Oil Film Interferometry for Boundary Layer Measurements in Aircraft Development

Abstract

Oil film interferometry (OFI) is a useful measurement method throughout the aircraft development process. From low-speed wind tunnel testing to high speed wind tunnel testing to flight testing, important boundary layer-related phenomenon can be identified and understood based on information obtained using OFI. The simplified OFI methods developed in the early 1990s are key to the practical implementation of OFI in the aircraft development process. In wind tunnel testing and in flight testing, having an easy to use, qualitative method to provide insight into detailed boundary layer behavior helps to answer questions much more quickly than is otherwise possible. The ability of OFI to also provide quantitative data further enhances it value. Nomenclature Cf = coefficient of skin friction Cf� = coefficient of skin friction nondimensionalized using freestream dynamic pressure mN = image constant for determining skin friction from interference patterns no = index of refraction of oil at reference pressure qref = freestream or reference dynamic pressure for determining image constant R = coefficient of thermal effect for silicone oil S = coefficient of thermal effect for silicone oil T = temperature t = time Tref = temperature for oil reference properties trun = elapse time of wind tunnel run for oil film measurements �sf,im = interference fringe spacing measured from images � = wavelength of light used to create interference images �O,ref = oil reference viscosity �O,nom = nominal oil reference in centistokes �O,ref = oil reference density � = off-normal angle of illumination of interference image