On Low-Frequency Pressure Pulsations and Static Pressure Distribution in Open Jet Automotive Wind Tunnels 1999-01-0813

Avoiding low-frequency pressure pulsations and establishing a good axial static pressure distribution are primary concerns for open jet wind tunnels. The current research was conducted to ensure the full scale Chrysler Aero-Acoustic Wind Tunnel (AAWT) design is consistent with good performance in these two areas. Experiments were conducted in two tunnels: a 1/3.6-scale closed-circuit tunnel and a 1/12-scale open-loop tunnel. Results from both are consistent, and a configuration that exhibits i) minimal pulsations for both empty test section and 15% vehicle blockage and ii) a good axial static pressure distribution has been identified for the AAWT. The results illustrate the effect of open jet length, collector geometry, and plenum geometry on pulsation levels and highlight the spatial variation of the pulsation levels within the plenum chamber. Pulsation levels were observed to increase with increasing open jet length and decreasing collector throat area. For high-pulsation configurations, the resonant nature of the phenomenon is indicated by local maxima in fluctuation level (C_p,rms) at specific wind speeds. Although the physics are not fully understood, pulsations represent a coupling between the open jet shear layer and acoustic modes of part or all of the tunnel circuit. An analytical model is presented that highlights two acoustic modes that can lead to resonant fluctuations. The model predicts pulsation frequencies as a function of wind tunnel geometry. Predictions exhibit reasonable agreement with measurements. Axial static pressure measurements were acquired in the empty test section for various open jet lengths and collector configurations. A region of adverse pressure gradient is present immediately upstream of the collector inlet. For open jet region begins at a similar distance upstream of the collector. Given this independence of axial pressure distribution and collector throat area for these conditions, the collector throat area can be set to minimize low-frequency pressure pulsations.