How to survive train rides in crosswinds?

Published: Posted on
 Syeda Anam Hashmi

Syeda Anam Hashmi

Syeda Anam Hashmi, a Doctoral Researcher in the School of Engineering at the University of Birmingham
, discusses how windbreak/acoustic barriers for railway lines can provide passenger safety and comfort.

We can already see, as outlined in David Soper’s blog, why aerodynamics is important. High-speed passenger trains are associated with a number of challenges where large steady and unsteady aerodynamic loads due to strong crosswinds can compromise the safety and integrity of a train body. Under strong crosswinds, trains are at a possible risk of derailment or even being overturned.

Primarily motivated by the frequent rail-related accidents under strong crosswinds, windbreaks are gaining an increasing amount of popularity as they are a cost friendly, feasible option that can be built easily to reduce turbulence and improve ride comfort and safety.

But, is it really this simple?
An important problem which has not been addressed in detail is the various structural designs of windbreak walls. Installing windbreak walls is deemed effective if the wind barrier is built entirely parallel to the track. Whereas, in reality it is not possible to develop an entirely uniform and continuous windbreak structure due to possible complexities in the surrounding terrain such as cuttings, which cannot be avoided. The windbreak wall would therefore consist of a discontinuous structure with a series of transition regions.

So, what are these transition regions?
Transition regions refer to the specific design implementations on the windbreak wall to tackle different topographies and terrains that exist along the railway lines. These designs induce vortices on the train surface and may cause stability issues. My research is aimed at understanding these aerodynamic flows that develop around a passenger train with respect to the different types of windbreak walls.

How is this research carried out?
This research is done in two parts: experimental and numerical.

The experimental research is performed at the University of Birmingham’s (UoB) wind tunnel facility. The UoB wind tunnel is used extensively to test the aerodynamic performance of model-scale vehicles along with other model-scale objects. A 1:25 model of the Class 390 Pendolino train (high-speed passenger train) is introduced inside the UoB wind tunnel with and without different geometries of the windbreak walls to investigate the influence of these windbreaks on the flow around a passenger train at varying yaw angles.

Images of the experimental set-up of the model inside the wind tunnel at yaw angle of 90°
Images of the experimental set-up of the model inside the wind tunnel at yaw angle of 90°

Multi-hole pressure probes are used to measure velocities of the flow and the static pressure. These were mounted to clamp stands, positioned at the designated measuring points, to provide the three components of time-varying (fluctuating) and time-averaged (mean) velocity in real-time. A Digital Pressure Measurement System (DPMS) is used for measuring the pressure on the surface of the model train as the system is capable of measuring both time varying and time-averaged pressures, thus making it suitable for the measurement of rapidly fluctuating pressure distributions as well.

The experimental results demonstrate that:

  • A series of different flow patterns can be identified to develop along the train length.
  • For a no windbreak wall case, the windward sides of the train are characterised by positive values of mean surface pressure, while on the roof, the mean surface pressure presents a negative gradient, signifying suction.
  • The shielding effect of the windbreak walls is apparent and results in the train body being surrounded by a negative pressure area.
  • The wall with a 90° transition region provides a lower negative pressure as compared to the wall with a 45° transition angle.
  • The tallest windbreak wall usually provided the lowest mean pressure distribution.
  • Overall, while windbreak walls cause varying pressure distributions on a train surface, they are successful in reducing the loads on a train surface significantly.
Overall mean aerodynamic load coefficients for the side (lateral), and lift (vertical) forces along with the rolling moments about the X-axis and leeward rail for different experimental test cases at a yaw angle of 90°
Overall mean aerodynamic load coefficients for the side (lateral), and lift (vertical) forces along with the rolling moments about the X-axis and leeward rail for different experimental test cases at a yaw angle of 90°

For the numerical investigation, Computational Fluid Dynamics (CFD) is used for investigating the effects of crosswinds on the model train under different scenarios. Once initial numerical simulations are validated through experimental results, numerical work is expanded to better understand the flow structures in order to form an enhanced understanding of the flow behaviour around trains with windbreak walls. Numerical work also takes into account the relative movement of the train and the windbreak wall. The numerical simulations help study the time-dependent behaviour of the flow structures and the resulting impact on the surface pressure of the train, along with the aerodynamic coefficients. In addition, further details are provided on the flow around the transition regions in windbreak walls. Overall, the numerical results demonstrate that the flow around a high-speed moving train, behind the windbreak wall, is three-dimensional and turbulent in nature. The analysis shows that there are potential flow accelerations and decelerations, existence of curved boundaries, flow separations and recirculation regions throughout the domain.

Flows around trains under crosswinds are mainly expected to show high/low pressure regions and vortex shedding from the roof and underbody regions. The crosswind flow is influenced by the windbreak wall and the transition region in three ways:

  • Modification of velocity streamlines
  • Reduction in the mean horizontal windspeeds
  • Alteration in the turbulent structures of the incident flow

The transition region in a windbreak wall can be observed to influence the flow in a significant manner. In fact, different flow features are visible behind the transition region due to the non-uniformity of the structure.

What’s next?
The University of Birmingham’s Transient Aerodynamic Investigation (TRAIN) rig is a unique facility built for measuring the aerodynamics of scaled moving models. The next step of this research is to verify the numerical results obtained on the moving model by performing an aerodynamic investigation at the TRAIN Rig. These experimentations can benefit engineering designers in the rail sector as such investigations provide to be an easier and inexpensive option as compared to field tests.

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