Unsteady boundary-layer transition measurements and computations on a rotating blade under cyclic pitch conditions

• Published in: Experiments in fluids 2020-02, Vol.61 (2), Article 61
• Main Authors: Weiss, Armin, Wolf, C. Christian, Kaufmann, Kurt, Braukmann, Johannes N., Heineck, James T., Raffel, Markus
• Format: Article
• Language: English
• Subjects: Actuation; Boundary layer transition; Computer simulation; Downstream effects; Engineering; Engineering Fluid Dynamics; Engineering Thermodynamics; Flow control; Fluid- and Aerodynamics; Forward flight; Heat and Mass Transfer; Helicopter performance; Infrared imaging; Mathematical models; Research Article; Suction; Thermography
• ISSN: 0723-4864; 1432-1114
• DOI: 10.1007/s00348-020-2899-7

The presented work tackles the lack of experimental investigations of unsteady laminar-turbulent boundary-layer transition on rotor blades at cyclic pitch actuation, which are important for accurate performance predictions of helicopters in forward flight. Unsteady transition positions were measured on the blade suction side of a four-bladed subscale rotor by means of non-intrusive differential infrared thermography (DIT). Experiments were conducted at different rotation rates corresponding to Mach and Reynolds numbers at 75% rotor radius of up to M 75 = 0.21 and Re 75 = 3.3 × 10 5 and with varying cyclic blade pitch settings. The setup allowed transition to be measured across the outer 54% of the rotor radius. For comparison, transition was also measured using conventional infrared thermography for steady cases with collective pitch settings only. The study is complemented by numerical simulations including boundary-layer transition modeling based on semi-empirical criteria. DIT results reveal the upstream and downstream motion of boundary-layer transition during upstroke and downstroke, a reasonable comparison to experimental results obtained using the already established σ c p method, and noticeable agreement with numerical simulations. The result is the first systematic study of unsteady boundary-layer transition on a rotor suction side by means of DIT including a comparison to numerical computations.