The aerodynamic, packaging, and styling requirements of today's passenger vehicles have resulted in smaller underhood compartments and smaller front openings. New components are being added to the already congested underhood region, further reducing the available cooling air flow and increasing temperature levels in the underhood region. The resulting environment can be too hot for the operation of sensitive components.

Solutions on meshes of 1.5 million cells can be obtained in a matter of hours using parallel processing

CFD analysis of the underhood thermal field has the potential to reduce the expense of prototype construction and testing. Problems can be identified early and remedied, significantly reducing the design time and cost, and increasing the reliability of the vehicle. Vehicle underhood regions pose a significant challenge for CFD because of their geometric complexity, however, and mesh generation time is prohibitive with conventional body-fitted hexahedral grids. Furthermore, the simultaneous solution of the external and underhood flows leads to large computational models, so the turnaround time for calculations is also critical.

Underhood Modeling Today

Engineers at several large automotive companies have been using FLUENT/UNS for underhood thermal analysis. To reduce the mesh generation time, tetrahedral meshes have been used with considerable success. Furthermore, it has been shown that solutions on meshes of 1.5 million cells can be obtained in a matter of hours, using the parallel processing capability of FLUENT/UNS.

The figures shown here illustrate the near-wall temperature prediction for a recent simulation of a small passenger sedan moving at 28 mph. The flow over the complete vehicle was solved in order to obtain the proper split between the underhood, underbody, and exterior flows, but no attempt was made to accurately resolve the fine details of the exterior flow. Radiative and conductive heat transfer were taken into account using a proprietary one-dimensional code, which was coupled to FLUENT/UNS for the simulation.

Automated meshing and parallel processing in FLUENT/UNS have made detailed underhood/underbody predictions like these (above and below) feasible within automotive design cycles.

Full Vehicle Simulations in the Future

For vehicle aerodynamicists, the ultimate CFD application combines accurate analysis of the exterior flow field with the underhood and underbody flows, yielding a true "full vehicle simulation". At Fluent, we're working toward that goal, with hybrid meshing to promote both accuracy and setup speed, and high-performance parallel computing to handle the calculation requirements of these ultimate models.

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