Wall Shear Stress Measurements on a Double-Decker Train

Abstract

The prediction of the total energy consumption of new or further developed rail vehicles is still a comprehensive task in the industrial development process. The aerodynamic drag is a dominant part of the driving resistance of today’s rail vehicles. An estimation of the energy consumption therefore requires information of the aerodynamically induced drag. Computational studies of the current state-of-the art do not meet the high accuracy level required for a reliable prediction of the total energy consumption of a rail vehicle. Scale model geometries in wind tunnel experiments do not allow for simulating realistic near wall flow conditions as a fundamental aspect to predict friction drag. Besides, no test facilities are available for a full-scale train configuration. Nowadays, coasting tests on a test track are generally used by the industry to predict the aerodynamic drag of rail vehicles. A main disadvantage of coasting tests is the strong impact of the environmental conditions on the measurement results. Therefore, a large number of test runs - associated with high costs - is necessary to produce reliable experimental results. A key interest from industry partners of DLR is the reduction of development costs by increasing the performance of computational studies to predict the aerodynamic drag of a rail vehicle. Besides pressure and vortex drag induced by add-on parts such as the pantograph, the bogies, the inter-car gaps and other supply systems and the pressure drag between the vehicle front and rear end, the major part of aerodynamic drag of a rail vehicle is friction drag as a result of the developing boundary layer on the train shape defined by the wall shear stress. This article describes an experimental study performed on an electric double-deck train KISS on a test track to measure the wall shear stress under real operating conditions. The results are compared to theoretical values and results published in the literature. The measured wall shear stresses are further used to improve the predictability of computational studies. The idea is to modify the surface rough-ness of the wall boundary condition of the vehicle geometry to reach the same wall shear stress as measured in the experiment. The oil film interferometry as a non-intrusive method to measure the wall shear stress was therefore used in the experiments.