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Computational Fluid Dynamics (CFD) has found increasing use by aerodynamicists in recent years. Highly affordable computer hardware coupled with advances in computational techniques and availability of commercial CFD codes support this trend. However, as is true with any computer simulation, there is always a need for comparing aerodynamic CFD predictions extensively to the results measured in wind tunnel experiments. One such calibration study has been initiated by Ford Motor Company to assess the predictive ability of commercially available CFD codes for the aerodynamic design of automobile shapes. Several codes have been checked against a set of detailed wind tunnel measurements on a number of car shapes. The work is being continued to date. This study has provided a significant information base for comparison of predicted and measured flow fields.

An extensive calibration study has been initiated to assess the predictive ability of CFD (Computational Fluid Dynamics) for the aerodynamic design of automotive shapes. Several codes are being checked against a set of detailed wind tunnel measurements on ten car-like shapes. The objective is to assess the ability of numerical analysis to predict the CD (drag coefficient) influence of the rear end configuration. The study also provides a significant base of information for investigating discrepancies between predicted and measured flow fields and for assessing new numerical techniques. This technical report compares STAR-CD predictions to the wind tunnel measurements. The initial results are quite encouraging. Calculated centerline pressure distributions on the front end, underbody and floor compare well for all ten shapes. Wake flow structures are in reasonable agreement for many of the configurations. Drag, lift, and pitching moment trends follow the experimental measurements.

An experimental investigation of the underhood cooling airflow of a production passenger car has been carried out with a 3/8-scale model in a water filled tow tank at Texas Tech University and with a production vehicle in the Maritime Dynamics Laboratory tow basin of SSPA Maritime Consulting AB in Gothenburg, Sweden. The primary objectives for both the 3/8 and production vehicle investigations were to obtain a better understanding of the cooling airflow behavior within the engine compartment of an automobile and to identify the major factors influencing the flow field. The tests consisted of running a fan on a stationary and moving vehicle with pressure measurements and extensive flow visualizations within the engine bay.

Wind tunnel force-balance and wake-traverse tests were made on .154-scale car models with various degrees of streamlining to determine the significance of ground treatment for increasing levels of aerodynamic cleanness. The wake-traverse analysis included investigations of spanwise distributions of vortex and viscous drag, which gave insight into the flow mechanisms involved. It was found from these tests that thick, uncontrolled tunnel-floor boundary layers yielded wakes with viscous “side-lobes” at floor level, which were absent with a moving ground representation. For “bluff car designs, this was the only effect. For “slippery” designs, more typical of modern design practice, other more significant changes were noted. Measured changes in trailing-vortex strength and the associated vortex drag suggested that very low-drag designs experience an increase in effective angle-of-attack when the ground is fixed.

Maskell's approach for performing drag integrals in far-field fully-developed three-dimensional wakes has been extended for near-field application. The extended theory has been applied to wake survey data at two traverse stations involving a stalled wing, and an idealized car model tested over a fixed ground at two yaw angles. As a spin-off, cross-flow velocity vector plots have been resolved into vorticity-driven and source-driven components, which give added insight into complicated flows. The new extension appears to work well in application to the test data, but correlations with balance measurements were compromised by fixed-floor flow problems. A limited review suggests that, while problems of wake definition may be soluble at zero yaw, changes in flow topology may negate comparable solutions with the model yawed. These difficulties are circumvented if a moving ground is employed.

The use of low aspect ratio, often untapered wings on agricultural aircraft leads to strong tip vortices, high induced drag and disruption of spray patterns. Wing tip devices which modify the trailing vortex offer performance and flow improvements. A systematic experimental program has lead to a patented vortex diffuser device for drag reduction which comprises a winglet-like vane mounted from a boom which trails a wing tip. Flight test experiments on an RCRV model indicate slight degradation of lateral stability with such vortex diffusers fitted but substantial degradation for winglets. Analysis indicates this is due to increases in Cℓβ with the tip devices present. The design of Vortex Diffuser Vanes for the “Thrush” agricultural aircraft is described. Instrumented flight tests are scheduled for Spring, 1981.