Low Noise Aeroacoustic Open Jet Wind Tunnel
Our new low noise wind tunnel is of the open test section type and is located in Cedaredge, Colorado.
Aeroacoustic measurements are typically performed in open jet wind tunnels. Sound reflected from the walls, which is a problem with closed section wind tunnels, can be reduced significantly with proper acoustic room treatments. At the same time, it is important to be aware of the effect / implications of open jet shear layers.
The wind tunnel is powered by two low noise single-stage axial fans. The primary fan generates approximately ~1,250 cfm and the secondary (return) fan generates approximately ~750 cfm. The fans we selected produce high cfm's and exceptional static pressure performance. Fan speeds are variable from 35% to 100% and digitally calibrated for wind speed accuracy and repeatability. Several different nozzles can be deployed to vary turbulence levels and increase flow velocities above 50 mph. A manometer with pitot probe is used to monitor internal pressure.
The wind tunnel is housed in an acoustically treated 6' by 28' room. Primary sections of the room are lined with acoustic paneling to help isolate the room from external sound and vibration. These panels are then covered with acoustic foam wedges. Remaining sections are selectively treated with sound absorbing panels and acoustic sound blankets. The combined sound proofing and sound absorption treatments create a testing environment that rivals the quietest of country roads and naturescape environments.
Audio Measurement Equipment
The low background noise environment (~35 dB) is perfect for aeroacoustic wind interaction studies and is supported by a very well-equipped acoustics laboratory. Multiple microphones, narrow band analyzers, signal generators, and required microphone calibration equipment is available.
Air Flow Measurement Equipment
Because accurate and precise measurement of air flow characteristics is so important, we use a variety of specialized anemometers. These anamometers allow us to evaluate approaching, encircling and close proximity air flow. We have also made equipment modifications to meet our very specialized requirements.
Generation of Wind Tunnel Turbulence
Atmospheric (intrinsic) turbulence intensities typically vary from 0 to 10% depending on wind conditions. In addition turbulence length scales can vary significantly. To take advantage of the controlled and more repeatable conditions experienced in the wind tunnel (vs. on the road), we employ a number approaches to simulate the effects of unsteady onset flow including several turbulence grids and the mini wind jet shown below.
Flow Visualization and Imaging Equipment
While our primary flow visualization equipment (hydrodynamic flume with floresceine dye) is located in New Jersey, our Cedaredge faculty is equipped with preliminary smoke visualization capabilities. This allows us to make initial observations - with more detailed flow analysis in New Jersey. Our new facility includes digital imaging with imaging software, focused smoke generation capabilities, and non-modulating laser light sheets.
Assessing the Human Response to Wind Noise
Because microphones and ears behave differently, we incorporate human testing in our new wind tunnel facility. This allows us to identify and compensate for related microphone vs. human ear differences. Understanding perceived wind noise helps ensure that our performance comparisons and conclusions are more accurate.
Independent BTU Aeroacoustic Wind Tunnel Tested
In addition, Cat-Ears have been independently tested, and performance validated, in the open jet aeroacoustic wind tunnel at Brandenburgische Technische Universität (Technical Acoustics Department) at Cottbus (Germany). "The aeroacoustic wind tunnel at BTU is a research facility for the experimental study of sound generation from bodies immersed in a fluid flow." The wind tunnel can be operated at very low noise levels and turbulence intensities. While the BTU wind tunnel is larger, the BTU and Cat-Ears wind tunnels have similar aeroacoustic testing capabilities.
Schema of the wind tunnel layout: (1a) baffle, (1b) return intake, (2) splitter silencer, (3) contraction, (4) bend, (5) straight pipe, (6) and (7) flexible-flange duct connectors, (8) square-to-round transition, (9) circular silencer, (10) diffuser silencer, (11) settling chamber with honeycomb and screens, (12) nozzle (all dimensions are given in mm).