The NASA-Langley Mach 6 Quiet Tunnel (M6QT) was built as part of the Langley Hypersonic Facility Complex (currently known as the Aerothermodynamic Laboratory Complex) to provide a low-disturbance environment required for boundary-layer transition and instability testing. This tunnel served as a compliment to other Langley hypersonic tunnels that did not have the same low-disturbance characteristics. The M6QT was designed using results from the Langley research and development program focused on laminar-flow control techniques for low-disturbance tunnels. More specifically, Blanchard et al (1997) lists a "suction slot upstream of the nozzle throat," a well polished nozzle throat and a "straight contour just downstream of the nozzle throat" as key elements in quiet tunnel technology. After several years of use, the tunnel was decommissioned and eventually relocated to Texas A&M University (TAMU) in 2005 where it is currently being reassembled.
A complete description of the tunnel is given in Chen et al (1991) and Blanchard et al (1997). The operating principle behind the quiet tunnel is shown schematically in Figure 1.
Figure 1: Schematic of the flow structure within the NASA Langley M6QT (Blachard et al, 1997)
Original operating characteristics of the M6QT include stagnation pressure and temperature, measured from the settling chamber, ranging from 80 to 130 psia (Horvath et al 2002) and 350 to 400°F respectively. At 130 psia, the upper limit for quiet flow, the unit Reynolds number is 2.8 x 106 /ft (Wilkinson 1997). The maximum unit Reynolds number (no quiet flow) is 8.6 x 105/inch at a stagnation pressure of 475 psia (Wilkinson 1992). The tunnel's reference Mach number is 5.91 with a relatively large area of uniform flow that extends downstream of the nozzle exit (Blanchard et al 1997). On average, facility runtime was between 30 and 60 minutes.
Newly incorporated into the TAMU National Aerothermochemistry Laboratory, the M6QT is an open-jet blow-down tunnel that shares a common pressure and vacuum system with other hypersonic tunnels in the laboratory. Figure 2 provides a layout of the laboratory.
Figure 2: National Aerothermochemistry Laboratory Layout
Figure 3 shows a close-up of the original M6QT components that are being incorporated into existing TAMU hardware. The only Langley piece not shown is the 1-micron filter upstream of the settling chamber.
Figure 3: Langley M6QT Components, Dimensions in Inches (Note: test section enclosure is being designed and built at TAMU)
An 848 ft³ tank pressurized up to 2351 psia drives air via steel tubing into a one-micron filter where particle impurities are removed from the gas, preventing damage to the nozzle surface finish and instrumentation. Two Chicago Pneumatic Air Compressors (500 SCFM) recharge the tank when depleted. Nominally, the air is dry and kept at -99.4 °F; however, a 500 kW Chromalox circulation heater can preheat the air up to 494 °F. The filtered and preheated air flows into a stainless steel settling chamber approximately 85 inches in length with an internal diameter of 11.5 inches (Figure 4).
Figure 4: Schematic of M6QT Components. a) General Arrangement, b) Settling Chamber (Chen et al, 1991)
The chamber is split into two regions, upstream and downstream, with several screens and plates in the upstream half. In order of flow direction, it includes a cone shaped Rigimesh®, a perforated plate, steel wool, another Rigimesh with filter paper on the upstream side, two 8x8 mesh/cm screens, two 12x12 mesh/cm screens and three 20x20 mesh/cm screens (Chen et al 1991). Figure 5 shows the major internal components of the upstream half of the settling chamber.
Figure 5: Upstream Settling Chamber Components (Langley 1991)
The nozzle is stainless-steel and weighs approximately 400 pounds. Its internal surface is treated with a nickel-phosphorus alloy to prevent oxidation. The expansion length starting from the throat minimum and exit diameter are 39.76 inches and 7.5 inches respectively (see Figure 6).
Figure 6: Mandrill for M6QT Nozzle (Langley 1988)
Nozzle throat minimum is 1.00 inch (Horvath 2002). In addition to a highly polished throat, the nozzle also houses lip suction slots at the entrance, which when open bleed off the turbulent boundary-layer from the settling chamber and contraction, allowing the nozzle boundary-layer flow to remain laminar for a longer period of time. The nozzle is surrounded by a test section that also houses the diffuser. Figures 7 and 8 show the original test chamber with the nozzle and diffuser installed.
Figure 7: Original Langley Nozzle Test Chamber (Langley 2005)
Figure 8: Close up of Original Langley Nozzle Test Chamber; Left: Nozzle Sleeve and Model Test Stand; Right: Model Test Stand and Diffuser (Langley 2005)
A new test chamber, more suited to current testing requirements is currently under design at TAMU. The test section is 13.63 inches long. The diffuser has an entrance diameter of 10.75 inches, which contracts to a 7.50 inch diameter 6.00 inches downstream. Total diffuser length is approximately 36 inches. The flow from the diffuser exits into a vacuum created by two-stage Fox Brand Ejector system, with a 2 ft diameter suction inlet, pictured in Figure 9.
Figure 9: Fox Ejector System Located Outside the National Aerothermochemistry Laboratory
Maximum run time for the tunnel is 2 minutes based on a fully charged tank. The available Reynolds number range within this system is approximately 1.0 x106 to 2.0 x 107 /m. The incoming pressurized air and outgoing ejector system are controlled through regulators placed inline with the air pipelines and monitored on gauges located inside the laboratory. Pressure is manually adjusted prior to the start of testing and is not changed during tunnel operation. A variety of experimental and computational equipment is available to support the M6QT operation, such as laser diagnostics, hot-wire anemometry and parallel computing.
References:Blanchard, AE, Lachowicz, JP,Wilkinson, SP 1997. NASA Langley Mach 6 Quiet-Tunnel Performance, AIAA J., 35, No. 1, pp 23-28.
Chen FJ, Wilkinson SP, Beckwith IE. 1991. Görtler Instability and Hypersonic Quiet Nozzle Design. AIAA Paper No. 91-1648.
Horvath TJ, Berry SA, Hollis BR, Chang CL, Singer BA. 2002. Boundary layer transition on slender cones in conventional and low disturbance Mach 6 wind tunnels.
AIAA Paper No. 2002-2743.
-- . 2005. Photos of NASA-Langley Research Center Mach 6 Quiet Tunnel Test Chamber. Personal communication from SP Wilkinson to R Bowersox.
-- . 1988. Photos of NASA-Langley Research Center Mach 6 Quiet Tunnel Nozzle. NASA Langley Photo No. 88-3891.
-- . 1988. Photos of NASA-Langley Research Center Mach 6 Quiet Tunnel Settling Chamber Components. NASA Langley Photo No. 91-15320.
