The aerodynamics of freight: approaches to save fuel by optimising the utilisation of container trains

Aerodynamic drag is approximately proportional to speed squared so the drag of slower moving freight trains has received less attention than that of higher-speed passenger trains. Key results of wind tunnel tests of European container trains were published in 1989 and are the basis for most assessments of drag of European container trains (American container trains usually have far higher drag due to double-stacking containers or transporting complete semi-trailers and were studied in research programmes at a similar time). The research reported here concerns a reappraisal of the European results and of more recent results obtained from the application of computational fluid dynamics modelling and the results of real-world and wind tunnel testing of the aerodynamics of container wagons. The paper presents empirical equations that can be used to predict the energy savings associated with different container loading scenarios within a fixed length train and the energy required for carrying aerodynamic features such as baffles or fairings. Illustrative examples are provided using data measured during freight operations. The effect on drag of side winds and their speed distributions are included as are representative vehicle speed profiles. Most previous authors have ignored both side winds and end effects; it is shown that the effects of these are opposite but of similar magnitudes so the results of these authors remain valid.