Evaluation of an Open-grill Vehicle Aerodynamics Simulation Method Considering Dirty CAD Geometries

In open-grille vehicle aerodynamics simulation using computational fluid dynamics, in addition to basic flow characteristics, such as turbulent flow with a Reynolds number of several million on the bluff body, it is important to accurately estimate the cooling air flow introduced from the front opening. It is therefore necessary to reproduce the detailed geometry of the entire vehicle including the engine bay as precisely as possible. However, there is a problem of generating a good-quality calculation grid with a small workload. It usually takes several days to a week for the pretreatment process to make the geometry data ‘clean’ or ‘watertight’. The authors proposed a computational method for complex geometries with a hierarchical Cartesian grid and a topology-independent immersed boundary method with dummy cells that discretize the geometry on a cell-by-cell basis and can set an imaginary point arbitrarily. It is possible to avoid problems of ‘dirty’ computer-aided-design data, such as a gap/overlap and a zero-thickness surface. The present study applied this method to the several full vehicle models. The results were compared with the results of a conventional method based on an unstructured grid. A resolution of about 6 mm was adopted, and the number of cells ranged from tens of millions to 100 million. Preprocessing in the conventional method takes 2 weeks, and improvements to the calculation speed using hundreds of cores have reached a limit. In the present method, preliminary processing was shortened to 10 minutes, and the calculation speed was scaled up to several thousand cores, thanks to the simplification of calculation by the Cartesian grid. Differences in the drag coefficient due to the effect of aerodynamic parts were almost comparable. Because the present method has higher parallel computing efficiency, it is suited to massively parallel environments that will emerge in the near future.