Unsteady Boundary Layer Transition Detection with Local Infrared Thermography

A new approach to measuring unsteady boundary layer transition in periodic processes with infrared thermography is introduced. The radiation from the heated suction surface of a pitching airfoil model is measured with an infrared camera. The extraction of the extrema of the measured radiation signal at fixed model locations yields instants of the motion phase that correlate with the occurrence of boundary layer transition. The analysis of the extrema of the signal’s gradient produces results that are equivalent to the results acquired by optimized differential infrared thermography, and it provides insight into the origins of the systematic error of that technique. It is demonstrated that the local infrared thermography approach can be readily extended to measuring two-dimensional boundary layer transition fronts.