, aerodynamic noise reduction mechanisms in bogie sections have not been clarified. Clarifying these reduction mechanisms is expected to inform further development of countermeasures to reduce aerodynamic noise in bogie sections.
In this study, numerical simulations were carried out to clarify one of the mechanisms of aerodynamic noise reduction by countermeasures to reduce aerodynamic noise in the bogie section. This report examines the flow field changes with and without the rounded-corner shape which was confirmed to be effective in wind tunnel tests and discusses the reduction mechanism. Chapter 2 gives an overview of the rounded-corner shape and Chapter 3 describes the analysis of the reduction mechanism by numerical simulation.
Figure 1 shows an overview of the rounded-corner shape. The rounded-corner shape is a simple countermeasure to reduce aerodynamic noise in the bogie section by filling the four corners of the rectangular bogie cavity, which is called baseline in this report. This shape is one of several reduction measures capable of being installed on a real vehicle without compromising the safety and maintainability of the bogie section, and it is the shape with the highest potential for practical application. The radius of the rounded corners is 700 mm in actual vehicle dimensions.
The effectiveness of the rounded-corner shape in reducing aerodynamic noise at the bogie section has been confirmed in several wind tunnel tests carried out in the past [2]. Table 1 shows the reduction in sound pressure level due to the rounded-corner shape, organized by representative frequencies for aerodynamic noise and pressure fluctuation, respectively.
| Pressure fluctuation | ||||
| Frequency band, Hz | < 16 | 25 | 50 | |
| Sound pressure level difference, dB | -0.8 | -0.1 | -0.3 | |
| Aerodynamic noise | ||||
| Frequency band, Hz | 250 | 500 | 1,000 | 2,000 |
| Sound pressure level difference, dB | -0.3 | -0.4 | -0.3 | 0.1 |
*Negative value means reduction of noise.
However, while wind tunnel tests have shown its effectiveness, the details of the mechanism of aerodynamic noise reduction in the bogie section by rounding the corners have not been clarified. One reason for this is the difficulty of obtaining experimental information on the entire flow field in the vicinity of the bogie section. The flow field near the bogie is very complicated, and the narrow bogie cavity makes it very difficult to measure flow velocity, pressure distribution, etc. without disturbing the flow field, making it difficult to conduct multi-point measurements of the flow field in wind tunnel tests and on-vehicle tests. If information on the entire flow field in the vicinity of the bogie is obtained and the reduction mechanism is clarified from the changes in the flow field when aerodynamic noise reduction measures are implemented on the bogie, such as rounding the corners, it is expected that the reduction measures can be further refined.
There are two general aerodynamic noise reduction mechanisms: reduction of the flow velocity and change in the vortex motion. The first mechanism, reduction of the flow velocity, is due to the fact that the power of aerodynamic noise is proportional to the 6-8th power of the flow velocity, so reducing the flow velocity leads to a reduction in aerodynamic noise. The second mechanism, change in the vortex motion, is due to the fact that the aerodynamic source is the unsteady motion of the vortex. In the case of the rounded-corner shape, no significant change in the flow velocity is expected, and it is thought that the reduction in aerodynamic noise is largely due to the change in the vortex motion. In the next section, we will focus on the change in vortex motion.
Numerical simulation on the flow field is useful to clarify the mechanism of aerodynamic noise reduction in the bogie section by rounding its corners. This chapter describes the results of numerical simulations based on computational fluid dynamics for the unsteady three-dimensional compressible Navier-Stokes equations to analyze the flow field near the bogie.
3.1 Methodology: computational model and conditionsFigure 2 shows a computational model. The model consists of a car body with the same dimensions as a real vehicle and a bogie that is located in the center of the car body. The computational domain is a rectangular domain, which includes a car body of uniform cross section with a bogie and rails. To reduce the calculation cost, the bogie was so simplified that it is composed of the bogie frame, wheel axles, main electric motors, gear units, and air springs, with only the main components that affect the aerodynamic noise of the bogie extracted from its shape. Calculations were carried out for two cases: the baseline and the rounded-corner shape.
