Infrared thermography for boundary layer transition measurements

Infrared thermography has been successfully adopted in the field of flow diagnostics over the last decades. Detecting the laminar–turbulent boundary layer transition through variations in the convective heat transfer is one of the primary applications due to its impact on the aerodynamic performance. Recent developments in fast–response infrared cameras allow unsteady measurement of fast–moving surfaces and moving transition positions, which must consider the thermal responsiveness of the surface material. Experimental results on moving boundary layer transition positions are highly valuable in the design and optimization of airfoils or rotor blades in unsteady applications, for example regarding helicopter main rotors in forward flight. This review article summarizes recent developments in steady and unsteady infrared thermography, particularly focusing on the development of differential infrared thermography (DIT). The new methods have also led to advances in the analysis of unmoving boundary layer transition for static airfoil test cases which were previously difficult to analyze using single–image methods.