Boundary-layer transition-detection in a cryogenic wind tunnel usinginfrared imaging

Abstract

Cryogenic wind tunnels are currently the only option available to test models of medium to large transport airplanes at high Reynolds numbers identical to those encountered in flight. This matching in Reynolds numbers is required for reproducing the flow viscous effects, most notably the laminar to turbulent boundary-layer transition. Infrared imaging is particularly attractive for transition detection in cryogenic wind tunnels because it is nonintrusive, fast, and provides a global view of the boundary-layer status on the model. An experimental method using a commercial 8- to 12-p infrared imaging system was developed for transition detection in cryogenic wind tunnels. So far, transition has been detected at temperatures down to 170 K. At this temperature, tunnels with a pressurization capability of only a few atmospheres can generate transonic flows at Reynolds numbers beyond 3 X108 per meter. Nomenclature M = Mach number Pt = total pressure R/m = Reynolds number per meter Rc = chord Reynolds number Tt = total temperature a = angle of attack ATt = total temperature step set in the wind-tunnel flow