Representing large boundary layers in slipstream moving model tests

Abstract

When a train drives it induces airflow along the side of the train and behind it, commonly denoted as slipstream. The induced airflow will impact the surrounding in the vicinity of the passing train and can be hazardous for people and objects on platforms or trackside workers. Therefore there exist limits which the train must comply to get authorisation to be placed in service. In Europe the Technical Specification for Interoperability (TSI), which has the level of law, gives requirements to fulfil. The air speed is measured in full scale tests at a specific distance from the track centreline (3.0 m), at two heights (0.2 m and 1.4 m). The upper location is representative of platforms and the lower of trackside workers. The train should be the longest configuration; if operated with multiple coupled units, at least two units. Due to the highly stochastic nature of the flow many independent measurements are needed to get representative values of the induced air flow with reasonable accuracy. As a balance between practicality and accuracy, 20 independent measurements are required. It was seen in the EU project on railway aerodynamics AeroTRAIN [1, 2] that even low wind speeds have a noticeable influence on the measured values, why for valid measurements the ambient wind needs to be equal or less than 2 m/s, which again is a balance between practicality and accuracy.