Abstract
In order to understand mechanism of aerodynamic noise generation in large turbulent flow fields, vortex structures of flow around a circular cylinder were visualized by using an active turbulence generator (ATG). To reduce the noise level of the active turbulence generator, passive silencers and low noise operation mode were developed. Therefore the aerodynamic noise from the developed ATG was only 10 dB larger than that of the conventional wind tunnel. It is well-known that small holes were used to reduce aerodynamic noise radiated from a circular cylinder. Noise reduction effect of the holes was estimated in the large-scale turbulent flow filed. The experimental result showed that the noise reduction effect of the small holes was decreased when the cylinder was placed at strong turbulent flow field. In the case of the laminar flow, small holes make the impinging jets just behind the cylinder. The jets destroy the Karman vortex and reduce the aerodynamic noise. On the other hand, the velocity of the impinging jets was reduced by uniform turbulent flow. Then, the noise reduction effect of the holes was reduced in a large-scale turbulent flow field.
