Search Results

A well-known problem with open-jet wind tunnels are low-frequency pulsations, which affect the flow quality and thus the quality of the measured data. This so-called wind tunnel buffeting is caused by large-scale vortices in the shear layer, which excite acoustic resonant modes of the wind tunnel circuit. This paper presents a novel approach to control low-frequency pulsations by means of active flow control at the nozzle exit. The final setup consists of oscillated flaps which are actuated by electrodynamic shakers. Single-frequency sinusoidal signals and band-limited random signals were used for excitation. The oscillated flaps prevent the synchronization of vortex shedding with acoustic resonance by generating forced perturbations in the shear layer at a frequency that is different from the acoustic resonance. The flaps - which were named FKFS-flaps by the authors - were installed at the IVK 1:4 model-scale wind tunnel.

Low-frequency pressure fluctuations which occur at certain flow speeds are an undesired feature in many open jet wind tunnels. This so called ‘wind tunnel pumping’ affects the aerodynamic quality of the flow and thus the quality of the measured data. In this paper a novel approach is presented to control the pulsation phenomenon by active damping of the acoustic resonant modes of the wind tunnel circuit. The acoustic mechanism of the resonance effect was investigated using a 1/20 scale pilot wind tunnel with a complete and detailed representation of test section, ducting, turning vanes and fan. The newly devised Active Resonance Control (ARC) System essentially consists of a microphone which picks up the pressure fluctuations in the plenum, a loudspeaker which is mounted in the tunnel wall and a time delay to adjust the phase relation between the microphone signal and the loudspeaker output.

The out-of-flow background noise spectrum in the test section of the full-scale low-noise automobile wind tunnel facility of BMW Technik GmbH in Munich showed a pronounced hump around 2 kHz Although this excess noise component was audible, it did not interfere with the automobile flow noise studies conducted in the tunnel An experimental study was performed to identify the source mechanism. It was found that the excess noise is generated by small cavities in the face side of the impeller blade tips When the cavities were closed the excess noise component completely dissappered The peak spectral level of the fan noise was reduced by as much as 30 dB The overall out-of-flow level reduction in the test section was 3 dB(A).