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Practical Implementation of the Two-Measurement Correction Method in Automotive Wind Tunnels
Abstract:
In recent years, there has been renewed attention focused on open jet correction methods, in particular on the two-measurement method of E. Mercker, K. Cooper, and co-workers. This method accounts for blockage and static pressure gradient effects in automotive wind tunnels and has been shown by both computations and experiments to appropriately adjust drag coefficients towards an on-road condition, thus allowing results from different wind tunnels to be compared on a more equitable basis. However, most wind tunnels have yet to adopt the method as standard practice due to difficulties in practical application. In particular, it is necessary to measure the aerodynamic forces on every vehicle configuration in two different static pressure gradients to capture that portion of the correction.
Building on earlier proof-of-concept work, this paper demonstrates a practical method for implementing the two-measurement procedure and demonstrates how it can be used for production testing. The approach requires a correlation of multiple vehicle shapes measured in two different static pressure gradients. For vehicles subsequently tested, the correlation can be used in place of the second measurement so that there is no impact to productivity in the wind tunnel operation. This correlation approach has been applied to both the full-scale and model-scale wind tunnels at Chrysler. The results presented show that this approach is quite accurate (although naturally depending on the vehicles selected for the correlation) and is suitable for day-to-day development testing. In addition, the agreement in results between these two wind tunnels is shown to improve considerably with application of the correction.