Experimental wind-tunnel results for radiation of sound from semi-buried ducts

Abstract

Semi-buried engines are part of of many future aircraft designs. From acoustic’s point of view the increased noise shielding is one of the potential benefits. But at the same time the fan will experience a disturbed inflow which can lead to additional noise. In order to assess such new concepts numerical predictions can help, but it at the same time it is important to validate the methods against experimental data. The current paper describes a wind tunnel experiment which can serve as a validation case. The setup consists of a duct which is mounted on a flat plate. The whole model is placed inside a wind tunnel and tested with different flow speeds. Upstream the duct, a boundary layer develops which enters the duct, imitating boundary layer ingestion. The sound is generated by a specially designed source that it can be easily translated to computational methods. All important design details are given which makes it possible to use it as a benchmark case for numerical methods. The sound radiation is measured upstream the duct. Since the sound source operates at dedicated frequencies, clear interference patterns become visible. Such patterns can help to make small configuration changes visible (e.g. different flow speeds). The results serve as a starting point for detailed studies of flow and boundary layer effects on sound radiation from a duct. Numerical methods can be validated against the given result and extend the parameter range e.g. to much thicker boundary layers as well as recording results at much larger evaluation planes. However, such numerical studies are beyond the scope of this paper.