Optimisation of Differential Infrared Thermography for Unsteady Boundary Layer Transition Measurement

Abstract

Differential infrared thermography (DIT) is a method of analyzing infrared images to measure the unsteady motion of the laminar-turbulent transition of a boundary layer. It uses the subtraction of two infrared images taken with a short time delay. DIT is a new technique which already demonstrated its validity in applications related to the unsteady aerodynamics of helicopter rotors in forward flight. The current study investigates a pitch-oscillating airfoil and proposes several optimizations of the original concept. These include the extension of DIT to steady test cases, a temperature compensation for long-term measurements, and a discussion of the proper infrared image separation distance. The current results also provide a deeper insight into the working principles of the technique. The results compare well to reference data acquired by unsteady pressure transducers, but at least for the current setup DIT results in an additional measurement-related lag for relevant pitching frequencies.