Simulating planes flying at Mach 6
Scramjets, or supersonic combustion ramjets, such as the X-51A aircraft being built by Pratt & Whitney and Boeing, should start to fly in 2009. And if everything goes according to plan, missiles flying at Mach 6 could be deployed by 2015. But designing such planes is not so easy. This is why Purdue University engineers have developed the only wind tunnel capable of running quietly at 'hypersonic' speeds. The Purdue engineers say that this 'quiet wind tunnel operation is critical for collecting data to show precisely how air flows over a vehicle's surface in flight' at hypersonic speeds. But read more...
You can see above a photo of Steven Schneider, "an aerospace engineer and professor in Purdue's School of Aeronautics and Astronautics, [operating] the university's one-of-a-kind wind tunnel, which runs quietly at Mach 6, or six times the speed of sound." (Credit for photo: David Umberger for Purdue University) This photo and its caption come from another Purdue University news release, 'Quiet' Mach 6 wind tunnel at Purdue helps shape future aircraft (January 5, 2006). And here is a link to a larger version of this picture. You also might to take a peek at this Boeing/AFOSR Mach-6 Quiet Tunnel Image Gallery.
Before going further, here are two links to wikipedia pages about scramjets and the Boeing X-51. As said Schneider in the latest Purdue news release, "the X-51A is a wedge-shaped vehicle with a scooplike cowl on its underbelly, where air rushes into the inlet of the engine's combustor. It is critical for air entering the inlet to be turbulent at hypersonic speeds, or the engine could 'unstart,' causing it to crash." So it is critical to simulate the behavior of such planes, which is being done in the Purdue wind tunnel.
You can see above a schematic of the Boeing/AFOSR Mach-6 Quiet Tunnel. "Quiet facilities require low levels of noise in the inviscid flow entering the nozzle through the throat, and laminar boundary layers on the nozzle walls. To reach these low noise levels, conventional blow-down facilities must be extensively modified. Requirements include a 1 micron particle filter, a highly polished nozzle with bleed slots for the contraction-wall boundary layer, and a large settling chamber with screens and sintered-mesh plates for noise-reduction. To reach these low noise levels in an affordable way, the Purdue facility has been designed as a Ludwieg tube. A Ludwieg tube is a long pipe with a converging-diverging nozzle on the end, from which flow exits into the nozzle, test section, and second throat." (Credit: Purdue University)
Here are some of the findings done by the Purdue engineers. "The research findings will enable engineers to determine precisely where to place the trips and how far they should be raised from the aircraft's skin, Schneider said. Experiments under quiet conditions yielded more accurate findings compared with experiments under noisy conditions. The quiet data indicated the trips should be raised about twice as high."
They also "are able to switch the wind tunnel back and forth from quiet to high-noise airflow, which allows them to compare the quality of data in the two modes. To measure the airflow velocity and turbulence, the researchers use a heated wire about one-tenth the diameter of a human hair. The higher the speed of the airflow, the more the wire is cooled and the greater the electrical current needed to maintain the wire's hot temperature. Monitoring the changing current needed to maintain the wire's temperature reveals the changing air speed at fluctuations of up to 250,000 times per second."
Their findings will be presented on January 8, 2007 during the 46th AIAA Aerospace Sciences Meeting and Exhibit which will be held in Reno, Nevada (January 7-10, 2008).
Schneider will present a paper named "Effect of Freestream Noise on Roughness-Induced Transition for the X-51A Forebody" (PDF format, 16 pages, 5.72 MB). Here is the abstract. "A 20%-scale X-51A forebody model was tested in the Boeing/AFOSR Mach-6 Quiet Tunnel. Repolishing the nozzle throat has restored quiet flow at high Reynolds numbers. The effect of a smooth blank and two different trip strips on windward-forebody transition was measured using temperature-sensitive paint and hot-wire anemometry. Reducing freestream noise from conventional to quiet levels increased the smooth-wall transition Reynolds number by a factor of at least 2.2. In addition, the transition Reynolds number based on the distance from the trips increased by a factor of 2.4 for the smaller trips and by a factor of 1.7 for the larger trips. Thus, tunnel noise had a substantial effect on roughness-induced transition."
The schematic of the Boeing/AFOSR Mach-6 Quiet Tunnel and its caption have been extracted from this paper. And if you're interested in in this subject, please note that Schneider is also one of the co-authors of two other papers presented during this conference, "Surface-Pressure Measurements of Second-Mode Instability in Quiet Hypersonic Flow" (PDF format, 14 pages) and "Hypersonic instability waves measured using fast-response heat-flux gauges" (PDF format, 11 pages).
Sources: Purdue University news release, January 2, 2008; and various websites
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