Unsteady Response of Natural Laminar Flow Airfoil Undergoing Small-Amplitude Pitch Oscillations

Abstract

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Onset of absolute instability on a pitching aerofoil

“…Of course, such sensitive dependence on operating conditions is not restricted to high Reynolds numbers, but may also be observed in the transitional regime, as has been documented by Pascazio et al (1996), Nati et al (2015) and Negi et al (2018), where small-amplitude pitch oscillations produced large variations in the boundary layer characteristics. The work of Negi et al (2018Negi et al ( , 2021 investigates these two regimes for the same aerofoil, and together they provide a fascinating contrast between the effects of small-amplitude forced pitch oscillations of an aerofoil in two different Reynolds number regimes. In the moderately high Reynolds number Re c = 750 000 (studied in Negi et al 2021), the unsteady flow dynamics could be qualitatively understood through quasi-steady assumptions leading to phase-lagged behaviour of the unsteady boundary layer.…”

“…The work of Negi et al (2018Negi et al ( , 2021 investigates these two regimes for the same aerofoil, and together they provide a fascinating contrast between the effects of small-amplitude forced pitch oscillations of an aerofoil in two different Reynolds number regimes. In the moderately high Reynolds number Re c = 750 000 (studied in Negi et al 2021), the unsteady flow dynamics could be qualitatively understood through quasi-steady assumptions leading to phase-lagged behaviour of the unsteady boundary layer. On the other hand, the transitional regime at Re c = 100 000, investigated in Negi et al (2018), displayed a richer dynamic behaviour of the unsteady boundary layer that was far from quasi-steady.…”

“…A series of investigations on aeroelasticity in the last decade has highlighted the critical role played by boundary layer transition on the aerodynamic forces experienced by the aerofoil during unsteady motion. Experiments done in moderately high Reynolds number compressible (Mai & Hebler 2011; Hebler, Schojda & Mai 2013) as well as incompressible (Lokatt 2017; Negi, Hanifi & Henningson 2021) regimes have shown that in natural laminar flow aerofoils, the laminar–turbulent transition point may have a sensitive dependence on the operating conditions, and small changes in angle of attack can result in large variations of the suction side transition location. The emergence of aerodynamic force nonlinearities was found to be linked intimately with this variation of the laminar–turbulent transition location.…”

“…The work of Negi et al. (2018, 2021) investigates these two regimes for the same aerofoil, and together they provide a fascinating contrast between the effects of small-amplitude forced pitch oscillations of an aerofoil in two different Reynolds number regimes. In the moderately high Reynolds number (studied in Negi et al.…”

“…In the moderately high Reynolds number (studied in Negi et al. 2021), the unsteady flow dynamics could be qualitatively understood through quasi-steady assumptions leading to phase-lagged behaviour of the unsteady boundary layer. On the other hand, the transitional regime at , investigated in Negi et al.…”