Insights into leading edge vortex formation and detachment on a pitching and plunging flat plate

Kissing, Johannes; Kriegseis, Jochen; Li, Zhenyao; Feng, Li-Hao; Hussong, Jeanette; Tropea, Cameron

doi:10.1007/s00348-020-03034-1

Cited by 17 publications

(10 citation statements)

References 31 publications

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“…It is defined as α * = αss c/U, where αss is the angular velocity of the pitching manoeuvre at the static stall angle. This parameter has been shown to describe the dynamic stall behaviour in various studies (Mulleners & Raffel 2012;Kissing et al 2020;Le Fouest, Deparday & Mulleners 2021). However, due to the symmetry of the sine function about its mean value, the magnitude of the instantaneous effective unsteadiness is identical for test cases at mean angles equidistant to the static stall angle.…”

Section: Measure Of Unsteadinessmentioning

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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Section: Measure Of Unsteadinessmentioning

confidence: 99%

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

et al. 2022

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“…As a result, the vorticity feeding rate of the dynamic stall vortex decreases and the vortex lifts-off from the surface. Vortex lift-off occurs when the vortex has grown strong enough such that secondary vortices of opposite-signed vorticity emerge between the vortex and the blade's surface (figure 6(a)) or when the effective inflow velocity decreases and drops below the self-induced velocity of the vortex [31,32]. Even though the dynamic stall vortex and the circulation in the field of view continues to increase for 0.25 ≤ t/T < 0.4, the tangential force coefficient drops as the vortex is no longer bound to the blade.…”

Section: Resultsmentioning

confidence: 99%

The dynamic stall dilemma for vertical-axis wind turbines

Fouest,

Mulleners

2022

Preprint

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Vertical-axis wind turbines (VAWT) are excellent candidates to complement traditional wind turbines and increase the total wind energy capacity. Development of VAWT has been hampered by their low efficiency and structural unreliability, which are related to the occurrence of dynamic stall. Dynamic stall consists of the formation, growth, and shedding of large-scale dynamic stall vortices, followed by massive flow separation. The vortex shedding is detrimental to the turbine's efficiency and causes significant load fluctuations that jeopardise the turbine's structural integrity. We present a comprehensive experimental characterisation of dynamic stall on a VAWT blade including time-resolved load and velocity field measurements. Particular attention is dedicated to the dilemma faced by VAWT to either operate at lower tip-speed ratios to maximise aerodynamic performance but experience dynamic stall, or to avoid dynamic stall at the cost of loosing performance. Based on the results, we map turbine operating conditions to one of three regimes: deep stall, light stall, and no stall. The light stall regime offers VAWT the best compromise in the dynamic stall dilemma as it yields positive tangential forces during the upwind and downwind rotation and reduces load transients by 75 % compared to the deep stall regime.

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“…As the pitch rate varies continuously for a sinusoidal motion, we use the instantaneous pitch rate at the time the static stall angle is exceeded as the representative effective pitch rate for the sinusoidal motions [12,14,13]. The effective pitch rates for the sinusoidal motions vary between 0.0035 and 0.02.…”

Section: Resultsmentioning

confidence: 99%

“…The angular delay of flow separation is considered one of the classical hallmarks of dynamic stall [5]. From a timing perspective however, high pitch rates promote flow separation and reduce the blade's reaction time relative to the static case [12,13].…”

Section: Introductionmentioning

confidence: 99%

The dynamics and timescales of static stall

Fouest,

Deparday,

Mulleners

2021

Preprint

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Airfoil stall plays a central role in the design of safe and efficient lifting surfaces. We typically distinguish between static and dynamic stall based on the unsteady rate of change of an airfoil's angle of attack. Despite the somewhat misleading denotation, the force and flow development of an airfoil undergoing static stall are highly unsteady and the boundary with dynamic stall is not clearly defined. We experimentally investigate the forces acting on a two-dimensional airfoil that is subjected to two manoeuvres leading to static stall: a slow continuous increase in angle of attack with a reduced pitch rate of 1.3 × 10 −4 and a step-wise increase in angle of attack from 14.2°to 14.8°within 0.04 convective times. We systematically quantify the stall reaction delay, or the timespan between the moment the blade exceeds its critical static stall angle and the onset of stall, for many repetitions of these two manoeuvres. The onset of flow stall is marked by the distinct drop in the lift coefficient. The reaction delay for the slow continuous ramp-up manoeuvre is not influenced by the blade kinematics and its occurrence histogram is normally distributed around 32 convective times.The static reaction delay is compared with dynamic stall delays for dynamic ramp-up motions with reduced pitch rates ranging from 9 × 10 −4 to 0.14 and for dynamic sinusoidal pitching motions of different airfoils at higher Reynolds numbers up to 1 × 10 6 . The stall delays for all conditions follows the same power law decrease from 32 convective times for the most steady case down to an asymptotic value of 3 convective times for reduced pitch rates above 0.04. Static stall is not phenomenologically different than dynamic stall and is merely a typical case of stall for low pitch rates where the onset of flow separation is not promoted by the blade kinematics. Based on our results, we suggest that conventional measurements of the static stall angle and the static load curves should be conducted using a continuous and uniform ramp-up motion at a reduced frequency around 1 × 10 −4 .

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Insights into leading edge vortex formation and detachment on a pitching and plunging flat plate

Cited by 17 publications

References 31 publications

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

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

The dynamic stall dilemma for vertical-axis wind turbines

The dynamics and timescales of static stall

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