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Critical Sources of Aerodynamic Resistance in a Medium Distance Urban Train: a CFD approach - pp. 111-124In order to improve the aerodynamic performanceof the train, it is important to keepin mind that an ideal geometry does not exists,understanding by ideal geometry one with a nildrag coefficient. Efforts are then targeted towardsobtaining a value as low as possible. If we look carefullyat the shape that the Metro de Medellin has,two characteristics are immediately highlighted;it’s flat shape and the other the peculiar hollow inthe zone of the windshield. These elements do notcontribute to the good aerodynamic performance,because they constitute a significant obstacle tothe flow, which suffers an abrupt deceleration invicinity of the surface, transforming all this kineticenergy into a force that opposes motion.Other important phenomenon equally undesiredis the detachment generated at the edges ofthe frontal face, where the flow, due to the rapidchange in direction (see Figure 12 A), suffers astrong acceleration and it is not able to adhereto the lateral surface. Consequently it detachesand generates an adverse pressure gradient, thatdown-stream is traduced into a big recirculationbubble (see Figure 12 B). Avoiding the generationof that bubble without a drastic interventionat the frontal geometry is very complicated.However, analyses indicate that the sudden andabrupt changes in the velocity at the vicinity ofthe frontal face are the main contributors to theaerodynamic drag increment. This is reflectedat the pressure figure along the train length (seeFigure 12 D), where a pressure a peak is generatedat the frontal surface due to direct impactof air, and the further pressure stability over thesurface, which means that the critical zone is thefrontal part of the train. Nevertheless, decreasingthe frontal impact is implicitly working in theturbulence generation downstream.Figure 12A) Flow redirection B) Recirculation C) Turbulence D) Pressure chart along the train surfaceRevista Científica Guillermo de Ockham. Vol. 11, No. 1. Enero - junio de 2013 - ISSN: 1794-192X ‣ 119