The focus of the paper was to revisit the basic definitions for the different reference pressures used in automotive wind tunnels. Basically, two different ways to define the dynamic pressure in the test section (and the adjacent static pressure) are available: the nozzle method, where all conditions are related to the situation in the settling chamber, and the plenum method, where the reference conditions are defined in the plenum chamber. In ideal zero blockage situations both methods deliver identical results, but major differences emerge under high blockage conditions. Therefore sources of error are qualified whether they are blockage related or not. A number of non-blockage related sources of error are discussed, such as the velocity dependence of tunnel factors, the interaction with boundary layer conditioning systems and local empty tunnel velocity differences. Usually these errors can be ruled out by reasonable technical efforts. The often discussed effect of the pressure build up in front of the model on local pressure taps is shown to be a general blockage effect, which does not depend on the position of the pressure taps. The second, blockage related class of errors is far less under control. At hand of new experimental data it is shown, that both nozzle and plenum method define reference conditions, which at high blockage need additional wind tunnel corrections to make the results comparable to results in infinite flow. In contradiction to order investigations, the plenum method seems to be less liable to nozzle blockage effects than the nozzle method. It is less sensitive to the position of the vehicle relative to the nozzle and should therefore be preferred for measurements of practical interest, as for example pressure distributions at cooling air inlets or measurements of wind loads on vehicle parts. If it is intended to judge the validity of force measurements in a wind tunnel, cp-measurements are not well suited, independent of the method used. The measured cp-values are blockage dependent to the same extend as the measured forces are. Blockage corrections are not treated in the paper. Although considerable effort hag been spent in the past on working out correction schemes for automotive applications, this remains - at least from the viewpoint of the authors - an unresolved item, especially for open jet wind tunnels.
Reference static and dynamic pressures in automotive wind tunnels
Bezugswerte für statische und dynamische Drücke in Fahrzeugwindkanälen
2003
15 Seiten, 18 Bilder, 12 Quellen
Aufsatz (Konferenz)
Englisch
Reference static and dynamic pressures in automotive wind tunnels
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