Analysis of Flow Timescales on a Periodically Pitching/Surging Airfoil

Dunne, Reeve; Schmid, Peter J.; McKeon, Beverley

doi:10.2514/1.j054784

Cited by 22 publications

(13 citation statements)

References 31 publications

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“…The simplicity of a constant Reynolds number based on the tangential velocity can aid in the tractability of the problem. This same definition of chord-based blade Reynolds number has been used previously by a variety of authors (Ferreira et al 2009;Bachant & Wosnik 2016;Dunne, Schmid & McKeon 2016).…”

Section: Introductionmentioning

confidence: 98%

Dynamic stall in vertical axis wind turbines: scaling and topological considerations

Buchner

Soria

Honnery³

et al. 2018

J. Fluid Mech.

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Vertical axis wind turbine blades are subject to rapid, cyclical variations in angle of attack and relative airspeed which can induce dynamic stall. This phenomenon poses an obstacle to the greater implementation of vertical axis wind turbines because dynamic stall can reduce turbine efficiency and induce structural vibrations and noise. This study seeks to provide a more comprehensive description of dynamic stall in vertical axis wind turbines, with an emphasis on understanding its parametric dependence and scaling behaviour. This problem is of practical relevance to vertical axis wind turbine design but the inherent coupling of the pitching and velocity scales in the blade kinematics makes this problem of more broad fundamental interest as well. Experiments are performed using particle image velocimetry in the vicinity of the blades of a straight-bladed gyromill-type vertical axis wind turbine at blade Reynolds numbers of between 50 000 and 140 000, tip speed ratios between $\unicode[STIX]{x1D706}=1$ to $\unicode[STIX]{x1D706}=5$, and dimensionless pitch rates of $0.10\leqslant K_{c}\leqslant 0.20$. The effect of these factors on the evolution, strength and timing of vortex shedding from the turbine blades is determined. It is found that tip speed ratio alone is insufficient to describe the circulation production and vortex shedding behaviour from vertical axis wind turbine blades, and a scaling incorporating the dimensionless pitch rate is proposed.

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Section: Introductionmentioning

confidence: 98%

Dynamic stall in vertical axis wind turbines: scaling and topological considerations

Buchner

Soria

Honnery³

et al. 2018

J. Fluid Mech.

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“…The surface pressure measurements revealed the presence and continued growth of a suction peak at the leading edge during dynamic stall development. The use of time-resolved particle image velocimetry allowed for accurate flow field measurements that were analysed to reveal the dominant role of the dynamic stall vortex and its characteristic time scales [14][15][16][17][18] . Recent high-fidelity large eddy simulations 19,20 as well as direct numerical simulations 21 of dynamic stall on a pitching airfoil show excellent agreement with experimental results 22 .…”

Section: Introductionmentioning

confidence: 99%

Modeling the interplay between the shear layer and leading edge suction during dynamic stall

Deparday

Mülleners

2019

Physics of Fluids

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The dynamic stall development on a pitching airfoil at Re = 10 6 was investigated by time-resolved surface pressure and velocity field measurements. Two stages were identified in the dynamic stall development based on the shear layer evolution. In the first stage, the flow detaches from the trailing edge and the separation point moves gradually upstream. The second stage is characterised by the roll up of the shear layer into a large scale dynamic stall vortex. The two-stage dynamic stall development was independently confirmed by global velocity field and local surface pressure measurements around the leading edge. The leading edge pressure signals were combined into a single leading edge suction parameter. We developed an improved model of the leading edge suction parameter based on thin airfoil theory that links the evolution of the leading edge suction and the shear layer growth during stall development. The shear layer development leads to a change in the effective camber and the effective angle of attack. By taking into account this twofold influence, the model accurately predicts the value and timing of the maximum leading edge suction on a pitching airfoil. The evolution of the experimentally obtained leading edge suction was further analysed for various sinusoidal motions revealing an increase of the critical value of the leading edge suction parameter with increasing pitch unsteadiness. The characteristic dynamic stall delay decreases with increasing unsteadiness and the dynamic stall onset is best assessed by critical values of the circulation and the shear layer height which are motion independent.

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“…The benefits of applying modal decomposition in general, and DMD in particular, to examine dynamic stall may be found in Dunne et al. (2016), Mohan et al. (2016) and Ramos (2019).…”

Section: Resultsmentioning

confidence: 99%

Analysis of the onset and evolution of a dynamic stall vortex on a periodic plunging aerofoil

et al. 2022

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The onset and evolution of the dynamic stall vortex (DSV) are analysed by means of large eddy simulations of an SD7003 aerofoil undergoing periodic plunging motion in a transitional Reynolds number flow ( $Re =6\times 10^{4}$ ). Interactions between upstream propagating Kelvin–Helmholtz instabilities and a shear layer formed at the leading edge trigger flow separation. The former appear to be related to acoustic waves scattered at the trailing edge due to initial vortex shedding. Two freestream Mach numbers ( $M_{\infty }=0.1$ and $0.4$ ) are employed to examine the flow differences due to compressibility variations. The existence of a common timing for the acoustic perturbations in both flows suggests a possible Mach number invariance for the birth of the Kelvin–Helmholtz instability. Increasing compressibility, however, induces earlier spanwise fluctuations, higher flow three-dimensionality and a weaker and more diffuse DSV, which is formed further downstream of the leading edge and has lower residency time. In order to better characterize the onset of the DSV, two empirical criteria are assessed: the leading edge suction parameter and the chord-normal shear layer height. Results demonstrate a higher robustness of the latter with respect to Mach number variations. Modal decomposition, performed with both the classical dynamic mode decomposition (DMD) and its multi-resolution variant (mrDMD), highlights key trends and demonstrates the capacity of the mrDMD to extract physically meaningful flow structures related to the stall onset. Such detailed characterization of the shear layer can be used for a systematic exploration of flow control strategies for unsteady aerofoils.

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Analysis of Flow Timescales on a Periodically Pitching/Surging Airfoil

Cited by 22 publications

References 31 publications

Dynamic stall in vertical axis wind turbines: scaling and topological considerations

Dynamic stall in vertical axis wind turbines: scaling and topological considerations

Modeling the interplay between the shear layer and leading edge suction during dynamic stall

Analysis of the onset and evolution of a dynamic stall vortex on a periodic plunging aerofoil

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