The Unsteady Wind Tunnel was originally located at the National Bureau of Standards in Gaithersburg, Maryland, where it was designed and built by Dr. Philip Klebanoff. It was moved to Arizona State University in 1984 and became operational during fall 1987 after a total investment of $1,000,000 to reassemble the facility and provide a building and instrumentation. With the addition of new computers, instrumentation, and models the replacement cost of the facility is now $2,500,000. In 2003, the tunnel was decommissioned at ASU and relocated to Texas A&M University where it is in the process of building construction and tunnel reassembly. Current scheduling indicates that the building construction will be complete in June of 2007 and the tunnel reconstruction will be finished by the end of 2007. A presentation describing the new tunnel facility is available by clicking here or by clicking on the Publications link to the left.
The tunnel is a low-turbulence, closed-return facility with a test section capable of generating oscillatory flows for the study of unsteady problems in low-speed aerodynamics. It can also be operated as a conventional low-turbulence wind tunnel with a steady speed range of 1 m/s to 36 m/s (Mach 0.1) that can be controlled within a 0.1% accuracy. Provision is also made for simulating gusts and lulls of varying amplitude and frontal duration and for varying the intensity and scale of the free stream turbulence. A unique design feature consists of two parallel ducts, one of which serves as the test section. The flow oscillations are controlled by a system of rotating shutters such that the oscillations produced in one duct are out of phase with those in the other duct. This results in a performance improvement compared to conventional single-duct facilities.
The flow in the test section can be oscillated sinusoidally over a continuous frequency range from 0.1 Hz to 25 Hz. The amplitude of the oscillation can be varied, for example, from 0 to 100% of mean speed at 0.1 Hz for speeds up to 14 m/s. The rate at which the amplitude attenuates with increasing frequency is determined by the time constant of the facility which is approximately 0.2 seconds. The facility is powered by a 112 kW (150 hp) variable-speed DC motor and single-stage axial blower. The shutter blades upstream of the secondary duct serve as trim shutters for speed control in the secondary duct.
The tunnel is actually a major modification of the original NBS facility. ASU purchased a new motor drive which has the capability of continuous speed variation over a 1:20 range. This obviates the need for the throttling shutter blades upstream of the fan. In order to improve the flow quality, the entire length is extended by 5 m. On the return leg of the tunnel, the diffuser is extended to obtain better pressure recovery and to minimize large-scale fluctuations. The leg upstream of the fan is internally contoured with rigid foam in order to provide smooth contraction and smooth square-to-circular transition at the fan entrance. Steel turning vanes with a 70 mm chord, spaced every 40 mm, are placed in each corner of the tunnel.
On the test length, the contraction cone is redesigned by using a fifth-degree polynomial with L/D of 1.25 and a contraction ratio of 5.33. It is fabricated from 3.2 mm (1/8in) thick steel sheet. The primary duct has seven screens that are uniformly spaced at 230 mm (9 in). The first five screens have an open area ratio of 0.70 and the last two have an open area ratio of 0.65. This last set of screens are seamless and have dimensions of 2.74 m x 3.66 m (9ft x 12 ft) with 0.165 mm (0.0065in) diameter stainless steel wire on a 30 wire/inch mesh. An aluminum honeycomb, with a 6.35 mm (0.25in) cell size and a L/D of 12, is located upstream of the screens. Along with these changes a 1.6 m long settling chamber has been added downstream of the screens. This helps to lower the turbulence levels from those in the original NBS tunnel over the entire velocity range.
Both the test section and the fan housing are completely vibration isolated from the rest of the tunnel by means of isolated concrete foundations and flexible couplings. The test section is easily removable and each major project has its own test section. The interior of the return section is contoured with a wooden frame, window screens, and urethane foam in order to provide smooth contraction and square-to-circular transition before the fan. To measure the various speeds along the test wing a 3-dimensionial traverse mechanism has been installed.
