The Influence of Wind Tunnel Grid Turbulence on Aerodynamic Coefficients of Trains C. N. Nayeri, C. Strangfeld, C. Zellmann, M. Schober, A. Tietze and C. O. Paschereit Abstract In the design process of trains wind tunnel tests are indispensable in or- der to assess the cross-wind sensitivity. The aerodynamic forces and moments act- ing on the leading cars or trailer cars can be measured and aerodynamic coeffi- cients determined. Usually the tests are carried out at low turbulence conditions ("smooth flow"). During recent wind tunnel tests on new high speed train models in the Hermann-Föttinger Institute Berlin undesired Reynolds number effects were observed. This paper describes an approach to reduce these effects by changing the turbulence conditions of the flow. Wind tunnel measurement on a high speed train model in scale 1:25 are carried out with different turbulence conditions: increased free stream turbulence and a tripped boundary layer on the model. The free stream turbulence level is varied from 0.5 % to 8% by using grids with different geometries up stream of the test section. The experiments showed that the Reynolds number dependency on the saftey relevant rolling moment can be well reduced by a simple rectangular grid installed in the contraction of the wind tunnel. 1 Introduction Cross wind sensitivity of trains is a safety relevant topic covered in European legisla- tion within the framework of the Technical Specification for Interoperability (TSI). Train manufacturers are therefore required to meet the safety criteria defined therein. However, in order to achieve maximum performance and to increase operating ef- C. N. Nayeri, e-mail: christian.nayeri@tu-berlin.de, C. Strangfeld , e-mail: strangfeld@tu- berlin.de, C. Zellmann e-mail: christoph@zellmann.de Hermann-Föttinger Institute (TU-Berlin), Müller-Breslau-Str.8, 10623 Berlin, Germany M. Schober e-mail: martin.schober@de.transport.bombardier.com, A. Tietze e-mail: an- dreas.tietze@de.transport.bombardier.com Bombardier Transportation, Am Rathenaupark, 16761 Hennigsdorf, Germany