Keywords: train aerodynamics, three-dimensional boundary layer, skin-shear lines, longitudinal vortices, oil-film visualization.

When a train is very streamlined and short (a single vehicle), the potential flow region dominates and a specific treatment of the boundary layer adds an important refinement to the inviscid analysis. A great challenge to the correct modelling of the boundary layer is the transverse pressure gradient, that is responsible for skewing an otherwise plane boundary layer. The transverse pressure gradient arises because, in general, the flow around trains is not axisymmetric as a result of the shape of the nose and the cross-wind component of the relative incoming velocity.

The three-dimensional model of the boundary layer has been compared with wind tunnel experiments, using oil-film visualizations, confirming the value of the numerical approach to study streamlined vehicles. The comparison was carried out with a concept train at different yaw angles. The basic shape of the concept train, succeeded in preserving a smooth attached flow over almost all the train up to high yaw angles: lateral wind gusts of the order of 50 km/h, orthogonal to a train running at 250 km/h. The visualizations revealed the characteristics of two longitudinal vortices: (a) at high yaw angles, immediately downstream of the curved edge between the lateral walls and the roof and (b) at all angles, near the bottom of the lateral leeward wall.

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