On a moving vehicle (1) an air flow (f1) generated by the draught caused by the moving vehicle is guided round the corner section (4) of the vehicle and along the latter's side section (8) in order to reduce the air resistance and cause the side section (8) to be cleaned by an air flow (f1) which counters its fouling from below by spray and dirt particles. The air flow (f) passes also through an engine space in order to cool an engine. The engine space being relatively cramped means that the air flow (f) is subjected to flow resistance and pressure drop. This leads to part of the air flow (f3) leaking from the engine space, e.g. in the region round a boarding stairway (16). The leaking air flow (f3) may weaken and alter the direction of the air flow (f1), thereby reducing the latter's cleaning effect and increasing the air resistance and the fuel consumption. A redirecting means (50) adapted to redirecting the air flow (f3) in a direction such as not to affect the air flow (f1).