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Results are presented from a study on the use of Computational Fluid Dynamics (CFD) for automotive underbody design. A diffuser-equipped bluff body with endplates was examined in ground effect at varying ride heights in configurations with and without wheels. The study was performed using commercial CFD, Fluent© 6.3.26. CFD data is compared to experimental work done with similar bodies by Cooper et al. [1, 2], George et al. [3, 4], Zhang et al. [5, 6], and others [7, 8, 9]. Emphasis is made on the study of vortex structures in bluff body flow. Various mesh geometries and solvers were explored with computational models designed to operate on single-processor workstations or small networks. Steady-state solutions were modeled for all cases; boundary layers were approximated with wall functions. CFD results for lift coefficient measured within 15-25% of experimental cases, dependent on solver. Qualitative results matched well with experimentally measured flow structures.

The validity and usefulness of low-complexity “fast-turnaround CFD” for motorsports design is investigated using results from three different combined experimental and CFD analyses of racing or high-speed vehicles. Analyses using both wind tunnel experiments and CFD simulations (with commercial software and moderate computing resources) found good agreement in some aspects of interest over a variety of applied situations. Key results were the ability for relatively simple CFD models to consistently predict CL in complex flows within 15-25% of experimental findings, predict the effect of design changes on flow, and accurately show qualitative flow phenomenon. However, CD values were not accurately predicted with the low-complexity simulations. Simulations were run using the commercial Fluent© 6.3 application. Experimental results were performed in the Cornell University 4 by 4 foot wind-tunnel.