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Because of the contradictions of the downsizing and the serious thermal-load of modern engine, thermal fatigue failure of the engine components easily happens due to excessive temperature gradient and thermal stress. During recent years, with the constantly advancing progress of high performance computer technology, many CFD software codes provide the ability to solve, or take part in the best possible solution of FVM/FEM coupled problems, and Fluid Structure Interaction (FSI) analyses have been developed. However, the unidirectional coupling of thermal flow and thermal stress as well as structural fatigue failure analyses in IC engine applications, especially transient case, has received less attention. In this paper, authors have studied unidirectional FSI in depth, including that transient high speed and high temperature flow strongly affects the engine exhaust manifold. These researches can be used in protection certain fatigue failures for general purpose.

Using the temperature distribution of solid structures obtained by a CFD (computational fluid dynamics) analysis, thermal-stress analyses can be conducted. In the paper, an FVM (finite volume method) based CFD software is used to solve for the temperature and flow field. Making complex mesh model, it uses a hybrid mesh consists of tetrahedron, hexahedron, prism and pyramid elements. And all variables are defined on nodes of elements. Because the hybrid mesh and variables on nodes are compatible between structural analysis FEM (finite element method) elements and node based CFD FVM elements, not only the temperature distribution of the solid but also the mesh can be directly applied to the thermal-stress analysis. The following three approaches are investigated for the best possible solution of FVM/FEM coupled problems. 1) The mesh and the temperature distribution of CFD are applied in a thermal-stress analysis directly, taking full advantage of the mesh compatibility.

In SAE paper 2004-01-1345, author focused on how to use a steady state temperature result obtained by a CFD analysis to conduct a thermal-stress analysis easily with 3 data transfer methods (Direct Conversion Method, Surface Mapping Method and Volume Mapping Method). The advantage and disadvantage of the 3 methods were compared in that paper and a steady state analysis of an engine exhaust manifold was used to show the accuracy, flexibility, efficiency and practicality. In this paper, how to simplify data transfer in a transient coupling is introduced. In transient couplings, 3 troublesome operations always happen. They are 1) multisteps coupling, 2) change of element from low to high order, and 3) estimation of mid node temperature loads of high order elements. In this paper, CFD (Computational Fluid Dynamics) software adopted is SC/Tetra [1], which generates hybrid mesh consisting of tetrahedron, hexahedron, and prism as well as pyramid elements.