Multiscale Vortex Characteristics of Dynamic Stall from Empirical Mode Decomposition

Ansell, Phillip J.; Mülleners, Karen

doi:10.2514/1.j057800

Cited by 18 publications

(14 citation statements)

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“…in Göttingen. The data from this measurement campaign was previously used to study the onset and development of dynamic stall 14,43,44 This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.…”

Section: Methodsmentioning

confidence: 99%

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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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Section: Methodsmentioning

confidence: 99%

“…c. The temporal evolution of the dynamic stall development has been discussed in detail in previous publications 14,43,44 and is summarised here for reference. From the beginning of…”

Section: Please Cite This Article As Doi:101063/15121312mentioning

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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“…In other experimental investigations, Deparday & Mulleners (2019) and Ansell & Mulleners (2020) estimated the LESP from surface pressure and time-resolved particle image velocimetry measurements of a sinusoidally pitching OA209 aerofoil (, , quarter-chord pivot) at a Reynolds number of 920 000. The trends from their work are similar to those seen in the current work, providing further experimental confirmation for the observations made in the current work.…”

Section: Post-leading-edge-separation Behaviour Of Lespmentioning

confidence: 99%

“…Although there was no proof at that time for the constant value of leading-edge suction during LEV shedding, the predictions for the low-Reynolds-number cases from the LDVM code using this implementation agreed well with CFD and experimental results for a variety of pitch and plunge motions (Ramesh et al 2014). Since then, the LESP criterion for modelling LEV formation has been successfully used to study aeroelastic problems (Ramesh, Murua & Gopalarathnam 2015), flow-energy harvesting approaches (Liu et al 2016), machine-learning approaches to modelling separated flows (Eldredge & Jones 2019; Hou, Darakananda & Eldredge 2019), finite-wing LEV formation (Hirato et al 2019) and to also explain some experimental results (Ansell & Mulleners 2020).…”

Section: Introductionmentioning

confidence: 99%

Variation of leading-edge suction during stall for unsteady aerofoil motions

et al. 2020

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“…In this study, the intermittent processes during dynamic stall were detected more efficiently using EMD, as compared to proper orthogonal decomposition. Ansell and Mulleners [32] used a combination of EMD and 2D Fourier transform on the experimentally-acquired velocity field about a sinusoidally pitching OA209 airfoil, to provide quantitative estimates of the dominant length scales associated with the dynamic stall flowfield. These length scales were then related to the characteristic flow structures in the off-body flowfield, including the formation of discrete vortices due to an instability in the separated shear layer, interactions between distinct vortical structures leading to a vortex-pairing process, and the large-scale roll-up behavior resulting in the emergence of the DSV.…”

Section: Literature Reviewmentioning

confidence: 99%

Unsteady Flow Physics of Airfoil Dynamic Stall

Gupta

Ansell²

2019

AIAA Journal

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A series of wind tunnel experiments were conducted on an NACA 0012 airfoil undergoing a linear pitch ramp maneuver at a fixed dimensionless pitch rate of Ω + = 0.05 and across three transitional Reynolds numbers, Re c = 0.2 × 10 6 , Re c = 0.5 × 10 6 , and Re c = 1.0 × 10 6. The primary objectives of these experiments were to perform a detailed analysis of the flow evolution, with particular emphasis on the underlying physical mechanisms, and to extract the dominant scales associated with the flow perturbations, for a canonical dynamic stall process. A series of unsteady surface pressure measurements, with a high sampling frequency, were acquired in order to investigate the time-dependent behavior of the flow in the immediate vicinity of the airfoil. These surface pressure measurements were used to identify the region of boundary layer transition during the initial stages of the dynamic stall process. A spatially-contracting laminar separation bubble was also identified near the airfoil leading edge from the characteristic pressure plateau in the surface pressure distribution. The dominant frequencies associated with the laminar separation bubble were extracted using a continuous wavelet transform technique. These frequencies were observed to span a wide range of chordbased Strouhal numbers between St = 50 and St = 105, at Re c = 0.5 × 10 6. The off-body flow evolution was inferred and described using a combination of surface pressure measurements and time-resolved particle image velocimetry. For Re c = 0.2 × 10 6 and Re c = 0.5 × 10 6 , the dynamic stall vortex was observed to emerge from a collective interaction of the discrete vortices that were ejected from the leading edge of the airfoil. At Re c = 1.0 × 10 6 , however, the near-wall vortices were observed to amalgamate into two regions, forming a distinct primary and a secondary coherent structure. After formation, these two structures were observed to interact with each other, following a co-rotating vortex merging process and resulting in the emergence of a single, coherent dynamic stall vortex. The process of emergence of the dynamic stall vortex at Re c = 1.0 × 10 6 , observed from the present experiments, is therefore quite distinct from the classical understanding of the dynamic stall vortex formation, which was observed at the lower Reynolds numbers. The time-dependent spectra of the velocity field were calculated using a combination of empirical mode decomposition and Hilbert transformation. From the velocity spectra, the fluctuations in the flow were observed to attain an amplified state during the initial ejection of vorticity from the leading-edge region of

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Multiscale Vortex Characteristics of Dynamic Stall from Empirical Mode Decomposition

Cited by 18 publications

References 75 publications

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

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

Variation of leading-edge suction during stall for unsteady aerofoil motions

Unsteady Flow Physics of Airfoil Dynamic Stall

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