Figure 3 shows the domain at which the computational grid is generated in the cross-section perpendicular to the rail direction at the center of the bogie and the maximum grid spacing on the boundary line. The maximum grid spacing is 12.5 mm in the area under the floor of the car, and the maximum grid spacing is increased as the distance from the center of the track increases. The maximum frequency of pressure waves propagating through space can be estimated using the value of the maximum grid spacing. Since approximately 8 grids per wavelength are required to resolve spatial pressure wave propagation, it is possible to resolve pressure waves of 3.4 kHz (= 340 m/s / (12.5 mm × 8)) or less under the floor of a vehicle. Assuming that the pressure fluctuations generated by the bogie cavity reach up to the area of 2.7 m away from the track center (maximum grid spacing of 75 mm), the pressure waves can be similarly resolved up to approximately 550 Hz (= 340 m/s / (75 mm × 8)).
The calculation conditions were set for a train running at 360 km/h (= 100 m/s). Since the freestream Mach number is about 0.3, compressible Navier-Stokes equations were used as the governing equations. Large Eddy Simulation was used for the turbulence model, and the standard Smagorinsky model was used for the sub-grid scale model. For the equations of motion, the discretization scheme for the advection term consisted of a second-order accurate central difference and a first-order accurate upwind difference with a mixing ratio of 8:2 (partly 5:5), and the Crank-Nicolson method was used for the time integration method. The inflow condition was a constant velocity of 100 m/s perpendicular to the inlet surface toward the interior of the computational domain, and the outflow condition was a constant pressure of 1.0 × 105 Pa at the outlet surface. On the solid surface, the Spalding law was applied, and the ground and rails were subjected to a travel speed of 100 m/s in the freestream direction, and the wheelsets were subjected to a rotational speed equivalent to 100 m/s travel. The movement of the ground and rails and rotation of the wheelsets are conditions that are difficult to simulate in wind tunnel tests, so there are advantages to performing numerical calculations in this respect as well. The free-slip condition was applied to the top and sides, which are the outer boundaries of the analysis domain. The time step was set to 2 × 10-6 s.
3.2 Results: flow field around bogieThis section describes the trends which are common to the two cases of the baseline and the rounded-corner shape, which is the basic flow field around the bogie. Here we show the visualization results of the vortex based on the Q values and the root mean square (RMS) of the pressure fluctuations on the bogie surface. Q values represent the second invariant of the velocity gradient tensor, and positive Q values represent the vortex strength. By drawing isosurfaces for positive Q values, the vortex distribution can be visualized.
Figure 4 shows the time series variation of the vortex distribution around the bogie section in 3 ms steps. Here, isosurfaces of Q = 10,000 were drawn to represent the distribution of the vortices. The general trend is that vortex-shedding from the leading edge of the bogie cavity and advection along the main flow can be seen. The vortex is a long transverse vortex in the sleeper direction and is periodically shed at a frequency of about 100 Hz. The vortex is transferred downstream over time, impinging on various parts of the bogie (wheels, motors, gearboxes, etc.) and being transformed. The vortex then repeatedly deforms before reaching the trailing edge of the bogie cavity, and the vortex gradually disappears after passing the trailing edge of the bogie cavity. The above trend is common for both the baseline and the rounded-corner shape. On the other hand, a detailed comparison of the figures reveals differences in the distribution of vortices in the vicinity of the side covers, as described in the next section.
Figure 5 shows the power spectral density of each directional component of the velocity variation acquired at the track center position of the front edge of the bogie cavity in the case of baseline. In the power spectral density at about 100 Hz, peaks are observed in the streamwise velocity v and in the vertical velocity w, while not in the velocity u in the sleeper direction. This confirms the presence of transverse vortices periodically shed from the leading edge of the bogie cavity.
Figure 6 shows the RMS of the pressure fluctuations on the bogie surface in the case of the baseline. The streamlines on the surface based on the average velocity are also shown here. Each statistic was calculated in 0.2 seconds. The areas with large RMS of the pressure fluctuation appear locally on the surface of the bogie. Such locations are as follows; (1) the upstream motor, (2) the downstream motor, (3) the downstream gear box, (4) the downstream wheels, (5) the trailing edge of bogie cavity, (6) the side cover edges, (7) the bogie frame corresponding to downstream axle boxes. The streamlines show that these locations are stagnation points, which are thought to be the locations where the advecting vortices collapse. These locations are generally consistent with the locations of the aerodynamic noise sources on the bogie confirmed in previous wind tunnel tests [3].
