Unsteady boundary layer transition measurements by differential infrared thermography

Maintaining laminar flow over a large portion of a rotor blade reduces drag appreciably. Therefore, the location of boundary layer transition is an important design parameter for rotor blades. Unsteady laminar/turbulent boundary layer transition is of special interest, since modern rotor blade design methodologies increasingly take dynamic performance of into account. Once the design is concluded, the real transition position on the blade is of great interest as the knowledge of the exact transition position is needed for both the validation of the design and the performance assessment. The paper demonstrates the detection of unsteady boundary layer transition with a high framing rate infrared camera and the differential infrared thermography (DIT) method. DIT is based on image intensity differences between two subsequently recorded measurement images. A pitching NACA0012 airfoil served as the test object. The measurement results are compared with the results of unsteady CFD simulations based on the DLR TAU code.