A generic train-underfloor experiment for CFD validation

Abstract: Within the DEUFRAKO research cooperation “Aerodynamics in Open Air” (AOA) a generic train underfloor geometry has been studied in the wind tunnel in scale 1:7 in order to provide reference data for the assessment of the suitability of various turbulence modeling approaches later to be used on full train geometries under 1:1 conditions. The geometry consists of a duct connecting three cavities opposite to a flat wall representing the trackbed. The first and the last cavities are equipped with realistic models of ICE 3 bogies in scale 1:7 whereas the central cavity is empty. It represents the intercargap. The inlet boundary conditions consist in steady air stream with 25 m/s and low turbulence intensity. Various profiles of all components of the mean velocity vector and of the turbulence intensities (normal components of the Reynolds stress tensor) have been collected using a twocomponent LDA. A sufficient accuracy was reached for the use of the data for validation of CFD predictions both on the level of mean flow and of turbulence intensities. Flow visualizations helped to identify flow features such as a strong flux out of the funnelshaped rearward end of the first bogie cavity and the existence of strong secondary flow in the central cavity. Downstream of the first bogie cavity the irregularity of the underfloor geometry had a noticeable effect on the flow near the trackbed. There, an increase of the axial speed by 30 % and of the turbulence intensity by 200 % was observed. Results can not directly be mapped to the real, full-scale situation of a passing train. Nevertheless, the generic configuration seems to pose sufficient challenges to allow for a meaningful comparison for different turbulence modeling approaches such as RANS, DES, and LES. Simulation results obtained are presented in an accompanying paper.