On the other hand, the RMS of pressure fluctuation does not necessarily increase at stagnation points. In other words, stagnation points are also observed on the upstream wheel and on the surface of the upstream gear unit, but the RMS of the pressure fluctuation does not increase at these locations. These locations are considered to be the positions where the vortices collide with each other, immediately after being shed from the leading edge of the bogie cavity.
Considering the above trends comprehensively, it can be assumed that aerodynamic noise is generated where vortices that were once disturbed by collisions with each part of the bogie, etc., collide again with each part of the bogie.
3.3 Results: change in flow field by rounding cornersThe change in vortex motion when the corners were rounded was seen in the vicinity of the side covers. Figure 7 shows a comparison of the instantaneous vortex distribution in the vicinity of the side cover when Q = 10,000. The visualization area is the area at least 1.1 m away from the center plane of the track as shown in Fig. 2(b). In the rounded-corner shape shown in Fig. 7(b), the vortex periodically shed from the upstream corner is advected downstream, keeping its shape. On the other hand, in the case of the baseline in Fig. 7(a), there are not only vortices periodically shedding from the upstream corner, but also vortices appearing as secondary vortices between those vortices. The number of secondary vortices is one or two between the periodically shedding vortices. In both shapes, the spacings between vortices periodically shedding from the corners are approximately equal, so the baseline with secondary vortices has narrower spacings between vortices than the rounded-corner shape. Therefore, it is thought that interference between vortices occurs in the baseline, resulting in fine deformation of the vortices near the downstream corner. The application of the rounded-corner shape is expected to have the effect of suppressing secondary vortex generation and associated vortex deformation.
Figure 8 shows the instantaneous vortex distribution in the vicinity of the side cover when the Q is further increased to 250,000 to see a stronger vortex distribution. The vortex distribution for each shape in Fig. 8 is at the same time as the vortex distribution shown in Fig. 7, with Fig. 8 drawn closer to the vortex core. There are vortices in the vicinity of the upstream corner in the baseline shown in Fig. 8(a), but not in the rounded-corner shape shown in Fig. 8(b). This suggests that the rounded-corner shape has an effect of reducing the strength of the vortices shed from the upstream corner.
It was found in Section 3.2 that the vortices, once disturbed, collide with various parts of the bogie and generate aerodynamic noise, and in Section 3.3 that the rounded-corner shape suppresses vortex deformation in the vicinity of the side covers and reduces the strength of the vortices shed from the corners. Taking these factors into account together, it can be assumed that turbulent vortices advecting in the vicinity of the side covers collide with the downstream corners of the bogie section and become aerodynamic sources, and that the aerodynamic noise generated here can be reduced by rounding the corners. This therefore suggests that the rounded-corner shape suppresses vortex deformation, which reduces the turbulence of vortices colliding with the downstream corners and thus contributes to the reduction of aerodynamic noise in the bogie section.
Numerical analyses were carried out to clarify the aerodynamic noise reduction mechanism on rounded-corner bogie sections, which is one of the countermeasures which can be used to reduce bogie aerodynamic noise. The basic flow field in the vicinity of the bogie is that transverse vortices, which are long in the sleeper direction, are periodically shed from the leading edge of the bogie cavity and advected along the freestream, repeatedly colliding with each part of the bogie and deforming, and finally the vortex gradually disappears after passing through the trailing edge of the bogie cavity. The vortex shedding frequency is approximately 100 Hz. Furthermore, the rounded-corner shape suppresses vortex deformation in the vicinity of the side covers and reduces the strength of the vortex shedding from the upstream corner. Taking the above into consideration, it is believed that the aerodynamic noise in the bogie generated by the downstream corner is reduced by rounding the corner.
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Tatsuya TONAI Researcher, Noise Analysis Laboratory, Environmental Engineering Division Research Areas: Railway Noise |
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Toki UDA, Ph.D. Senior Chief Researcher, Head of Noise Analysis Laboratory, Environmental Engineering Division Research Areas: Railway Noise |
