Noise is a significant issue facing aviation today. The reduction of aircraft generated noise is difficult without a comprehensive understanding of the mechanisms behind aeroacoustic noise generation. Experimental aeroacoustic facilities play a critical role in elucidating the fundamental phenomena that result in the generation and propagation of such disturbances.
In order to provide the best possible conditions for this type of research, the open jet, anechoic wind tunnel at the University of Toronto Institute for Aerospace Studies has recently undergone refurbishment to improve the flow and acoustic properties of the facility. This facility is used to support aeroacoustic investigations of various aircraft components, including landing gear and high lift devices. Exterior and plan views of the wind tunnel are shown in Figures 1 and 2 respectively. The major upgrades to the facility include resurfacing of flow facing components, redesign and replacement of flow conditioning honeycomb and screens, design and installation of secondary contraction and replacement of anechoic wedges. This paper outlines the design process and characterisation of the improved facility.
Obtaining steady and uniform flow in the acoustic wind tunnel is critical for the proposed research applications. The bell-mouth pieces, settling chamber and primary contraction were resurfaced to provide a smooth flow-facing surface. A newly designed honeycomb screen with 3/8" cell diameter, based on the guidelines by Lindgren & Johansson (2002) and Mehta & Bradshaw (1979), were installed to provide flow straightening. To improve flow uniformity and reduce freestream turbulence intensity, five mesh flow conditioning screens were installed downstream of the honeycomb. These 32 mesh screens with open area ratios of 62.7% were designed in accordance with Laws & Livesey (1978) and Mehta & Bradshaw (1979). The original circular nozzle produced an undesirable, non uniform potential core with regions of accelerated flow near the shear layer. In order to approach the more desirable top hat velocity profile for the potential core and to increase maximum flow velocity, a secondary contraction was designed to replace the original. The contraction profile was based on recommendations by Mathew (2006) and Su (1991), and the new velocity profile was characterised using a traversing hot wire probe. Comparison of the old and new potential core velocity profiles and freestream turbulence intensity will be provided in the final manuscript.
Quality anechoic wind tunnel measurements require the test facility to have a minimal noise floor, reverberation time and low frequency cutoff. To address these requirements for current projects and future research initiatives, the previously installed fibreglass anechoic insulation has been replaced with open cell polyurethane foam wedges (see Figure 3). Results of the acoustic characterisation efforts using a point noise source and traversing free field microphone will be presented in the final manuscript.
References
Laws, E. M. & Livesey, J. L. Flow through screens. Ann. Rev. Fluid Mech., vol. 10, pp. 247-266, 1978.
Lindgren, B & Johansson, A. V. Design and evaluation of a low-speed wind- tunnel with expanding corners. Technical reports from Department of Mechanics, Royal Institute of Technology, Sweden, 2002.
Mathew, J. Design, fabrication, and characterization of an anechoic wind tunnel facility. PhD thesis, University of Florida, 2006.
Mehta, R. D. & Bradshaw, P. Design rules for small low speed wind tunnels. Aeronautical Journal of the Royal Aeronautical Society, 1979.
Su, Y. X. Flow analysis and design of three-dimensional wind tunnel contractions. AIAA Journal, vol. 29, no. 11, pp. 1912-1920, 1991.
