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Dr. Christoph Heine and Gerd Matschke, Deutsche Bahn AG, Munich, Germany
Modern trains are lighter than those of past years. This is due in part
to the replacement of a power car at the rear of the train with an unpowered
driving trailer. This change has meant lower axle loads, reduced wear
on ballast, and increased passenger capacity, since the end car can now
be filled with seats.
Oil-flow path lines, colored by pressure, show the flow patterns on the
end car surface
For a light-bodied driving trailer, the unsteady aerodynamic loads may
become significant for the running behavior, and this effect has become
a concern for a number of railway operators in Europe. In the BriteEuram-funded
research project RAPIDE (Railway Aerodynamics of Passing and Interaction
with Dynamic Effects), the partners have joined forces to investigate
the boundary layer development along a modern high-speed train and the
wake flow characteristics behind the end car using CFD.
The CFD investigation was divided into three parts, corresponding to
three sections of a moving train: the front car, the six midcars, and
the trailing car. The boundary layer grows in thickness from the front
to the trailing car, and when this thick boundary layer separates behind
the trailing car, the points of separation on the train surface can periodically
shift. This gives rise to aerodynamic oscillations about the longitudinal
axis, which can cause discomfort to the passengers riding in the trailing
car. The European organizations MIRA and SNCF performed boundary layer
development calculations on the front and midcar sections, respectively.
Their results were then used by Deutsche Bahn to simulate the unsteady
flow around and behind the German ICE 2 end car.
The end section modeled was 40m in length and positioned in a the flow
patterns on the end car surface domain of length 60m, width 20m, and height
15m. A volumetric mesh of tetrahedral and prismatic cells was used. The
profiles along the sides and on top of the train generated by the other
partners in the project were used as inlet boundary conditions. The ground
under the train was given a uniform speed equal to that of the moving
train.
Comparison of surface pressure for the steady and unsteady cases
Path lines and planes showing velocity magnitude contours behind the train
A steady-state simulation using the k-e turbulence model was initially
performed on multiple processors. The symmetric solution showed low pressure
on the shoulder areas of the end car and a high pressure region on the
back face that results from the onset of separation. A transient calculation
was then initiated using the steady solution as a starting point. Using
time steps of up to 0.01s, unsteady flow developed with a period of oscillation
on the order of 1 Hz. This frequency was found to be in good agreement
with measurements reported by a Japanese railway company1 . Further
runs were done using smaller time steps and a higher order turbulence
model (RSM), yielding identical oscillations in the flow. Based on the
CFD results, the aerodynamic coefficients were calculated. These forces
and moments served as an input for Multi Body Systems (MBS) calculations
performed by Bombardier Transportation, and the running comfort was evaluated.
Luckily, the oscillations were found to be far too weak to cause vehicle
movements, so they would not cause any passenger discomfort.
References :
1. Kohama, Y., Koshikawa, T. and Okude, Wake Characteristics of a High
Speed Train in Relation to Tail Coach Oscillations, Vehicle Aerodynamics
Conference, Loughbuough Univ., UK, 1994.
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