Active Suppression of Buffeting at the Audi AAWT: Operational Experiences and Enhancements of the Control Scheme 2004-01-0804
In order to suppress the well-documented low frequency pressure fluctuations in open jet wind tunnels, termed ‘wind tunnel buffeting’, an Active Resonance Control (ARC) System was implemented in the Audi aero-acoustic wind tunnel several years ago. This ARC-Sys-tem reduces the periodic pressure fluctuations by up to 23 dB and completely eliminates the periodic velocity fluctuations using a simple feedback control scheme.
To set up the ARC system in practice, the system's parameters are optimised once for each critical flow velocity, when the vortex shedding frequency coincides with an acoustic resonance mode of the wind tunnel.
Due to the fact that both frequency and amplitude of the excited resonances not only depend on flow velocity but also on other parameters such as collector position and test-car geometry, the system has to be adjusted with regard to each of these cases. Furthermore, experimental data suggests that multiple resonances are excited simultaneously under certain conditions.
For these reasons a new, self-adapting control scheme was developed which combines the utilisation of the same physical mechanism of damping with greater control flexibility.
The first part of this paper deals with experimentally measured pressure fluctuations and the influence different variables have on this phenomenon. The configuration of the self-adapting Enhanced Active Resonance Control (EARC) together with experimental results is presented in the second part of the paper.