IMPROVING COST-EFFECTIVENESS OF LES BY USING NON-REFLECTING BOUNDARY CONDITIONS : THE FLOW AROUND SIMPLIFIED ICE 2

The importance of having proper open or outflow boundary conditions for the calculation domain is a known issue in Computational Fluid Dynamics (CFD) for a long time and some recent developments were done originally without having Large Eddy Simulation (LES) applications in mind. An impact of it on LES results is even greater than in the case of some other modelling approaches due to the number of vortices which may pass through the outflow boundary. The flow may enter and leave the computational domain at the same time and at the same boundary. In such circumstances, it is important that numerical implementation of boundary conditions enforces certain physical constraints. This paper evaluates recently proposed non-reflecting boundary conditions on improving costeffectiveness of LES calculations by reducing calculation domain and with that, also reducing the number of calculation cells which by the rule leads to a shorter computational time. The results will be shown for the flow around simplified ICE2 high-speed train subjected to a constant cross wind used in the previous work of Hemida and Krajnovi (2009). INTRODUCTION The interest in using time-dependent simulations such as LES in vehicle aerodynamics has increased during last ten years as a result of success with LES simulations of flows around Ahmed body (Krajnovi and Davidson, 2005a, 2005b) and some other external vehicle flows. However, most of these simulations were performed at Reynolds numbers that have lower power than real vehicle flows due to insufficient computer resources and required simulation time for real vehicle flows. There is ongoing research today in decreasing requirements for computational effort of LES by modelling the near wall flow rather than computing it. The objective of the present paper is to demonstrate how computational effort of LES can be decreased by shortening the computational domain and using outlet boundary conditions different from convective boundary conditions which have been a standard in LES of bluff body flows. The largest interest in using time-dependent simulations in external vehicle aerodynamics is for flows that are inherently transient and very difficult to study using experimental facilities due to practical difficulties to obtain correct boundary conditions. An example of such situations is the flow around trains under the influence of cross winds. In the present paper we choose to demonstrate our approach on a flow around simplified ICE2 high-speed train subjected to a constant cross wind. Despite a significant increase in computing power, there is almost always lack of computational cells when it comes to complex industrial applications. It is sometimes necessary to decrease the Reynolds-number as it was done in calculations presented here. However, and in this case, measurements of Orellano and Schober (2006) also show a very little influence of changing the Reynolds number on aerodynamic coefficients. They investigated three different Sixth International Symposium on Turbulence and Shear Flow Phenomena Seoul, Korea, 22-24 June 2009