Development and Testing of a Novel Acoustic Wind Tunnel Concept

*† ‡ § A new anechoic wind tunnel test section concept was developed and tested. The new concept involves the construction and installation of a test section with walls formed largely from tensioned Kevlar cloth embedded in an anechoic chamber. This acoustically open design has the potential to eliminate the need for a jet catcher and reduce interference effects. The concept was tested in the Virginia Tech Stability Wind Tunnel. The existing wind tunnel test section was modified to incorporate prototype acoustic treatment, two large Kevlar side-walls and surrounding acoustic enclosures. In-flow microphone measurements over the whole speed range of the facility showed that the acoustic treatment was effective in reducing noise levels by 8 to 15 dB. A NACA0012 airfoil model was placed inside the modified test section to perform aerodynamic and aeroacoustic measurements for proof-ofconcept. Tests with and without an airfoil model showed the Kevlar side-walls can quietly and stably contain the flow. Furthermore, they significantly reduce lift interference. Phased array measurements of the trailing edge noise produced by the airfoil demonstrate the acoustically transparency of the Kevlar walls and the practicality of making noise measurements through them. I. INTRODUCTION This paper describes the preliminary development and testing of a new anechoic wind tunnel test section design. The design uses large areas of ballistic Kevlar cloth to provide a stable flow boundary. Sound generated in the flow can propagate through the Kevlar cloth into a surrounding anechoic chamber, where detailed noise measurements can be made. The specific application of this work is to the acoustic upgrade of the Virginia Tech Stability Wind Tunnel. This facility can generate flows of up to 80 m/s through its 1.83 m x1.83 m square test section. The use of Kevlar walls in the test section of this facility offers the possibility of providing anechoic capability without the complication and noise generated by a jet catcher. This approach also holds the promise of controlling lift interference, which can be a large factor in open-jet wind tunnels. The objective of this work is a wind tunnel design that, in the case of the Stability Wind Tunnel, will allow the testing of the aerodynamics and aeroacoustics of large scale lifting configurations (particularly airfoils) at realistic Reynolds numbers. In this paper we describe the design of the prototype acoustic test section, including acoustic treatment, acoustic enclosures and the Kevlar cloth walls. We also describe the development of a 63-microphone phased array system used to make measurements through these acoustic windows. We report on the aerodynamic behavior of the Kevlar walls over the full speed range of the facility, and the effects of the windows and treatment on in-flow noise levels. We also report on the aerodynamic interference generated by the cloth walls observed through mean pressure distributions measured on a large NACA 0012 airfoil as a function of angle of attack and flow speed. Finally we demonstrate that trailing-edge noise measurements of the same model can be made through the Kevlar walls with the phased array system. The results show a significant reduction in the background noise level of the flow. More importantly, they also show that large Kevlar panels can be used to quietly and stably contain the flow eliminating