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Technical Paper

Measurement of Reference Dynamic Pressure in Open-Jet Automotive Wind Tunnels

1992-02-01
920344
In automotive open-jet wind tunnels reference velocity is usually measured in terms of a static pressure difference between two different cross-sectional areas of the tunnel. Most commonly used are two sections within the nozzle (Method 1: ΔP-Nozzle). Sometimes, the reference velocity is deduced from the static pressure difference between settling chamber and plenum (Method 2: ΔP-Plenum). Investigations in three full-scale open-jet automotive wind tunnels have clearly shown that determination of reference dynamic pressure according to ΔP-Plenum is physically incorrect. Basically, all aerodynamic coefficients, including drag coefficient, obtained by this method are too low. For test objects like cars and vans it was found that the error ΔcD depends on the test object's drag blockage in an open-jet wind tunnel.
Technical Paper

Influence of Test-Section Length and Collector Area on Measurements in ¾-Open-Jet Automotive Wind Tunnels

1988-02-01
880251
A detailed investigation of aerodynamic car testing in ¾-open-test-sections was made. Herein two main influencing variables (dimensionless length scales) could be identified: first, the relative length of the ¾-open-test-section influences the static pressure gradient along the x-axis and, second, the relative collector area has large effects on the wake of a car. The measured values (i.e. drag) are mainly determined by a combined effect of these two parameters. The basic investigation was made in the Porsche 1:4 model-wind-tunnel with two different types of vehicles {sportscar and van) and in two different scales (1:4, 1:5). The results are graphically summarized. This diagram can predict the differences between full size open-jet-wind-tunnels. These predictions were verified by measuring Porsche production cars and the Porsche calibration car in three other automotive wind-tunnels.
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