NASA's Langley Research Center. The Langley 16-Foot Transonic Tunnel (16-Ft TT) is an atmospheric, closed circuit tunnel with a Mach number range of 0.2 to 1.25. The test section of the tunnel is octagonal with a distance of 15.5 ft across the flats. The twin 34-ft diameter drive fans form a two-stage axial flow compressor with counterrotating blades and no stator. Boundary layer control during transonic operation is achieved with a 35,000-hp axial flow compressor that is able to remove up to 4.5 percent of the tunnel flow from the plenum that surrounds the test section.