During World War II NACA engineers at Ames decided to try combining three desirable wind tunnel...
....characteristics in a single tunnel. These coveted qualities were and still are:
1. High Reynolds numbers
2. High subsonic speeds
3. Very low airstream turbulence.
Previously, tunnel designers had set their sights on only one or two of these objectives in a tunnel design, mainly because each goal requires considerable engineering finesse. Striving for all three in the same tunnel posed too big a challenge in the early days of tunnel design.
The new tunnel that took shape on the Ames drawing boards certainly had a different look about it. Basically, it was a 12-foot tunnel at the test section, but just before the test section was a rather grotesque bulge some 43 feet in diameter. Low-turbulence screens located here smoothed out the airflow, thus achieving one of the goals. High-speed flow was obtained by brute force-a 12 000-horsepower electric drive system. By examining the tunnel corners, one can discover a clue as to how high Reynolds numbers were achieved. Instead of the usual sharp, mitered 90-degree corners, the tunnel high-speed airstream is turned in small angular steps. Such stepwise construction withstands high pressures much better than sharp-angled turns. Obviously (at least to an engineer), high Reynolds numbers were reached by pressurizing the tunnel. Six atmospheres pressure was the goal, although this specification was later reduced to five. Shell plate thickness approached 2 inches in many places. The total structural weight was 3000 tons.
The high-pressure integrity of this massive shell was tested by filling it with water-a rather novel idea to the layman but a wonderful idea to safety engineers. Multiply the miniature violence of a pricked rubber balloon millions of times and you will understand why no one wanted to pump an untested tunnel up to 6 atmospheres of air pressure, and especially not to the 9 atmospheres (50 percent overpressure) required to prove safety. The energy of the compressed air at 9 atmospheres would have been...
....sufficient to blow the entire 3000-ton tunnel 1/2 mile high. While an air-filled tunnel was a bomb, a water-filled tunnel was safe because water is essentially incompressible. Of course, it might rupture like a harpooned waterbed, but the results would be far less catastrophic than chunks of 2- inch-thick steel plate raining down on the Sunnyvale countryside.
The filling took a week: 5 000 000 gallons (20 800 tons) of water. Of course, the tunnel foundations had to be built with this immense temporary load in mind. The foundation survived the first pressure test, but as the internal pressure crept up toward 9 atmospheres, there was a terrific report, and a highpressure jet of water sprayed the area. A steel plate had ruptured.
Inspection and analysis proved that the rupture resulted from stress concentration at the joint of two plates of different thickness. The failure was not serious, and repairs were made promptly.
On the second try the tunnel passed the hydrostatic test successfully. Both NACA and later NASA profited from this experience. Thirty years later, during the design of the National Transonic Facility, NASA reviewed hydrostatic pressure tests and decided to apply similar tests to the new facility, which had to withstand an operating pressure of 9 atmospheres.
When the 12-foot pressure tunnel commenced operation in July 1946, all eyes were focused on tunnel calibration, air velocity, air turbulence, and flow uniformity. All performance requirements were met, assuring a long productive life of aerodynamic research.
The unique capabilities of the 12-foot tunnel- large size, high Reynolds numbers, and low turbulence-were used to explore the performance of low-aspect-ratio straight wings, swept wings, and delta wings with different cambers and flap systems. This effort led to important performance increases on the Convair F-102 and F-106 fighters through the use of conical camber on the leading edges of the delta wings. Space reentry vehicles depended heavily on the 12-foot tunnel for the assessment of scale effects, even though the tunnel tests were limited to subsonic speeds.
Perhaps the tunnel's greatest contribution was in the development and testing of wing landing- flap systems at high Reynolds numbers. Almost all our modern military and commercial aircraft have benefited from this research. More recently, the low-turbulence qualities of the tunnel have been exploited in critical laminar-flow-control experiments in the development of fuel-efficient, long-range transports.