Background

The sum of the resistive forces acting on trains in the direction of travel is usually expressed as

F = A + B x V + C x V^2                         (1)

Here, it is assumed that the train does not accelerate and that it travels on flat ground and that the railway is straight. Otherwise, terms for the forces needed to overcome the resistance of acceleration, the gravitational force and mechanical curving resistance have to be included in Eq. 1. The term A on the right hand side contains mechanical resistances that are constant with respect to the speed of train, V but dependent on the mass of the train. The second term contains resistances that are considered to be linearly proportional to speed. The last term contains the aerodynamic resistance of the train which is proportional to the square of speed. A freight train normally consists of a large amount of wagons of different sizes, shapes and purposes. For a freight train the coefficient C in Eq. 1 is the sum of the contribution to the aerodynamic drag from the locomotive and all wagons in the train. The size of the contribution to C from each wagon depends on the position of the wagon in the train

Shown in Fig. 1 is the drag coefficient of a closed top gondola-type of freight wagon depending on the position in the train. Results are shown for 0, 5 and 10 degrees of yaw angle. It is seen that after the initial 3-4 wagons, the drag coefficient will reach some steady value that is some 20-50% less than the drag coefficient of the second wagon. The contribution to the total drag of the entire train from the locomotive will in turn be higher than the drag of the second wagon due to the contribution from the stagnation pressure of the air on the front of the locomotive. The majority of all wagons in a freight train will experience an aerodynamic drag force slightly lower than that experienced by the first wagon in the train.

In the literature there are some experimental wind tunnel studies on different types of freight wagons reported. Wind tunnel simulations were made in Ref. [1] on 1:10 scale wagons. The objective of the study was to determine how a "typical" wagon in a train could be simulated in the wind tunnel. With "typical" means that the wagon should not experience effects in the measured flow quantities from the front or the rear of the train. In the study it was found that for a wagon not to experience effects in the measured drag coefficient from the locomotive and or the end of the train one and a half dummy wagon was needed ahead of the wagon being studied. One half dummy wagon was needed downstream.

Previously, the supervisor of this thesis has carried out simulation of the flow around one single standing freight wagon [2]. As seen above, just studying the flow around one wagon is not representative of the flow around a real wagon in a freight train. In the proposed thesis, the student will carry out numerical simulations of the flow around one wagon with wagons ahead of the wagon and behind it.

Aim of the thesis

The aim is to accurately simulate the flow around 3-4 four container wagons in order get “typical” flow conditions around the middle wagon. The flow around that wagon will be analyzed and the sources of aerodynamic drag should be localized. If the outcome from the simulations is well and the student makes progress quickly, some means of reducing the aerodynamic drag of the wagon will be investigated in the project. Computational grids around the wagons will be constructed by the student by using the commercial grid generator software Ansys ICEM CFD. The simulations will be carried out on large scale computers clusters with the CFD software AVL Fire. The grids are expected to reach +20 million grid points with high geometrical details. Thus, it will be required by the student to acquire high skill level in grid generation using ICEM CFD. The analysis of the flow will be carried out with the EnSight visualization package.

The student

This project is suitable for one highly motivated Master student with interest in vehicle aerodynamics. This is an excellent opportunity to learn the necessary softwares used in the CFD process as well as getting state-of-the-art knowledge about applied Large Eddy Simulations in vehicle aerodynamics. The extent of the project is 30 credit points. Working environment will be provided at campus Johanneberg at the division of Fluid Dynamics. The interested student should send an e-mail to supervisor with an extract of his/her passed courses.

References

[1] S. Watkins, J. Saunders, H. Kumar. “Aerodynamic drag reduction of goods trains”. Journal of Wind Engineering and Industrial Aerodynamics, 40: 147 – 178, 1992.

[2] J. Östh, S. Krajnovic. “Large Eddy Simulation of the flow around of single-stacked container freight wagon”. Proceedings of the First International Conference on Railway Technology: Research, Development and Maintenance, J. Pombo (Editor), 2012. (to appear)