Audi Aero-Acoustic Wind Tunnel 930300

The present paper reveals the design concept as well as results of experimental investigations, which were conducted in the early design stage of the planned AUDI Aero-Acoustic Wind Tunnel. This low-noise open-jet facility, featuring a nozzle exit area of 11 m² and a top speed of approximately 60 m/s, enables aerodynamic as well as acoustic testing of both, full-scale and model-scale ground vehicles. Ground simulation is provided by means of a moving-belt rig. The surrounding plenum is designed as a semi-anechoic chamber to simulate acoustic free-field conditions around the vehicle. Fan noise will be attenuated below the noise level of the open jet.

The work reported herein, comprises 1/8-scale pilot-tunnel experiments of aerodynamic and acoustic configurations which were carried out at the University of Darmstadt. One of the primary goals was minimisation of collector induced broad-band noise and avoidance of discrete frequency open-jet excitations due to shear layer impingement on the collector. Furthermore, the pressure distribution along the empty test section and its pressure-loss had to be optimized. To match these requirements, a relatively long test section and a large collector appeared to be especially advantageous. Extensive experimental investigations were conducted to optimize the collector's aerodynamic as well as acoustic performance. The results suggested a test-section length of 10 m (=3.1 hydraulic nozzle diameters) and a funnel-type collector featuring a collector-to-nozzle area of about 3.7. Tests with models in the test section showed that even with a blockage ratio of 18 %, reliable aerodynamic data can be obtained. It was found that pressure distributions along comparison models, taken in a large tunnel at low blockage (1.9 %), could be reproduced satisfactorily, if the model was positioned at about 40 % of the test-section length.