The influence of airfoil kinematics on the formation of leading-edge vortices in bio-inspired flight

Rival, David E.; Prangemeier, Tim; Tropea, Cameron

doi:10.1007/s00348-008-0586-1

Cited by 97 publications

(38 citation statements)

References 12 publications

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“…Similar pitching experiments on a flat plate at a lower Reynolds number between Re 5 × 10 3 and 2 × 10 4 performed by Baik et al [16] showed that the instantaneous angle of attack and reduced frequency k determined flow evolution and that the LEV separation occurred later in the motion period, or at a higher angle of attack, with increased k, in agreement with the results of Rival et al [17]. They showed that leading-edge vortex circulation tended to increase linearly with the phase of the airfoil motion, with a faster growth corresponding to a lower reduced frequency k.…”

Section: Doi: 102514/1j054784supporting

confidence: 86%

Analysis of Flow Timescales on a Periodically Pitching/Surging Airfoil

2016

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Time-resolved velocity fields around a pitching and surging NACA 0018 airfoil were analyzed to investigate the influence of three independent timescales associated with the unsteady flowfield. The first of these timescales, the period of the pitch/surge motion, is directly linked to the development of dynamic stall. A simplified model of the flow using only a time constant mode and the first two harmonics of the pitch surge frequency has been shown to accurately model the flow. Full stall and leading-edge flow separation, however, were found to take place before the maximum angle of attack, indicating that a different timescale was associated with leading-edge vortex formation. This second, leading-edge vortex, timescale was found to depend on the airfoil convection time and compare well with the universal vortex formation time. Finally, instantaneous non-phase-averaged measurements were investigated to identify behavior not directly coupled to the airfoil motion. From this analysis, a third timescale associated with quasi-periodic Strouhal vortex shedding was found before flow separation. The interplay between these three timescales is discussed in detail, particularly as they relate to the periodic velocity and angle-of-attack change apparent to the blades of a vertical axis wind turbine.

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Section: Doi: 102514/1j054784supporting

confidence: 86%

Analysis of Flow Timescales on a Periodically Pitching/Surging Airfoil

2016

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“…Garmann and Visbal [25] and Granlund et al [29] have investigated vortex structures on pitching flat plates in detail through computational and experimental methods, respectively. Pitt and Babinsky [55], Baik et al [3], and Rival et al [61] have studied the effects of leading edge vortices using experimental techniques. Widmann and Tropea [75] have recently used experiments to investigate mechanisms responsible for the formation and detachment of LEVs.…”

Section: Introductionmentioning

confidence: 99%

Leading-edge flow criticality as a governing factor in leading-edge vortex initiation in unsteady airfoil flows

Ramesh

Granlund

et al. 2017

Theor. Comput. Fluid Dyn.

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A leading-edge suction parameter (LESP) that is derived from potential flow theory as a measure of suction at the airfoil leading edge is used to study initiation of leading-edge vortex (LEV) formation in this article. The LESP hypothesis is presented, which states that LEV formation in unsteady flows for specified airfoil shape and Reynolds number occurs at a critical constant value of LESP, regardless of motion kinematics. This hypothesis is tested and validated against a large set of data from CFD and experimental studies of flows with LEV formation. The hypothesis is seen to hold except in cases with slow-rate kinematics which evince significant trailing-edge separation (which refers here to separation leading to reversed flow on the aft portion of the upper surface), thereby establishing the envelope of validity. The implication is that the critical LESP value for an airfoil-Reynolds number combination may be calibrated using CFD or experiment for just one motion and then employed to predict LEV initiation for any other (fast-rate) motion. It is also shown that the LESP concept may be used in an inverse mode to generate motion kinematics that would either prevent LEV formation or trigger the same as per aerodynamic requirements.

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“…A common experimental and computational model of flapping flight is the sinusoidal pitching and/or heaving motion of an airfoil geometry in which an LEV is generated, and the effects of lift and drag can be carefully documented. Many experiments have looked at symmetric airfoils in sinusoidal heaving motion [11,12], or pitching motion [13], finding the boundary layer separation and formation of a LEV can greatly impact the lift generation. Experiments by Cleaver et.…”

Section: Introductionmentioning

confidence: 99%

Unsteady high-lift mechanisms from heaving flat plate simulations

Franck

Breuer

2017

International Journal of Heat and Fluid Flow

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Flapping animal flight is often modeled as a combined pitching and heaving motion in order to investigate the unsteady flow structures and resulting forces that could augment the animal's lift and propulsive capabilities. This work isolates the heaving motion of flapping flight in order to numerically investigate the flow physics at a Reynolds number of 40,000, a regime typical for large birds and bats and challenging to simulate due to the added complexity of laminar to turbulent transition in which boundary layer separation and reattachment are traditionally more difficult to predict. Periodic heaving of a thin flat plate at fixed angles of attacks of 1, 5, 9, 13, and 18 degrees are simulated using a large-eddy simulation (LES). The heaving motion significantly increases the average lift compared with the steady flow, and also surpasses the quasi-steady predictions due to the formation of a leading edge vortex (LEV) that persists well into the static stall region. The progression of the high-lift mechanisms throughout the heaving cycle is presented over the range of angles of attack. Lift enhancement compared with the equivalent steady state flow was found to be up to 17% greater, and up to 24% greater than that predicted by a quasi-steady analysis. For the range of kinematics explored it is found that maximum lift enhancement occurs at an angle of attack of 13 degrees, with a maximum lift coefficient of 2.1, a mean lift coefficient of 1.04.

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The influence of airfoil kinematics on the formation of leading-edge vortices in bio-inspired flight

Cited by 97 publications

References 12 publications

Analysis of Flow Timescales on a Periodically Pitching/Surging Airfoil

Analysis of Flow Timescales on a Periodically Pitching/Surging Airfoil

Leading-edge flow criticality as a governing factor in leading-edge vortex initiation in unsteady airfoil flows

Unsteady high-lift mechanisms from heaving flat plate simulations

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