AERODYNAMIC STABILITY OF A RAILWAY COACH

Abstract

Many years of operating experience has shown that train travel is very safe by comparison with other modes of surface transport. Railway operations have high tolerance of adverse weather conditions. However, there have been serious accidents caused by the adverse weather conditions. Low temperature lead to icing of the points and icicle formation in tunnels; high temperature leads to the buckling of rails and engine overheating failures; snow blocks lines and has been known in certain circumstances to be ingested into electric motors and to jam doors; heavy rains lead to landslips and flooding of the track; high tides to bridge collapse and erosion of the track bed. Experience in various parts of the world has shown that trains may overturn in very high winds. In the recent years there has been a considerable reduction in the weight of new trains owing to the use of lightweight material construction techniques and concern has arisen that such trains would be blown over in high winds at exposed sites such as embankments and viaducts. In order to be able to assess the probability of wind induced derailment, knowledge of aerodynamic forces and moments acting on trains in cross winds is required. The conventional way of obtaining such data is to mount a model of the train statically on ground board in a wind tunnel and to measure the forces and moments using conventional wind tunnel balances. However, there are some major inadequacies in this type of test. In particular, the effect of motion of the train is not modelled and also the effect of atmospheric boundary layer is neglected. To overcome these difficulties, a series of tests have been carried out in which the model of train was installed on leaf-spring transducers so that it simulates approximately the vibrations of a running train, The tests were carried out for sub-urban type of terrain iii described as terrain category 2 in IS: 875 (Part 3) the wind profile for which was generated. The aerodynamic moments could be measured by the leaf-spring transducers. The moment causing overturning of the train could thus be determined. The aerodynamic force and moment coefficients are sensitive to the nature of field conditions simulated in the experiment. The aerodynamic overturning moment is balanced by the gravity restoring moment minus the centrifugal overturning moment due to curving and track cant. The wind flow is strongly affected by embankments, viaducts, and topographical features like hills, valleys, etc. The effect of embankment is also important. On an 8 metre high embankment the wind speed is about 25 percent higher than on flat ground. Thus, the forces experienced by trains on exposed embankments are significantly larger than on flat sites. The estimation of overturning wind speed for a particular vehicle is usually based on simple statics and steady reliable aerodynamic force data (vehicle force, moment coefficients and force and moment admittances). However, the critical wind speed, i.e., the wind speed which would cause the overturning of the train depends on the severity of exposure to wind which is a major variable. The present experimental work has been carried out in the Boundary Layer Wind Tunnel at the Centre of Wind Engineering, University of Roorkee, Roorkee. Wind tunnel experimentals remain the primary tool for realistic investigation of the interaction of atmospheric boundary layer flows with the built-in environment. A 300 IN passenger coach was chosen for the study and it has been found to overturn at a wind velocity of 54.75 misec. The results of the test are also corroborated with theoretical results where overturning took place under a 3-sec gust of 54.75 m/sec.