Dynamic stall at high Reynolds numbers due to variant types of airfoil motion

Kiefer, Janik; Brunner, Claudia; Hultmark, Marcus; Hansen, Morten Hartvig

doi:10.1088/1742-6596/1618/5/052028

Cited by 4 publications

(5 citation statements)

References 6 publications

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“…The above observations reveal that the reduced frequency does not sufficiently characterise the dynamic stall process of airfoils undergoing differing pitch motions. This supports the findings of [22]. Because the pitch velocity varies significantly between the upstroke and the downstroke, the unsteadiness of the flow is more aptly described by two separate reduced frequencies, one corresponding to the upward pitch and the other to the downward pitch.…”

Section: Tip Speed Ratio Correctionsupporting

confidence: 87%

Comparison of dynamic stall on an airfoil undergoing sinusoidal and VAWT-shaped pitch motions

Brunner

Kiefer

Hultmark

2022

J. Phys.: Conf. Ser.

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The aerodynamics of vertical axis wind turbines (VAWTs) are inherently unsteady because the blades experience large angle of attack variations throughout a full turbine revolution. At low tip speed ratios, this can lead to a phenomenon known as dynamic stall. To better characterise the unsteady aerodynamics and represent them in models and simulations, data from studies of individual static or pitching airfoils are often applied to VAWT blades. However, these studies often involve sinusoidally pitching airfoils, whereas the pitching motions experienced by VAWTs are more complex. Here, the pressures and forces on an airfoil undergoing VAWT-shaped pitch motions corresponding to various tip speed ratios are compared to those of a sinusoidally pitching airfoil in order to assess whether a sinusoidal motion represents a reasonable approximation of the motions of a VAWT blade. While the lift development induced by the sinusoidal motion yields good agreement with that induced by the VAWT-shaped motion at the higher tip speed ratios, notable discrepancies exist at the lower tip speed ratios, where the VAWT motion itself deviates more from the sinusoid. Comparison with sinusoidal motions at reduced frequencies corresponding to the upstroke or downstroke of the VAWT-shaped motion yield better agreement in terms of the angle of stall onset.

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Section: Tip Speed Ratio Correctionsupporting

confidence: 87%

Comparison of dynamic stall on an airfoil undergoing sinusoidal and VAWT-shaped pitch motions

Brunner

Kiefer

Hultmark

2022

J. Phys.: Conf. Ser.

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“…This motion control, as opposed to the use of a lever mechanism to produce the angle variation, provided a high degree of freedom in the design of the waveform, so that not only sinusoidal but any arbitrary oscillations could be generated (Kiefer et al. 2020). The set-up, however, was limited by the torque provided by the stepper motor to overcome the aerodynamic moment induced in a given test condition.…”

Section: Experimental Set-upmentioning

confidence: 99%

Dynamic stall at high Reynolds numbers induced by ramp-type pitching motions

et al. 2022

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The transient pressure field around a moderately thick airfoil is studied as it undergoes ramp-type pitching motions at high Reynolds numbers and low Mach numbers. A unique set of laboratory experiments were performed in a high-pressure wind tunnel to investigate dynamic stall at chord Reynolds numbers in the range of $0.5\times 10^6\leq Re _c\leq 5.5\times 10^6$ in the absence of compressibility effects. In addition to variations of mean angle and amplitude, pitching manoeuvres at reduced frequencies in the range of $0.01\leq k\leq 0.40$ were studied by means of surface-pressure measurements. Independently of the parameter variations, all test cases exhibit a nearly identical stall behaviour characterized by a gradual trailing-edge stall, in which the dynamic stall vortex forms approximately at mid-chord. The location of the pitching window with respect to the Reynolds-number-dependent static stall angle is found to define the temporal development of the stall process. The time until stall onset is characterized by a power law, where a small excess of the static stall angle results in a drastically prolonged stall delay. The reduced frequency exhibits a decrease in impact on the stall development in the case of angle-limited pitching manoeuvres. Beyond a critical reduced frequency, both load magnitudes and vortex evolution become reduced frequency independent and instead depend on the geometry of the motion and the convective time scale, respectively. Overall, the characteristics of vortex evolution induced by dynamic stall show remarkable similarities to the framework of optimal vortex formation reported in Gharib et al. (J. Fluid Mech., vol. 360, 1998, pp. 121–140). The data from this study are publicly available at https://doi.org/10.34770/b3vq-sw14.

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“…The surface term of the integral equation includes the convective fluxes F c and the viscous fluxes F v . Both vectors are presented in Equation (3). In this Equation, V n = υ • n is the fluid velocity projected onto the surface normal n = (n x , n y , n z ) and ∆V is the difference between the velocity V n and the projected grid velocity…”

Section: Governing Equationsmentioning

confidence: 99%

“…Compared to these types of turbines, flapping foils have certain benefits. Firstly, the efficiency of flapping foils remains high even in very large angles of attack with the flow, due to the phenomenon of dynamic stalling which insures that lift remains temporarily high [3]. This also means that oscillating hydrofoils can harvest energy from a wider range of current velocities [4] compared to conventional turbines with a specific design point.…”

Section: Introductionmentioning

confidence: 99%

Investigation of Submergence Depth and Wave-Induced Effects on the Performance of a Fully Passive Energy Harvesting Flapping Foil Operating Beneath the Free Surface

Petikidis,

Papadakis

2023

JMSE

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This paper investigates the performance of a fully passive flapping foil device for energy harvesting in a free surface flow. The study uses numerical simulations to examine the effects of varying submergence depths and the impact of monochromatic waves on the foil’s performance. For the numerical simulations, a in-house artificial compressibility two-phase solver is employed and coupled with a rigid body dynamic solver. The results show that the fully passive flapping foil device can achieve high efficiency for submergence depths between 4 and 9 chords, with an “optimum” submergence depth where the flapping foil performance is maximised. The effects of regular waves on the foil’s performance were also investigated, showing that waves with a frequency close to that of the natural frequency of the flapping foil-aided energy harvesting. Overall, this study provides insights that could be useful for future design improvements for fully passive flapping foil devices for energy harvesting operating near the free surface.

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Dynamic stall at high Reynolds numbers due to variant types of airfoil motion

Cited by 4 publications

References 6 publications

Comparison of dynamic stall on an airfoil undergoing sinusoidal and VAWT-shaped pitch motions

Comparison of dynamic stall on an airfoil undergoing sinusoidal and VAWT-shaped pitch motions

Dynamic stall at high Reynolds numbers induced by ramp-type pitching motions

Investigation of Submergence Depth and Wave-Induced Effects on the Performance of a Fully Passive Energy Harvesting Flapping Foil Operating Beneath the Free Surface

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