With the Langley 8-foot and 16-foot high-speed tunnels now operational with slotted walls, tunnel designers could turn their attention to three other problems plaguing the operation of transonic wind tunnels: (1) high humidity and fog, (2) high turbulence levels in main stream flow, and (3) relatively low Reynolds numbers.
Fog and moisture in the tunnels were the most pressing problems. All the early tunnels operating near the speed of sound drew in outside air for cooling purposes. During the humid summer days, when moist outside air mixed with the mainstream flow,...
....cooling due to expansion in the high-speed nozzle was sufficient to create fog so dense that the model was obscured. Droplets of water condensing on the model and instruments hampered data collection and upset tunnel calibration. This was hardly the controlled environment customarily promised by wind tunnels. The obvious solution eliminated the source of moisture, that is, the cooling air from outside. In the new 8-foot transonic pressure tunnel, which became operational at Langley in 1953, a finegrid water-cooled coil in the airstream removed excess heat but added no moisture to the circulating air. This fine grid plus an array of screens and a high tunnel contraction ratio smoothed out the turbulent air to partially solve the second operational problem. To increase Reynolds numbers (the third problem), the tunnel was operated at pressures up to 2 atmospheres.
The designs of a long series of aircraft, including the Grumman F-9F and Convair B-58 supersonic bomber, were experimentally validated in the 8-foot transonic pressure tunnel. Later, rocket launch vehicles, reentry nose cones, the Viking spacecraft, the Space Shuttle, and many other famous craft did their tours in this tunnel.
On the west coast, NACA engineers quickly exploited the slotted wall. The Ames 16-foot highspeed tunnel, which had been operating since 1941 with a 27 000-horsepower drive, was repowered to 110 000 horsepower. The quadrupling of the power level made it possible to operate in the transonic range with a 14-foot ventilated test section wall. As the adjective "ventilated" implies, the Ames transonic tunnel differed markedly from the Langley slotted-wall approach. Replacing the slots were a mesh of holes in the test section walls which attenuated the reflections of shock waves generated by the model. Another unique feature was the flexible wall upstream of the test section. By activating a jack, tunnel operators could adjust the shape of the supersonic nozzle and thus attain various supersonic airspeeds.
The Ames 14-foot transonic tunnel-the most powerful in the world-commenced operation in late 1955. It had cost only $2 million to build in 1941; but the repowering, the new transonic test section, and other modifications cost an additional $9 million in 1955. The investment was worthwhile. With its large size and attainable speeds of Mach 0.6 to Mach...
...1.2, it produced accurate simulation of air inlets operating propellers, and even full-scale missiles. The 14-foot tunnel was particularly useful in solving the stability problems of large, fuel-laden rocket launch vehicles. These thin-skinned structures carried millions of pounds of sloshing fuel as they pushed through the aerodynamically critical transonic range. With accurate wind tunnel data, launch vehicle designers could better guarantee the structural integrity of large rockets.