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  Vehicle Aerodynamics:
Passing vehicle dynamic loads and gust effects   (printable version)
  A common problem in high speed vehicle dynamics are the transient loads that occur when vehicles pass one another, pass a fixed object or are subject to wind gusts. These effects have been observed in high speed train operations and they are also commonly observed in highway vehicles. In this study, a series of analyses were used to study the effect of the high speed operation of the AMTRAK Acela passenger train on rights of way that place Acela close to slower freight trains. A series of analyses were used to find the dynamic loads exerted by Acela on the freight train. A subsequent dynamic response analysis of the freight train was used to assess the risk of freight train derailment. The prediction of the aerodynamic loads calculated using AcuSolve is supported by direct comparison with experiments that were conducted at the Transportation Technology Center at Pueblo, Colorado using an Acela locomotive.

The passing vehicles problem is characterized by large changes in problem geometry due to the passing action. In AcuSolve this geometry change is accommodated using a sliding fluid-fluid interface to affect the motion. This technology permits two mesh domains to slide past one another while assuring full communication across the interface. The AcuSolve fluid-fluid interface is designed to be fully conservative, preserving all quantities in the solution.

The experiments in support of the analysis model were conducted using an Acela locomotive and an instrumented double stack container car as Shown in Figures 1 and 2. Instrumentation on the container car consisted of pressure gages on the front and back surfaces of the cars. In the experiments, the container car was stationary and the locomotive passed the container car at high speed. The predicted pressures from AcuSolve simulations were compared with the measured pressures to validate the method.

 
   

Figure 1. Acela and container car

Figure 2. Container car with instrumentation

 
   

Figure 3 compares measured and predicted pressures at three locations on the near side of the container car showing good agreement between the analysis and experiment. With this verification AcuSolve could be used to predict the loads on container consists under a variety of passing train speeds (with both trains moving) and wind speed and direction scenarios.

 
   
Figure 3. Pressures a three pressures sensors located along the length of
the container car are compared to the CFD analysis.

 
   

Figure 4 shows a sequence of flow visualizations from the simulation of the train passing problem. Pressure contours plotted on the trains and the ground show the variation in pressure on the container car during passing. The simulations showed that the resultant forces and moments on the car change sign during the passing maneuver creating a push-pull response. This response is most dangerous if the pulse length of the response is close to one of the natural response frequencies of the train structure.

 
   
  1 2
 
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  Figure 4. Pressure contours during train passing sequence at 0.05s intervals.
 
   

More complete descriptions of this work can be found in the following references.
  1. Measurement of the Aerodynamic Pressures Produced by Passing Trains", Proceedings of: The 2002 ASME/IEEE Joint Rail Conference, JRC2002 (23 April 2002)
  2. "High-Speed Passenger and Intercity Train Aerodynamic Computer Modeling," in Rail Transportation 2000 ed. S. K. Punwani, ASME Publication 2000 168 pp. ISBN: 0-7918-1926-4 RTD-Vol. 19 (2000)
 
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