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Typical production vehicle development includes road testing of a vehicle towing a trailer to evaluate powertrain thermal performance. In order to correlate tests with simulations, the aerodynamic effects of pulling a trailer behind a vehicle must be estimated. During real world operation a vehicle often encounters cross winds. Therefore, the effects of cross winds on the drag of a vehicle–trailer combination should be taken into account. Improving the accuracy of aerodynamic load prediction for a vehicle-trailer combination should in turn lead to improved simulations and better thermal performance. In order to best simulate conditions for real world trailer towing, a study was performed using reduced scale models of a Sport Utility Vehicle (SUV) and a Pickup Truck (PT) towing a medium size cargo trailer. The scale model vehicle and trailer combinations were tested in a full scale wind tunnel.

In an environment of tougher engineering constraints to deliver tomorrow's aerodynamic vehicles, evaluation of aerodynamics early in the design process using digital prototypes and simulation tools has become more crucial for meeting cost and performance targets. Engineering needs have increased the demands on simulation software to provide robust solutions under a range of operating conditions and with detailed geometry representation. In this paper the application of simulation tools to wheel design in on-road operating conditions is explored. Typically, wheel and wheel cover design is investigated using physical tests very late in the development process, and requires costly testing of many sets of wheels in an on-road testing environment (either coast-down testing or a moving-ground wind-tunnel).

A correlation of aerodynamic wind tunnels was initiated between Chrysler, Ford and General Motors under the umbrella of the United States Council for Automotive Research (USCAR). The wind tunnels used in this correlation were the open jet tunnel at Chrysler's Aero Acoustic Wind Tunnel (AAWT), the open jet tunnel at the Jacobs Drivability Test Facility (DTF) that Ford uses, and the closed jet tunnel at General Motors Aerodynamics Laboratory (GMAL). Initially, existing non-competitive aerodynamic data was compared to determine the feasibility of facility correlation. Once feasibility was established, a series of standardized tests with six vehicles were conducted at the three wind tunnels. The size and body styles of the six vehicles were selected to cover the spectrum of production vehicles produced by the three companies. All vehicles were tested at EPA loading conditions. Despite the significant differences between the three facilities, the correlation results were very good.

In the development of several outside mirror designs for vehicles, a high frequency noise (whistling) phenomenon was experienced. First impression was that this might be due to another source on the vehicle (such as water management channels) or a cavity noise; however, upon further investigation the source was found to be the mirror housing. This “laminar whistle” is related to the separation of a laminar boundary layer near the trailing edges of the mirror housing. When there is a free stream impingement on the mirror housing, the boundary layer starts out as laminar, but as the boundary layer travels from the impingement point, distance, speed, and roughness combine to trigger the transition turbulent. However, when the transition is not complete, pressure fluctuations can cause rapidly changing flow patterns that sound like a whistle to the observer. Because the laminar boundary layer has very little energy, it does not allow the flow to stay attached on curved surfaces.

Experimental and computational simulation techniques were concurrently employed throughout the aerodynamic development of the NASCAR Dodge Intrepid R/T in order to achieve a greater understanding of the complex flow fields involved. With less than 500 days to design, understand, and build a competitive vehicle, the development team utilized a closed loop approach to testing. Scale wind tunnel models and Computational Fluid Dynamics (CFD) were used to identify program direction and to speed the development cycle versus the traditional process of full scale testing. This paper will detail the process and application of both the experimental and computational techniques used in the aerodynamic development of the Intrepid R/T race vehicle, primarily focusing on the earlier stages that led to its competition introduction at the start of the 2001 season.