AIAA 2005-3052
11th AIAA/CEAS Aeroacoustics Conference (26th AIAA Aeroacoustics Conference) 23 - 25 May 2005, Monterey, California
Design, Fabrication, and Characterization of an Anechoic Wind Tunnel Facility J. Mathew*, C. Bahr*, B. Carroll†, M. Sheplak†, L. Cattafesta† University of Florida, Gainesville, FL, 32611-6250, (352) 846-3017, FAX (352) 392-7303,
[email protected]
The design, fabrication, and preliminary characterization of an anechoic wind tunnel facility at the University of Florida are presented. A previously existing and ISO 3745 validated 100 Hz anechoic chamber is upgraded to incorporate an open-jet anechoic wind tunnel facility suitable for airframe noise studies. The wind tunnel is driven by a 224 kW ( 300 HP ) , 69 m3 s (147, 000 cfm ) centrifugal fan controlled by a variable frequency drive. Airflow enters the wind tunnel through a settling duct with a honeycomb section and a set of four screens for the purposes of flow straightening and turbulence reduction, respectively. An optimized, minimum length, 3-D contraction designed using various computational methods accelerates the flow into a rectangular test section that measures 0.74 m ( 29") by 1.12 m
( 44")
by 1.83 m
( 6 ft ) .
is constructed using 19 mm
The contraction shape consists of matched polynomials and
( 0.75")
thick reinforced fiberglass.
Static pressure
measurements along the contraction length validate the design procedure. The estimated maximum velocity attainable in the test section is ~ 76 m / s ( 250 ft / s ) ; thus the maximum Reynolds number based on chord (=2/3 span) is Rec = 3 − 4 × 106 . Preliminary facility characterization experiments at a test section velocity of U TS ≅ 17 m / s indicate the rms flow non-uniformity and the turbulence intensity level at the nozzle exit are < 0.7% and < 0.11%, respectively. The flow leaving the test section enters a 2-D diffuser, turns a 90° corner using fiberglass, rubber-filled turning vanes, and then enters a second 2-D diffuser. The flow leaving the second diffuser then enters the fan through a vibration-isolated rectangular-toround transition section. The two diffusers and the corner sections are lined with a metal screen or perforate bounding 30.5 cm (12") thick bulk fiberglass to attenuate fan noise. The fan and its silencer rest on a vibration isolated concrete mass base located outside the building to minimize vibrations and the resulting noise that propagates to the chamber. Preliminary background noise level measurements with an empty test section reveal an overall SPL from 100 Hz – 20 kHz of 49.9 dB ( re 20 µ Pa ) , with a peak 1/3 octave-band level of 46 dB at 100 Hz that decreases to 29.9 dB at 1 kHz .
I.
Introduction
T
he aim of this research is to design, fabricate, and characterize an anechoic wind tunnel facility at the University of Florida (UF). An existing 100 Hz anechoic facility at UF 1 has been upgraded to an anechoic wind tunnel. The purpose of this effort is to permit high quality fluid dynamic and aeroacoustic experiments on airframe noise, with an initial focus on trailing edge noise. Strict regulations on the noise from commercial aircraft has increased the emphasis on airframe noise,2 which is often a significant portion of the aircraft noise during approach and landing. A reduction in aircraft noise will require attenuation of airframe noise produced by aircraft components, such as airfoil trailing edges, landing gear, airfoil flaps and slats, and wing tips. A fundamental understanding of the noise generation mechanisms will provide the ability to model and predict the emitted noise and may enable researchers to devise effective schemes to reduce * †
Graduate Research Assistant, MAE Department, P.O. Box 116250, Student Member AIAA. Associate Professor, MAE Department, P.O. Box 116250, Associate Fellow AIAA. 1 American Institute of Aeronautics and Astronautics
Copyright © 2005 by the author(s). Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.
airframe noise. However, appropriate experiments conducted in an anechoic wind tunnel are required to achieve significant advances. The existing trend in wind tunnel design is to construct large facilities with low background noise levels.3 Large facilities can provide high Reynolds numbers and low turbulence intensity in the test section. However, facility cost and power consumption increase dramatically with size, making these large facilities prohibitively expensive, particularly at a university scale. Currently, few high-quality, university-scale facilities exist within the United States.4 Our objective is to fabricate a university-scale, anechoic wind tunnel suitable for high quality airframe flow and noise measurements at higher Reynolds numbers than have been previously reported in the literature. The outline of this paper is as follows. Section II summarizes the design considerations and methodology and the fabrication of the facility components. Section III presents preliminary flow quality and noise measurements at U TS ≅ 17 m / s . Section IV offers preliminary conclusions and discusses future work.
II.
Design Considerations
The design of any wind tunnel presents numerous choices and inevitable compromises. Most airframe noise studies have been limited to chord Reynolds numbers, Rec , on the order of a million or less. Our overriding objective was to achieve Rec = 3 − 4 × 106 , as shown in Figure 1. Budgetary and fan size constraints, combined with a restriction on a model wing aspect ratio to maintain nominal 2-D flow (i.e., span/chord ≥ 1.5), established the test section size of 0.82 m 2 (1276 in 2 ) using a 224 kW ( 300 HP ) centrifugal fan capable of providing a maximum flow rate of 69 m3 s (147, 000 cfm ) with a pressure rise of 1993 Pa in the test section is 76 m / s
( 250 ft / s ) , or a Mach number
(8" H 2O ) .
The resulting maximum velocity
M = 0.22 , corresponding to a typical approach speed.
Clearly, the flow uniformity in the test section should be high, and our objective was to achieve
