The simulation of unsteady aerodynamic cross wind forces on trains

This paper describes an investigation of the effect of cross winds on train dynamic systems. It considers the nature of the fluctuating cross wind and how such cross winds produce fluctuating forces. A simple method for calculating the wind time history at the position of a moving vehicle is firstly set out, that is computationally much less expensive than existing methods. The concepts of aerodynamic admittance and weighting function are then introduced, and methods outlined for parameterising experimental values of aerodynamic admittance and obtaining simple closed form weighting function solutions from these expressions. The use of the weighting function in obtaining unsteady force time histories is then set out. It is shown that different formulations of this method are necessary for stationary and moving vehicles. A statistical analysis of the results produced by this method is then presented, which compares the results to those of simple quasi-steady calculations. An appendix describes how the cross wind forces develop in a variety of different idealised gust shapes. It is concluded that, if the dynamic phenomenon of interest has a time period of <0.5 s, then a simulation of the type described in this paper is required, as the quasi-steady approach produces unduly conservative force values. If, however, the time period of the dynamic phenomenon is of the order of 1 s or more, then quasi-steady calculations are more than adequate, particularly at the higher vehicle speeds.