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

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 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 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. Abbreviations AHD Boundary-layer transition criterion according to Arnal, Habiballah and Delcourt CFD Computational fluid dynamics DIT Differential infrared thermography

Infrared thermography for boundary layer transition measurements

“…The method has been demonstrated for pitching airfoils at both high and low inflow velocities [57,59], and for pitching finite wings in both swept and unswept configurations [60,61]. An application to a rotor with cyclic pitch is also possible [62,63] but the sensor integration requires additional effort, for example by means of a telemetry system. Gao et al [64] and Wei et al [65] proposed an approach which is similar to the σC p -idea but was developed independently.…”

“…• Surface heating by external radiation flux sources: general purpose spotlights [63,80], infrared emitter [81], infrared laser [21], or sunlight [68,82]. • Surface heating by internal resistance heating: electrically conductive paint [83], carbon nano tube materials [84], heat foils [81], etc.…”

Influence of Formulations on Characteristics of Ruthenium-Based Temperature-Sensitive Paints

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Characterisation of boundary layer transition over a low Reynolds number rotor

“…They used the spatially averaged pixel intensity as a marker, suggesting that a steep drop in intensity corresponded to the onset of transition. DIT was also used by Weiss et al [18] to measure the transition position on a rotating blade under cyclic pitch conditions. Miozzi et al [19] investigated an LSB (Re = 8 × 10 4 ) over a NACA 0015 aerofoil and were able to characterize a coherent structure from time-resolved IR temperature measurements.…”