Investigating side-wind stability of high speed trains using high resolution large eddy simulations and hybrid models

Modern high speed trains are of considerable risk of overturning or derailment when operating in the presence of strong cross winds. In order to assess the safety of such vehicles a detailed understanding of the aerodynamic forces is required. Currently, there are three different methodologies for evaluating the aerodynamics of trains: full-scale measurements, physical modeling using wind-tunnel experiments and nu-merical modeling using computational fluid mechanics (CFD). Latter has the potential advantage of providing a cost-effective tool to gain a deep understanding of the aer-odynamic flow around the vehicle for arbitrary operating and geometry conditions. However, there are still many difficulties associated with CFD, most of which are as-sociated with modeling and resolving turbulent dynamics on multiple scales. In the case of high speed trains operating in cross winds at large yaw angle, substan-tial unsteady vortex shedding on the roof and underfloor side of the train is expected. The complexity is even enhanced by the influence of the train nose, which renders the flow truly three-dimensional. Small scale unsteady effects are furthermore ex-pected to result from the interaction of the flow with the bogie cavity and the wheels. In such a scenario, a simple Reynolds averaged Navier-Stokes (RANS) assumption representing all turbulence effects on the mean flow field with a simple, isotropic model is generally violated and forces derived from this flow field tend to show size-able discrepancies to experimental values