Differential Infrared Thermography for Transition Detection on Rotor Blades

Abstract

Differential Infrared Thermography (DIT) was applied and investigated for the detection of unsteady boundary layer transition locations on a pitching rotor blade airfoil and on a rotating blade under cyclic pitch. The paper demonstrates the unsteady transition detection with a high framing rate infrared camera and the differential infrared thermography method. DIT is based on image intensity differences between two subsequently recorded images. A pitching NACA0012 airfoil served as the first test object. The recorded images were used in order to investigate and to further improve evaluation strategies for periodically moving boundary layer transition lines. The gained knowledge has then been used for the optical measurement of unsteady transition locations on helicopter rotor blade models under cyclic pitch and rotation for the first time. Image de-rotation for tracking the blade was employed using a rotating mirror to increase exposure time without causing motion blur. The paper describes the challenges that occurred during the recording and evaluation of the data in detail. The results obtained were discussed and found to be encouraging to further improve the method towards the measurement of unsteady boundary layer transition lines on helicopter rotor models in forward flight condition without the need of blade instrumentation.