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The determination of local motor vehicle surface temperatures in pre-prototype development has for many years been a critical problem. It is often difficult using numerical models to understand whether limitations lie in the modelling assumptions, or knowledge of the boundary conditions existing in vehicle test conditions. In this paper, we use numerical and experimental techniques to investigate possible accuracy obtainable with RANS-based CFD modeling techniques for local temperature prediction using a well-controlled thermal experiment representative of an underbody situation, operated over several air speeds, exhaust temperatures, and heat shield properties. We will examine the physical phenomena which lie behind observed temperature results using results from tests, RANS and DES simulation, and understand the current best level of correlation obtainable by RANS modeling.

A coupled steady-state CFD and thermal study was undertaken at full-vehicle scale using the Low-Reynolds formulation of the k-epsilon turbulence model, with hybrid wall function modification. The separate thermal model included radiative and conductive heat transfer. Road testing (simulated hill climb using towing dynamometer) was performed to provide both boundary conditions for exhaust temperature and detailed local temperatures (air and surface) to enable correlation. CFD and thermal models were alternately iterated until overall convergence was achieved. Measured air temperatures were utilized in the “control” thermal model to provide a best possible non-CFD solution. Coupled model results show reduction in local surface temperature prediction error due to the inclusion of the detailed convection modeling, but cause concerns that the heat transfer mechanism in the exhaust tunnel is not correctly represented.