Abstract
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 {mathrm {Re}}_{75} = 3.3 times 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 sigma 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.
Highlights
Laminar-turbulent boundary-layer transition strongly affects the power requirement of helicopter rotors
Numerical simulations of rotor aerodynamics often do not consider boundary-layer transition. This is due to a lack of appropriate transition models, which must account for the complex three-dimensional and unsteady flow conditions of a rotor in forward flight, and need experimental data for validation
Measured data are further compared to unsteady boundary-layer transition computations using the DLR-TAU code and the rotor blade transition (RBT) tool as recently applied by Kaufmann et al (2019) The result is the first systematic study of unsteady boundary-layer transition on a rotor suction side by means of differential infrared thermography (DIT) including a comparison to numerical prediction capabilities at DLR
Summary
Laminar-turbulent boundary-layer transition strongly affects the power requirement of helicopter rotors. Many experiments reduce the complexity of a rotor setup by investigating boundary-layer transition on periodically pitching airfoils equipped with locally installed fast-response hot-film sensors (Lorber and Carta 1992) or dynamic pressure transducers (Gardner and Richter 2015). The application of these techniques in the rotating frame demands the laborious effort of integrating the sensor into the model, yields results at only discrete locations, and possibly disturbs the boundary-layer flow. The techniques have been demonstrated in experiments using hot-film sensors integrated into the blades of a subscale helicopter model (Raffel et al 2011), or pressure transducers in dynamically pitching Mach-scaled rotor blades (Schwermer et al 2019)
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