Reduction of 3/4 open jet low-frequency fluctuations in the S2A wind tunnel

3/4 open jet wind tunnels are subjected to undesired low frequency pressure and velocity fluctuations at distinct flow velocities. The aim of this paper is to present recent research done in the full scale automotive aero-acoustic facility of the S2A wind tunnel complex in order to characterise the physical mechanism causing the fluctuations, to clarify their impact on the aerodynamic measurements and to validate solutions that allow high quality aerodynamic and acoustic measurements over the entire velocity range of the wind tunnel. Four areas of significant oscillations have been detected in the full scale S2A wind tunnel for wind velocities between 10 and 60 m/s. Those fluctuations are due to successive lock-ins of the wind speed dependent shear layer instability of the jet, with acoustic resonant frequencies of the wind tunnel. Various solutions have been tested in order to assess their impact on the fluctuations and on the quality of the aerodynamic measurements. A less protruding solution than classical vortex generator has been validated. It is made of two rectangular flaps along the upper edge of the nozzle exit, with a small plunging angle.