Experimental and Numerical Investigations of Unsteady Boundary-Layer Transition on a Dynamically Pitching Subscale Helicopter Rotor Blade

The presented work tackles the lack of experimental investigations of unsteady boundary-layer transition on rotor blades at cyclic pitch actuation, which are important for accurate performance predictions, for instance for helicopters in forward flight. Unsteady boundary-layer transition positions were measured on the blade suction side of a four-bladed subscale rotor by means of the non-intrusive Differential Infrared Thermography (DIT), which does not require instrumented rotor blades. Experiments were conducted at rotation rates corresponding to Mach and Reynolds numbers at 75% rotor radius of M_75 = 0.11, 0.22 and Re_75=10*5 = 1.7, 3.5 and at varying cyclic blade pitch settings. The setup allowed to sample boundary-layer transition across the outer 55% of the rotor radius. The study is complemented by numerical simulations including boundary-layer transition modelling based on semi-empirical transition criteria. For the first time, the work successfully applies DIT to capture unsteady boundary-layer transition on the blade suction side on rotating blades. Promising results reveal a plausible development of detected boundary-layer transition positions over the pitch cycle, a reasonable comparison to experimental results obtained using the established sigma c_p method and noticeable agreement to numerical simulations.