Full scale measurements of train underbody flows and track forces

This paper presents the results of full scale measurements of the underbody flow characteristics and track behaviour beneath a high speed train on a line in the south of England. Simultaneous measurements were made of the flow velocities and pressures beneath the train, sleeper displacements and ballast accelerations. The results showed that, while the mechanical measurements of displacement and acceleration were essentially deterministic and varied little from one train passage to another, the aerodynamic measurements were highly variable with substantial run-to-run variation. Vertical ballast accelerations with peaks greater than the acceleration due to gravity were measured for short durations. Velocity and turbulence intensity profiles were derived, from which aerodynamic shear forces at the ballast surface were estimated. Assessment of the magnitudes of the different forces acting on the ballast showed that the mechanical forces associated with track displacement and the aerodynamic forces were of similar order of magnitude on well performing track. For poorly maintained track with higher accelerations, the effects of track vibration on the initiation of ballast motion are likley to be much more significant.