Critical evolution of leading edge suction during dynamic stall

Deparday, Julien; Mülleners, Karen

doi:10.1088/1742-6596/1037/2/022017

Cited by 9 publications

(8 citation statements)

References 8 publications

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“…Beyond stall development, a vortex-induced separation mechanism was identified by these authors, as the governing physical process that was responsible for the onset of dynamic stall. The two-stage stall development process was also observed by Deparday and Mulleners [16,17]. Additionally, these authors were able to demonstrate a strong mathematical correlation between the surface pressure field near the airfoil leading edge and the state of the shear layer during the stall development phase of the dynamic stall process.…”

Section: Literature Reviewsupporting

confidence: 62%

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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Section: Literature Reviewsupporting

confidence: 62%

Unsteady Flow Physics of Airfoil Dynamic Stall

Gupta

Ansell²

2019

AIAA Journal

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“…LEADING EDGE SUCTION DURING DYNAMIC STALL A. Leading edge suction parameterThe leading edge suction parameter for our experimental data is derived from the leading edge suction vector S LE which is determined by integrating the pressure signals from the 13 unsteady pressure sensors located in the front 10 % of the airfoil (figure 2)42 . Approximately 40 % of the lift and 90 % of the chord-wise force component during the pitch-up motion are generated by the front 10 % of the airfoil.Figure 5 presents the phase-averaged evolution of the leading edge suction vector during pitch-up for the example dynamic stall case presented in figure 3.…”

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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 invariance of the maximum value of LESP with motion kinematics for a given aerofoil geometry and Re c was found to hold for low Reynolds numbers and higher pitch rates (Ramesh et al 2018;Hirato et al 2021). Deparday & Mulleners (2018) identified a primary instability stage and vortex formation stage, by a maximum in the leading edge suction between the two stages, also delineated by a change in shear layer growth rate.…”

Section: Introductionmentioning

confidence: 99%

“…2021). Deparday & Mulleners (2018) identified a primary instability stage and vortex formation stage, by a maximum in the leading edge suction between the two stages, also delineated by a change in shear layer growth rate.…”

Section: Introductionmentioning

confidence: 99%

A vorticity-based criterion to characterise leading edge dynamic stall onset

2022

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We propose a more conservative, physically-intuitive criterion, namely, the boundary enstrophy flux ( $BEF$ ), to characterise leading-edge-type dynamic stall onset in incompressible flows. Our results are based on wall-resolved large-eddy simulations of pitching aerofoils, with fine spatial and temporal resolution around stall onset. We observe that $|BEF|$ reaches a maximum within the stall onset regime identified. By decomposing the contribution to $BEF$ from the flow field, we find that the dominant contribution arises from the laminar leading edge region, due to the combined effect of large clockwise vorticity and favourable pressure gradient. A relatively small contribution originates from the transitional/turbulent laminar separation bubble (LSB) region, due to LSB-induced counter-clockwise vorticity and adverse pressure gradient. This results in $BEF$ being nearly independent of the integration length as long as the region very close to the leading edge is included. This characteristic of $BEF$ yields a major advantage in that the effect of partial or complete inclusion of the noisy LSB region can be filtered out, without changing the $BEF$ peak location in time significantly. Next, we analytically relate $BEF$ to the net wall shear and show that its critical value ( $=\max (|BEF|)$ ) corresponds to the instant of maximum net shear prevailing at the wall. Finally, we have also compared $BEF$ with the leading edge suction parameter ( $LESP$ ) (Ramesh et al., J. Fluid Mech., vol. 751, 2014, pp. 500–538) and find that the former reaches its maximum value between $0.3^{\circ }$ and $0.8^{\circ }$ of rotation earlier.

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Critical evolution of leading edge suction during dynamic stall

Cited by 9 publications

References 8 publications

Unsteady Flow Physics of Airfoil Dynamic Stall

Unsteady Flow Physics of Airfoil Dynamic Stall

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

A vorticity-based criterion to characterise leading edge dynamic stall onset

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