Evolution and Development of Cryogenic Wind Tunnels

Abstract

Starting in the early 1970s, researchers at the NASA Langley Research Center developed an improved way to increase the Reynolds number capability of large research wind tunnels. Cooling the test gas to cryogenic temperatures by spraying liquid nitrogen into the tunnel increases Reynolds number by as much as a factor of 7 with no increase in dynamic pressure and with a reduction in drive power. The purpose of this paper is to review the evolution and development of cryogenic wind tunnels used for aerodynamic research. I. Introduction Wind tunnels have played an important part in the development of every airplane from the 1903 Wright Flyer to our most advanced aircraft. Even in this age of supercomputers and advanced computational fluid dynamics, the need for good experimental data from wind tunnels increases with the development of each new aircraft. However, our ability to get good data is often compromised by things seemingly beyond our control. Until recently, test Reynolds number has been much too low. This can cause major problems, especially in high-speed wind tunnels. Ideally, tests in wind tunnels should be made at flight values of Reynolds number to be sure of having the proper flow over the model. The first practical solution to the problem of low Reynolds number in large research wind tunnels came as a result of combining cryogenic technology with wind-tunnel technology. The solution came early in the 1970s with the development of wind tunnels cooled to cryogenic temperatures by injecting liquid nitrogen (LN 2 ) into the tunnel circuit. The resulting cold gaseous nitrogen gives us an increase in Reynolds number by up to a factor of 7 with no increase in dynamic pressure and with a reduction in drive power. Work on two small cryogenic tunnels at the U.S. National Aeronautics and Space Administration (NASA) Langley Research Center (Langley) in the early 1970s led to the building of a very large cryogenic wind tunnel some 10 years later, the U.S. National Transonic Facility (NTF). The NTF combines a modern wind tunnel with large-scale cryogenic engineering. The NTF has a 2.5 x 2.5 m test section, which is large for transonic tunnels. It operates at pressures up to about 9 bar, which is very high for transonic tunnels. It is cooled by injecting liquid nitrogen at rates up to 450 kg/sec, which makes the NTF a fairly large and complex piece of cryogenic laboratory equipment. The success of the work at NASA Langley has prompted researchers around the world to design and build other cryogenic wind tunnels. In this paper I review the evolution and development of liquid-nitrogen cooled cryogenic wind tunnels used for aerodynamic research.